Transcriber’s Note


Italic text is enclosed in _underscores_; subscripts are enclosed in
{curly brackets}. Additional notes will be found near the end of this
ebook.




                           THE MANUFACTURE OF
                             EARTH COLOURS




                                  THE
                             MANUFACTURE OF
                             EARTH COLOURS

                                   BY
                            DR. JOSEF BERSCH

              TRANSLATED FROM THE THIRD GERMAN EDITION (AS
                  REVISED BY PROF. DR. WILHELM BERSCH)

                                   BY
                             CHARLES SALTER

                    _WITH THIRTY-ONE ILLUSTRATIONS_


                                 LONDON
                         SCOTT, GREENWOOD & SON
                       8 BROADWAY, LUDGATE, E.C.4
                                  1921

        [_The sole rights of translation in English remain with
                       Scott, Greenwood & Son._]




                      PRINTED IN GREAT BRITAIN BY
                     RICHARD CLAY & SONS, LIMITED,
                  PARIS GARDEN, STAMFORD ST., S.E. 1,
                          AND BUNGAY, SUFFOLK.




PREFACE


Originally issued as a volume of the series on pigments and colouring
matters by the present author’s father, the necessity for a new edition
afforded a welcome opportunity of revising “_Earth Colours_.” Although,
in the nature of things, little progress has been made in this subject
itself, there was a good deal to add in connection with the mechanical
appliances for treating the colour earths and manufacturing them into
pigments. In other respects, too, the work has been carefully gone
through and brought up to date, with new and additional illustrations.

The author desires to express his thanks to the various firms who
have afforded him assistance in his task by furnishing illustrations
and descriptions of new machinery, together with other information.
It is hoped that this third edition will meet the approval of those
interested in the subject; and the author will be glad to receive
supplementary information to render the work more complete in the event
of a future edition being found advisable.

                                             PROF. DR. WILHELM BERSCH.

1918.




CONTENTS


  CHAPTER I

                                                           PAGE
  INTRODUCTORY                                                1


  CHAPTER II

  THE RAW MATERIALS FOR EARTH COLOURS                         8

      (A) White Raw Materials and Pigmentary Earths          11

          Limestone (Calcite, Limestone, Chalk)              11

          Gypsum (Alabaster)                                 18

          Barytes, or Heavy Spar                             19

          Talc, Soapstone, Steatite                          20

          Clay                                               21

      (B) Yellow Earths                                      23

          Brown Ironstone                                    23

          Ochre                                              25

          Yellow Earth                                       26

          Terra di Siena                                     27

      (C) The Red Earths                                     27

          Red Ironstone                                      28

          Bole                                               31

          Alum Sludge                                        32

          Mine Sludge                                        32

      (D) Blue Earths                                        33

          Azurite, or Ultramarine                            33

          Vivianite                                          33

      (E) Green Earth Pigments                               34
          Green Earth                                        34
          Malachite                                          35

      (F) Brown Earth Pigments                               36
          Umber                                              36
          Asphaltum                                          37

      (G) Black Earth                                        38
          Black Schist                                       38
          Graphite                                           38


  CHAPTER III

  THE PREPARATION OF THE COLOUR EARTHS                       40
      Crushing Machinery                                     43
      Crushing and Sifting                                   77
      Calcining                                              81
      Mixing and Improving                                   81
      Moulding                                               85


  CHAPTER IV

  WHITE EARTH COLOURS                                        87
      Caustic Lime                                           87
      Pearl White                                            94
      Vienna White                                           95
      Chalk                                                  98
      Precipitated Chalk                                    107
      Calcareous Marl                                       110
      Gypsum                                                111
      Kaolin, Pipeclay                                      112
      Barytes, or Heavy Spar                                119
      Carbonate of Magnesia                                 123
      Talc                                                  124
      Steatite or Soapstone                                 125


  CHAPTER V

  YELLOW EARTH COLOURS                                      127
      The Ochres                                            128
      Calcining (Burning) Ochre                             132
      Ochres from Various Deposits                          136
      Artificial Ochres                                     138
      Ochres as By-products                                 146


  CHAPTER VI

  RED EARTH COLOURS                                         151
      Bole                                                  152
      Native Ferric Oxide as a Pigment                      154
      Iron Glance                                           154
      Hematite                                              155
      Raddle                                                155
      Burnt Ferric Oxide and Ochres                         158
        (_a_) Burning in the Muffle                         158
        (_b_) Caput Mortuum, Colcothar                      160
        (_c_) Calcining Ferric Oxide                        161
      Ferric Oxide Pigments from Alum Sludge                164


  CHAPTER VII

  BROWN EARTH COLOURS                                       168
      Terra di Siena                                        168
      True Umber                                            170
      Cologne Earth (Cologne Umber)                         173
      Asphaltum Brown (Bitumen)                             174


  CHAPTER VIII

  GREEN EARTH COLOURS                                       176
      Green Earth, or Celadon Green                         176
      Artificial Green Earth (Green Ochre)                  180
      Malachite Green                                       181


  CHAPTER IX

  BLUE EARTH COLOURS                                        183
      Malachite Blue (Lazulite)                             183
      Vivianite or Blue Ochre                               184


  CHAPTER X

  BLACK EARTH COLOURS                                       185
      Graphite                                              185
      Black Chalk                                           194


  CHAPTER XI

  THE COMMERCIAL NOMENCLATURE OF THE EARTH COLOURS          197
      White Earth Colours                                   198
      Yellow Earth Colours                                  200
      Red Earth Colours                                     200
      Brown Earth Colours                                   200
      Green Earth Colours                                   201
      Blue Earth Colours                                    202
      Grey Earth Colours                                    202
      Black Earth Colours                                   202

  INDEX                                                     203




EARTH COLOURS




CHAPTER I

INTRODUCTORY


Both from the chemical and practical standpoint it is necessary
to divide pigments into clearly defined groups, the following
classification being adopted on the basis and natural history of the
substances concerned:--

(1) Pigments occurring native in a finished condition, and only
requiring mechanical preparation to fit them for use as painters’
colours. (2) Pigments which are not ready formed in Nature, but
contain some metallic compound as pigmentary material, which requires
certain chemical treatment for its full development. (3) Pigments
which, in contrast to these two groups, contain only organic, and no
inorganic, constituents. This last class comprises all the natural
vegetable pigments, together with the large group of colours obtained
artificially from tar products, fresh groups of which are being
continually introduced. Nowadays, there is no longer any strict line
of demarcation between the natural and artificial organic colouring
matters, it being possible to produce even those of the vegetable
series, such as madder and indigo, by artificial means.

Whilst this group of colours exhibits the greatest variety, and is
constantly being enriched and increased by the progress of colour
chemistry, the case is different with the first group, the natural
earth pigments. Here we have chiefly to do with the preparation of
materials occurring in Nature, or with bringing about certain chemical
results, so that, consequently, the range of variety is far more
restricted, and there is little or no possibility of increasing the
number of these colours by the manufacture of really new products.
The earth colours nevertheless have a high technical and economic
importance, on account of their extremely valuable properties, coupled,
for the most part, with low cost.

If the term “earth colours” were strictly adhered to, the present work
would have to be confined to a description of the physical and chemical
properties of the various pigments, and of the various means by which
they can be brought into suitable condition for use in paints.

However, of late, the term has found wider application than formerly,
since it has been found practicable to modify (shade) certain of the
earth colours by simple operations, and thus considerably increase
the range of tones of the substances known as earth colours. The
progress of chemical industry has also largely increased the number
of the so-called earth colours, certain methods of chemical treatment
having enabled substances that are of little use for other purposes,
to be employed, in large quantities, as pigments. The application of
these--usually cheap--by-products is still further facilitated by the
fact that they can be transformed, by a simple chemical treatment, into
pigments which are distinguished by their beauty of colour and at the
same time possess the great advantages of durability and cheapness.

As an example of this, mention may be made of iron oxide, which occurs
in Nature in the form of various minerals which can be made into
pigments by mechanical treatment. In many cases, this treatment has
already been carried out by Nature, and deposits of iron oxide are
found in which the material has only to be incorporated with a vehicle
to make it fit for immediate use as a painters’ colour.

Moreover, the same oxide is obtained, in large quantities, as a
by-product of the treatment of other minerals. From the point of view
of chemical composition, this by-product is of very low value, by
reason of the large supplies of native oxide available. By means of
a very simple chemical treatment, however, this by-product oxide can
be considerably improved in commercial value, being, in many cases,
convertible, by merely heating it to certain temperatures, into a
variety of colours which sell at remunerative prices.

Consequently, in view of the present condition of the chemical
industry, the term “earth colours” can be enlarged to include a number
of waste products which fetch good prices as colours, though otherwise
practically valueless in themselves.

The number of earth pigments is very large, and comprises
representatives of all the principal colours. For painting purposes,
few pigments beyond the earth colours were known to the ancients;
and most of the colours in the paintings which have come down to us
from antiquity are pure earth pigments, thus affording proof of their
great durability, having retained their freshness unimpaired for
hundreds--and some for thousands--of years.

The earth colours might be divided into such as occur ready-formed
in Nature, and require only mechanical preparation, and which either
require special treatment (_e. g._ calcining), or are artificial
products (like the iron oxide mentioned above). Since, however, such
a classification would not advantage our knowledge of the nature of
this class of colours, it appears useless and superfluous, and we will
therefore simply confine ourselves to arranging the earth pigments
according to their colour--white, yellow, red, etc.

Adopting this classification, the following minerals and chemical
products may be considered as earth colours:--

_White._--These include the varieties of calcium carbonate, such as
chalk, marble, precipitated chalk, calcium phosphate, calcium sulphate
(in the form of gypsum, alabaster, muriacite and the precipitated
gypsum produced as a by-product in many chemical works), heavy spar,
the different varieties of clay, and magnesia.

_Yellow._--This group comprises ferric hydroxide (hydrated oxide of
iron) in the form of the various minerals known as ochre; all the
preparations chiefly composed of this hydroxide, and all those prepared
by artificial means. A very important member of this group is orpiment;
the other arsenical compounds frequently met with native, being
however, on account of their poisonous properties, no longer used as
pigments.

_Red._--Chief among the red earth colours are those consisting of
ferric oxide (iron oxide), under various names. The only other member
of the group is the far rarer vermilion.

_Blue._--The blue earth colours are few in number and of no particular
beauty; but they are of importance on account of their cheapness and
because all the artificial blue pigments are rather expensive. Two
products in particular merit attention in this connection, namely,
ultramarine, and the mineral known as blue ochre or blue ironstone.
The latter, as a matter of fact, cannot be used for anything else
than a painters’ colour, and can be obtained at a low price; whereas
ultramarine also forms a valuable raw material for the recovery of
copper, and is therefore dearer.

_Green._--This group, again, contains only two members, viz. malachite
green (chrysocolla), and the green earths (seladonite), known as
Verona, etc., green. These occur fairly often in Nature, and the green
earths in particular find a wide industrial application by reason
of their low price. Malachite green is very similar, in chemical
constitution, to ultramarine; and both form sources of copper and are
consequently expensive.

It should be mentioned that both ultramarine and malachite green
can only be profitably made into pigments where the minerals can
be obtained cheaply, since both of them can be manufactured where
artificial pigments are produced, and are put on the market under the
same names as the native articles. The very low price of the green
earths makes them highly popular as colouring matters in certain
branches of industry, and they are very largely used by wall-paper
manufacturers.

_Brown._--This is a large group, and the pigments composing it are
specially distinguished for their beauty and depth of colour, on
which account they are used in the finest paintings. Here, again,
it is ferric oxide, in combination with water--and therefore ferric
hydroxide--that furnishes a large number of the members of the group.
Like the renowned Siena earth, the artists’ colours known as Vandyck
brown, bole, Lemnos earth, umber, etc., mainly consist of more or
less pure ferric hydroxide. These minerals are, moreover, specially
important to the colour manufacturer, inasmuch as most of them enable a
large number of different shades to be obtained by a simple method of
treatment consisting merely of the application of heat in a suitable
manner; and these colours are among the most excellent we possess, by
reason of their beauty and permanence. Amongst this series must also be
classed native manganese brown, which chiefly consists of a mixture of
manganese oxide and the hydrated peroxide of the same metal.

_Black._--There is really only one member of this class, which,
however, is frequently used, viz. that form of carbon occurring as
hexagonal crystals and known as graphite. Another natural black natural
product, occasionally used as a painters’ colour is the so-called black
chalk. However, since black pigments can be produced very cheaply by
artificial means, the natural colours find only a limited application;
and only in one instance is graphite used alone, viz. for making
blacklead pencils.

As already mentioned, certain chemical industries furnish by-products
which are of very little value in themselves, and many of them, indeed,
may be classed as worthless, since chemical manufacturers naturally
endeavour to get everything possible out of their materials in the
course of manufacture.

Some of these by-products, however, can advantageously be used as
pigments, a good example of this being afforded by the iron oxide
formed as a by-product in the manufacture of fuming sulphuric acid
(Nordhausen oil of vitriol), by the old process, from green vitriol
(ferrous sulphate). In itself, this oxide is practically valueless,
but, by very simple treatment, it can be converted into very valuable
pigments which have a market value far in excess of the original
material. Although it has hitherto been the custom to confine the term
earth colours to such as occur ready-formed in Nature and only require
simple mechanical treatment to make them ready for immediate use as
pigments, the author is nevertheless of opinion that a book dealing
exhaustively with earth colours should also make some mention of all
the mineral colouring matters which can be easily made into pigments
by simple processes, such as calcination or bringing into association
with other substances. In accordance with this view, the present work
will describe all the pigments that are obtainable in this manner.
Most of the earth colours consist of decomposition products of certain
minerals; and this applies particularly to such of them as contain
iron oxide. According as the decomposition of the original mineral has
been more or less extensive, the natural product exhibits different
properties; and the manufacturer must consequently endeavour to treat
them in such a manner as to ensure that the pigment obtained will be
as uniform as possible in shade and permanence. In order to accomplish
this it is essential to have an accurate knowledge of the origin of
the raw material under treatment, and of its chemical and physical
properties. In view of this, the author considers it necessary to deal
more fully with the pigmentary earths forming the raw materials of the
earth colours, before passing on to the preparation of the colours
themselves.




CHAPTER II

THE RAW MATERIALS FOR EARTH COLOURS


The minerals constituting the raw materials for the preparation of the
earth colours occur under very divergent conditions in Nature. Some
of them, such as chalk, form immense deposits, even whole mountains,
whilst in other cases, _e. g._ the blue ferruginous earths, the
occurrence is connected with certain local conditions, and many are
found only in isolated deposits, as pockets or beds. This last is the
case, for instance, with the handsome brown iron pigments; and indeed
the names by which they are known indicate that they are only found in
well-defined localities, or that they are met with of special quality
there. The brown earth colour known to all painters as Terra di Siena,
is found at many other places as well as near Siena, but the product
from that city acquired aforetime a special reputation for beauty, and
therefore all similar earths, provided they are equal to that from
Siena, also bear the same name in commerce.

A number of raw materials for the preparation of earth colours are
found, it is true, in many deposits, but their utilisation depends, in
turn, on local conditions. For example, many copper mines contain, in
addition to the other cupriferous minerals, those used, in the powdered
state, as ultramarine or ultramarine green, and not infrequently lumps
of mineral are found containing both blue and green together. However,
it is only when these minerals occur in sufficient quantity to make the
necessary sorting profitable that their manufacture into pigments can
be regarded as practicable.

Before commencing to work a deposit it is essential to make sure
whether the raw material, or pigmentary earth, is actually suitable
for the manufacture of earth colour. Even the general character of the
material is important, those of soft, earthy consistency being much
easier to treat, and the cost of preparation smaller, than if the raw
material be hard, tough and crystalline.

The extent and thickness of the deposit, and the ease with which it can
be worked, also play an important, and even decisive part, since, other
conditions being equal, it will not pay to erect a colour works unless
the raw material is available in sufficient quantity and is cheap.
Generally, the deposit is not homogeneous throughout, the mineral being
purer in some places and more contaminated with gangue in others.
The percentage of moisture also varies, and in short, a number of
circumstances must be taken into consideration in forming a conclusion
as to whether a deposit is workable or not.

In order to arrive at a reliable opinion on all these conditions,
sampling is indispensable. If the samples are of uniform character,
they can be mixed together to make an average sample. But if they
differ considerably in appearance, general character, proportion
of gangue, etc., it is preferable to examine them separately, more
especially when the area which each represents is large.

The examination should extend, on the one hand, to the natural
percentage of moisture, and, on the other, to the purity of the
material. The water content is determined by thoroughly drying a
weighed sample, bearing, however, in mind the fact that pigmentary
earths of a clayey nature vary in water content according to the time
of year, besides changing in accordance with the weather when the won
material is stored in the open.

The purity can only be ascertained by an examination in which a
sample of the soft, clayey material is crushed and passed through a
narrow-mesh gauze sieve, the amount of the coarse particles--sand,
small stones, etc.--remaining on the sieve being determined. A more
accurate method, of course, is to separate the true pigmentary earth
from the gangue by levigation. For this purpose, a weighed quantity
of the crushed, air-dry sample is placed in a relatively narrow glass
vessel and thoroughly mixed with water, the turbid supernatant liquid
being poured off after a short interval. The residue is repeatedly
treated in the same way, until no more fine particles remain in
suspension, the residue then consisting of impurities, or gangue. Of
course, the washings can be collected, the suspended matter allowed to
settle, and finally weighed in an air-dry condition. By this means an
approximate idea of the yield of earth colour can be obtained at the
same time.

Raw materials which are not amorphous, soft and clayey must first be
crushed, an operation facilitated by heating to redness and quenching
in cold water. Oftentimes the heating causes a change of colour and
improves the covering power--a point to which reference will be made
later on.

In the following description of the various raw materials, the
chemical composition of the pure minerals will be given, together with
an enumeration of the most common impurities.


(A) WHITE RAW MATERIALS AND PIGMENTARY EARTHS

_Limestone_ (_Calcite_, _Limestone_, _Chalk_)

The number of materials furnishing white earth colours is comparatively
large, and these colours are particularly important, because, not
only are they extensively used by themselves, but they also serve as
adjuncts to other colours and for the production of special shades. The
chief raw material for the preparation of white earth colours is the
mineral calcite in its numerous modifications.

Calcite, or calc spar, occurs very frequently in Nature, and is one
of the most highly diversified minerals known. In its purest state
it appears as “double spar” (calcite), in the form of water-white
crystals, which are very remarkable for certain optical properties.
White marble is also a very pure variety of calcite, in which the
individual crystals are very small. The various coloured marbles owe
their appearance to certain admixtures of extraneous substances,
chiefly metallic oxides.

No sharp line of demarcation separates marble from ordinary limestone,
the difference between them really consisting only in the degree of
fineness of grain; and all limestones which grind and polish well
may be classed as marble. As is the case with marble, there are also
limestones of various colours, grey being, however, the most common.
This grey limestone forms huge mountain masses which, in Europe,
follow for example, the Alpine chain on its northern and southern
edges.

A few other examples of calcite may be mentioned which occur in
certain localities and, in part, are still in course of formation. To
these belong the stalactites and stalagmites, which sometimes consist
of extremely pure calcite. They are formed by the action of water,
containing carbonic acid in solution, which trickles through cracks and
cavities in limestone rock and dissolves out calcium carbonate from the
adjacent stone. On prolonged exposure to the air such water gives off
its free carbonic acid again; and as the calcium carbonate is insoluble
in pure water, it separates out in crystalline form. The masses formed
in this way usually resemble icicles in shape, and the finest examples
are to be found in the well-known stalactite grottoes at Krain, whilst
the grotto at Adelsberg is renowned for its beautiful stalactites.
Occasionally, stalactites have an opaque yellow or brownish tinge,
which they owe to the presence of iron oxide.

A formation similar in its origin to stalactites is the so-called
calc sinter and calcareous tuff. The former often occurs in cavities
as irregular masses which, in some places, enclose large quantities
of fossil animal bones, in which case they form “bone breccia”
(crag breccia). Calcareous tuff is deposited from numerous springs,
occasionally in very large quantities, enveloping plants and sometimes
forming thick deposits in which the structure of the plants can be
clearly recognised.

In some places a more or less pure white, extremely friable variety
of calcite is met with under the name “mountain milk” or “mountain
chalk” (earthy calcite), which seems to be a decomposition product,
and consists of a mixture of arragonite and chalk. Arragonite--which
will be referred to later--is completely identical, chemically, with
calcite--both being composed of calcium carbonate--the sole difference
being their crystalline form.

The most important for the colour-maker, however, is the variety known
as chalk. This is really a fossil product, _i. e._ it consists of the
microscopic shells of marine animals united into solid masses. Despite
the smallness of these animals, their epoch lasted long enough for
their shells to form entire mountains which are encountered all over
the world. A large part of the coast of England, the island of Rügen,
and many other localities, consist entirely of chalk.

In many cases, chalk is found interspersed with nodular masses of
flint, and in some places it also contains great quantities of the
remains of other marine animals, such as sea urchins, the spines of
which occur in such numbers in certain kinds of chalk as to unfit them
entirely for use as a pigment.

The foregoing varieties of calc spar are the most important, and also
occur in large quantities; but, to complete the tale, it is necessary
to mention also a few others which, however, are only found in small
amounts. To these belong, for example, anthracolite, a limestone
stained quite black by coal; the oolithic limestones or roe stones,
which are composed of granules resembling fish roe; muschelkalk,
which is also of fossil character and is almost entirely composed of
mussel shells cemented together with lime; the marls, which consist
of calc spar mixed with varying quantities of clay and consequently
often bear a great resemblance to loam in their properties. A few of
these varieties find extensive employment for certain purposes, some
marls for instance being used for making hydraulic lime, whilst all
modifications of calc spar that are sufficiently pure can be burned for
quick lime.

It has already been stated that the mineral arragonite is identical,
chemically, with calc spar, since both consist of calcium carbonate,
but differ in their crystalline habit. Thus, whereas the crystals of
calc spar belong to the rhombohedral or hexagonal system, those of
arragonite are always rhombic. This occurrence of one and the same
substance in two different crystalline forms is known as dimorphism,
and calcium carbonate is therefore dimorphous. Whether calcium
carbonate assumes the form of calcite or arragonite depends entirely on
physical causes. When the deposition of the carbonate takes place from
a cold solution the shape of the crystals is always one belonging to
the hexagonal or rhombohedral system; but when it is from hot solution,
rhombic crystals are invariably formed, calc spar resulting in the
former case and arragonite in the latter.

These different methods of formation which can be carried out in the
laboratory by producing the requisite conditions, occur on the large
scale in many parts of the world. Wherever a hot spring comes to the
surface, containing considerable amounts of lime in solution, this
separates out in the form of arragonite, which received its name from
the circumstance that specially handsome crystals of this mineral are
found in Arragon.

One of the best-known places where the formation of arragonite can be
observed at the present time is Carlsbad in Bohemia. The hot springs
there deposit a very large amount of lime, which is stained more or
less yellow or red by the presence of varying quantities of iron
oxide, and, under the name of “sprudelstein” is used for producing
various works of art. When the hot springs bring up particles of sand,
the lime substance incrusts these sand grains, forming globular masses
resembling peas, and consequently named pisolite.

In chemical composition, calcite and arragonite consist of a
combination of calcium oxide (lime) and carbonic acid, the formula
being expressed by CaCO{3}. Calcium carbonate is insoluble in pure
water, but dissolves somewhat freely in water charged with free
carbonic acid. It is assumed that a compound is formed, which is known
as calcium bi- (or acid) carbonate, is very unstable and can only exist
in a state of solution. When a solution of calcium bicarbonate--which
can be prepared by passing carbonic acid gas through water containing
finely divided calcium carbonate in suspension--is exposed for some
time to the air, it soon becomes cloudy, and a deposit of calcium
carbonate settles down at the bottom of the vessel, because, in the
air the dissolved calcium bicarbonate is decomposed into free carbonic
acid gas and calcium carbonate, which latter, as has been mentioned,
is quite insoluble in water. It has already been stated that this
phenomenon goes on in Nature in the formation of stalactites, lime
sinter and calcareous tuff.

Calcium carbonate is readily soluble in acids, the contained carbonic
acid being liberated (as carbon dioxide) with effervescence. When such
acids are employed for solution as form readily soluble salts with
lime, such as hydrochloric, nitric, acetic, etc. acids, a perfectly
clear solution is obtained; but if sulphuric acid is used, a white
pulpy mass is formed, consisting of calcium sulphate, or gypsum,
which, owing to its low solubility, separates out as small crystals.
Any sandy residue left when calcium carbonate is dissolved, mostly
consists of quartz sand. In dissolving dark-coloured limestones, grey,
or even black, flakes are left, which consist of organic material
very high in carbon. On limestone being subjected to fairly strong
calcination, all the carbonic acid is expelled, leaving behind the
so-called quick or burnt lime, which is, chemically, calcium oxide:--

  CaCO{3}           = CaO        + CO{2}
  Calcium carbonate = Quick lime + Carbon dioxide

If burnt lime be left exposed to the air for some time, it again
gradually absorbs carbon dioxide and is reconverted into calcium
carbonate. When burnt lime is sprinkled with water it takes up the
latter avidly, becoming very hot and finally crumbling down to a very
friable white powder, consisting of slaked or hydrated lime (calcium
hydroxide, Ca(OH){2}). The considerable rise of temperature in
quenching the lime is due to the chemical combination of the calcium
oxide and water.

Both quick and slaked lime dissolve to a certain extent in water, and
impart strongly alkaline properties thereto, lime being one of the
strongest of bases. On exposure to the air, the solution of quick lime
in water (lime-water) quickly forms an opalescent superficial film
of calcium carbonate, and in a short time no more lime is present in
solution, the whole having been transformed into calcium carbonate,
which settles down to the bottom of the vessel as a very fine powder.

Limestone that consists entirely of calcium oxide and carbon dioxide
is of rare occurrence in Nature, foreign substances being nearly
always present. Since the nature of these admixtures is of the greatest
importance to the colour-maker, owing to the considerable influence
they exert on the suitability of the minerals for his purposes, it
is necessary that these extraneous substances occurring in limestone
should be more closely described.

Nearly all varieties of limestone contain certain proportions of
ferrous and ferric oxides. The presence of ferrous oxide, when the
relative amount is but small, cannot be detected by mere inspection;
and even many limestones containing really appreciable quantities of
ferrous oxide are pure white in colour so long as they are in large
lumps. If, however, such a limestone be reduced to powder and exposed
to the air for a short time, it gradually assumes a yellow tinge, the
depth of which increases with the length of exposure.

The cause of this change is due to the fact that ferrous oxide has a
great affinity for oxygen, by absorbing which it changes into ferric
oxide. (Ferrous oxide consists of FeO, ferric oxide of Fe{2}O{3}.)
Ferrous oxide and its compounds are of a pale green colour which is
not very noticeable, whereas ferric oxide has a very powerful yellow
colour, and consequently the limestone, when its superficial area has
been greatly increased by reduction to powder, assumes the yellow tinge
due to ferric oxide. A limestone exhibiting this property can evidently
not be used for making white earth colours, but is, at best, only
suitable for mixing with other colours.

Occasionally, limestone contains varying quantities of magnesia, and
when this oxide is present in large amount, changes into another
mineral known as dolomite. In many places this dolomite forms large
masses of rock, which, however, is not employed for making colours,
owing to the yellow shade imparted by the fairly large amount of ferric
oxide present.


_Gypsum_ (_Alabaster_)

This mineral occurs native in many places, and is frequently worked
for a number of purposes. Gypsum occurs in Nature in a great variety
of forms. The purest kind is met with either as water-clear crystals,
which cleave readily in two directions, or as transparent tabular
masses (selenite) which also cleave easily. Micro-crystalline
fine-grained gypsum is milk-white in colour, highly translucent and is
largely used, under the name of alabaster, in sculpture. Owing to its
low hardness, alabaster can be readily cut with a knife, and on this
account is frequently shaped by planing or lathe-turning.

Gypsum is generally met with in dense masses, which may be of any
colour, grey, blue and reddish shades being the most common, whilst
pure white is rarer. The dark-coloured varieties can only be used for
manurial purposes; but the white finds a twofold application as a
pigment, and, in the calcined state, for making plaster casts.

In point of chemical composition, gypsum consists of sulphate of lime,
or calcium sulphate (CaSO{4} + 2H{2}O). It is soluble in water, but
only in such small quantity that over 400 parts of the latter are
needed to dissolve one part of gypsum. On being heated to between 120°
and 130° C., gypsum parts with its two molecules of combined water and
becomes anhydrous calcium sulphate or burnt gypsum. When this latter
is stirred with water to a pulp, it takes up the water again, with
considerable evolution of heat, swelling up considerably and setting
quickly to a solid mass.

The number of substances exhibiting this property being small, burnt
gypsum is very frequently used for making casts of statuary, and for
stucco work in building. Finely ground white gypsum can also be used as
a pigment, but is inferior to calcium carbonate in covering power, and
is therefore seldom employed for this purpose, though frequently added
to other colours. The mineral known as muriacite or anhydrite consists
of anhydrous calcium sulphate; and is therefore similar in composition
to burnt gypsum; but it lacks the property of combining with water when
brought into contact therewith.


_Barytes, or Heavy Spar_

The mineral known as heavy spar occurs in very large quantities and in
numerous localities. It forms rhombic crystals, which are very often
extremely well developed and form flat plates of considerable size. A
remarkable peculiarity of this mineral is its high specific gravity,
which is due to the barium content. It is found native in all colours,
white being the most common.

Chemically, heavy spar is barium sulphate, BaSO{4}. It can be used as a
pigment _per se_, but only when prepared artificially, the trade name
for the product being permanent white, or blanc fixe. Powdered native
heavy spar, even when ground ever so fine, has not enough covering
power, this property being peculiar to the artificial product.

When it is desired to mix other pigments with a white substance, to
lighten the shade, permanent white can be specially recommended, since
it is quite insensitive to atmospheric influences and has no chemical
action on the colour, so that it can be used with even the most
delicate colours without risk. In this way, not only can the colours
be considerably cheapened, but over-dark colours can be shaded to the
desired extent. Another advantage of such mixtures is that a smaller
quantity of oil or varnish is required, barytes only needing about 8%
of its own weight of vehicle to form a workable mixture, whilst other
pigments take five times as much, or even more. In many cases the low
covering power of barytes enables large quantities to be added, and
this reacts favourably on the consumption of varnish.

Another barium mineral is witherite, or barium carbonate. This is not
used direct as a pigment, but--in contrast to heavy spar--is readily
soluble in hydrochloric acid, and therefore serves as raw material for
the preparation of artificial barytes and other barium compounds, the
first-named being obtained by treating a solution of barium chloride
with sulphuric acid, insoluble barium sulphate being precipitated.


_Talc_, _Soapstone_, _Steatite_

Talc occurs in Nature either as a pure white mass, of greasy lustre, or
occasionally as yellow, green or grey masses, all distinguished by a
peculiar greasy appearance and a soapy feel. This appearance is common
to all the minerals of the steatite group, and is the cause of their
generic name, soapstone. Although the steatites have a very low degree
of hardness--most of them can be scratched by the finger-nail--some
difficulty is encountered in reducing them to fine powder. Calcination
usually increases the hardness considerably, so that, in some cases,
the calcined mineral gives off sparks when struck with a steel
instrument.

Soapstone is composed of magnesium silicates, containing varying
proportions of magnesia and silica, together with a small quantity
of water, apparently in a state of chemical combination, a very high
temperature, approaching white heat, being required to effect its
complete expulsion, the residue then attaining the aforesaid high
degree of hardness. The composition of talc can be expressed by the
symbol H{2}Mg{2}(SiO{3}){4}, corresponding to 63·52% of silica,
31·72% of magnesia, and 4·76% of water. In some varieties of talc, a
portion (1–5%) of the magnesia is replaced by ferrous oxide. Talc is
quite unaffected by the action of dilute acids, boiling concentrated
sulphuric acid being required to decompose it, with separation of
silica.

Owing to its low specific gravity and chemical indifference, talc is
suitable for lightening the shade of certain lake pigments. It can also
be used as a pigment by itself, and also as a gloss on wall-paper, for
mixing with paper pulp, and for various other purposes.


_Clay_

The mineral known as clay is, in all cases, a product of the
decomposition of other minerals, mainly felspar. This substance is a
double silicate of alumina and potash, K{2}O.Al{2}O{3}.(SiO{2}){6}.
Pure kaolin is Al{2}O{3}(SiO{2}){2} + 2H{2}O, or 46·50% silica, 39·56%
alumina, 13·9% water.

Clay may be supposed to have been formed by the conversion of felspar,
under the action of air and water, into silicate of alumina, the
silicate of potash being dissolved out. Being insoluble, the silicate
of alumina would be transported by the water, in a very fine state of
division, and finally deposited as a sediment, which in course of time
became a solid mass. This, when again brought into contact with water,
forms a very plastic pulp which, when dried and baked, forms a solid
mass, brick, which is no longer affected by water. Perfectly pure clay
forms a white mass, which, under the name of China clay or kaolin, is
used for making porcelain, and is only occasionally met with in large
quantities.

Pure kaolin is characterised by its great chemical indifference, being
decomposed only by strong alkalis and sulphuric acid. At the high
temperature of the pottery kiln, kaolin sinters to a very compact
mass, but cannot be fused, except when small quantities are subjected
to the intense heat of the oxyhydrogen flame, whereupon it fuses to a
colourless glass of great hardness.

In an impure state, silicate of alumina occurs frequently in Nature,
and then forms the minerals known under the generic names of clay,
loam, marl, etc. These impure clays contain varying proportions of
extraneous minerals which produce changes in the physical and chemical
properties. They are grey, blue or yellow in colour, the grey and blue
varieties mostly containing appreciable quantities of ferrous oxide,
whilst the yellow kinds contain ferric oxide. When fired, all of them
become yellow or red, the ferrous oxide being transformed into ferric
oxide by the heat. Some fairly white clays are high in lime, which
makes them fusible at high temperatures. In some very impure kinds,
even the comparatively low heat of the brick-kiln is sufficient to
cause partial fusion. For colour-making, the white clays, especially
kaolin and pipeclay, form a highly important material, being
procurable at very low prices and fairly easy to prepare.

The white clays are either used as pigments by themselves, or for
mixing with other colours of low specific gravity.


(B) YELLOW EARTHS

The number of yellow earths is large, but most of them exhibit a
certain similarity in chemical composition, the pigmentary principle in
the majority being either ferric oxide or ferric hydroxide. The former
is yellow, the latter brown, and the colour of the minerals resembles
that of the preponderating iron compound.


_Brown Ironstone_

The mineral known as brown ironstone consists of ferric hydroxide, and
usually forms compact masses, no decided crystals having, so far, been
observed. The lumps have an irregular or earthy fracture, a hardness
of 5–5·5, and a sp. gr. between 3·40 and 3·95. The colour ranges, in
the different varieties, from yellowish (rusty) brown, through cinnamon
to blackish-brown. The chemical composition of the pure lumps may be
expressed by the symbol 2Fe{2}O{3} + 3H{2}O; but a little manganese
oxide and silica is generally present even in the pure kinds.

The chief varieties of this mineral are:--

(_a_) Fibrous brown iron ore, or brown hematite, mostly forming
reniform or stalactitic masses.

(_b_) Compact brown ironstone, usually in dense masses, and not
infrequently also appearing in pseudo-morphs of other minerals.

(_c_) Ochreous brown ironstone. This variety is the most important to
the colour-maker, for whose purposes it is preferably used. It nearly
always forms very loose, earthy masses, yellow or brown in colour.

(_d_) Clay ironstone. This consists of a mixture of the above-mentioned
varieties with variable proportions of other minerals, clay being the
most common ingredient. Nodular iron ore, oölitic, bog and siliceous
ore belong to this class, as also the minette ores that are found in
great abundance in Alsace-Lorraine, Belgium and Luxemburg, and are
classed with the oölitic brown ironstones.

In most cases, the varieties enumerated are found together, and are
used for the production of iron. The ochre constituting the most
interesting member to the colour-maker often occurs as deposits
embedded in dense masses of brown ironstone, though in many places it
is found by itself.


CHEMICAL COMPOSITION OF VARIOUS BROWN IRONSTONES

The following analyses of brown ironstone from different deposits will
give an idea of the composition of these minerals.


_Ordinary Brown Ironstone_

  +-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  |             |  1. |  2. |  3. |  4. |  5. |  6. |  7. |  8. |  9. |  10.|
  +-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  |Ferric oxide |76·76|73·75|77·54|78·50|78·42|48·25|  -- |  -- |  -- |  -- |
  |Manganese    |     |     |     |     |     |     |     |     |     |     |
  | oxide       |16·56|10·50| 2·70| 1·95| 1·30|24·73|  -- |  -- |  -- |  -- |
  |Iron         |  -- |  -- |  -- |  -- |  -- |  -- |33·9 |37·88|54·80|55·04|
  |Manganese    |  -- |  -- |  -- |  -- |  -- |  -- | 0·15| 0·17| 0·57| 0·20|
  |Alumina      |  -- |  -- |  -- |  -- | 1·13| 2·33|10·03| 0·88| 1·15| 2·50|
  |Lime         | 0·60| 2·75| 0·48| 5·08| 3·55| 2·85| 0·41| 0·32| 0·50| 0·34|
  |Magnesia     | 6·44|  -- | 1·25| 4·50| 0·18| 0·90| 0·67|  -- | 0·02| 0·38|
  |Silica       |  -- |  -- | 3·55| 0·85  5·48|11·35|28·29|33·38| 0·02| 0·38|
  |SO{3}        |  -- |  -- |  -- |  -- |  -- | 0·09|  -- |  -- |  -- |  -- |
  |P{2}O{5}     |  -- |  -- |  -- |  -- | 0·09| 0·08|  -- |  -- |  -- |  -- |
  |Sulphur      |  -- |  -- |  -- |  -- |  -- |  -- | 0·33| 0·06| 0·04|Trace|
  |Phosphorus   |  -- |  -- |  -- |  -- |  -- |  -- | 0·04| 0·56| 0·02| 0·06|
  |Loss on      |     |     |     |     |     |     |     |     |     |     |
  | incineration| 5·65|14·00|14·51| 9·12| 9·10| 9·80| 9·88| 7·77|10·55|10·71|
  +-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

Deposits: (1) Hamm; (2) Schmalkalden; (3) Hüttenberg (Carynthia); (4)
Styria; (5) and (9) Bilbao; (6) Algeria; (7) Schwelm (Westphalia); (8)
Elbingerode (Harz); (10) Pennsylvania.


_Argillaceous Brown Ironstone_

  +-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  |             |_a._ |_b._ |_c._ |_d._ |_e._ |_f._ |_g._ |_h._ |_i._ |_k._ |
  +-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  |Ferric oxide |80·76|19·4 |55·39|66·33|57·32|52·50|39·50|75·67| --  | --  |
  |Iron         | --  | --  | --  | --  | --  | --  | --  | --  |40·90|21·69|
  |Manganese    |     |     |     |     |     |     |     |     |     |     |
  | oxide       | --  | 8·2 | --  | 6·42| --  | 5·49| 6·12| 0·72| --  | --  |
  |Manganese    | --  | --  | --  | --  | --  | --  | --  | --  | --  | --  |
  |Zinc oxide   | 0·92| 1·6 | --  | --  | 0·47| --  | --  | --  | --  | --  |
  |Alumina      | 2·36|11·0 |12·80| 7·74| 1·68| 5·23| 9·89| 3·10| 4·95| 3·88|
  |Lime         | --  | 2·6 |Trace| 0·41| 0·13| 3·36|20·34| 5·01| 5·59|21·25|
  |Magnesia     | --  | 0·2 | --  | 0·37| --  | 0·36| --  | --  | 0·49| 0·30|
  |Silica       |4·58 |48·61|22·73|12·97|30·64| 8·64| 5·22| 8·70|16·63|14·71|
  |P{2}O{5}     | --  | --  | --  | --  | 0·32| 3·86| 2·19| 3·68|  -- | --  |
  |Phosphorus   | --  | --  | --  | 0·02| --  | --  | --  | --  | 1·13| 0·48|
  |SO{3}        | --  | --  | --  | 0·03|Trace| --  | --  | --  | --  | --  |
  |Sulphur      | --  | --  | --  |  -- | --  | --  | --  | --  | 0·10| 0·05|
  |Loss on      |     |     |     |     |     |     |     |     |     |     |
  | incineration|12·71| 9·1 | 8·50|11·77|12·70|20·55|25·74| --  |16·04|28·70|
  +-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

(_a_) Oölitic (pea) ore from Elligserbrick (Brunswick); (_b_) from
Durlach (Baden); (_c_) and (_d_) Ore from Esslingen; (_e_) Oölitic ore
from Siptingen (Baden); (_f_) from Adenstedt, nr. Pirna (argillaceous);
(_g_) _Ibid._ (calcareous); (_h_) Minette from Esch; (_i_) Red minette
from Dolvaux; (_k_) Brown minette from Redange.


_Limonite (Bog Iron Ore)_

  +----------------+-----+-----+-----+-----+-----+-----+
  |                |  1. |  2. |  3. |  4. |  5. |  6. |
  +----------------+-----+-----+-----+-----+-----+-----+
  |Ferric oxide    |61·00|67·46|65·66|67·59|70·05|62·20|
  |Manganese oxide | 7·00| 3·19| 3·87| 1·45| 1·78| 7·60|
  |P{2}O{5}        | 2·00| 0·67| 1·13| 0·18| 0·34| 1·60|
  |SO{3} --        | 3·07|Trace| 0·21|Trace|Trace|     |
  |Silica          | 6·00| 7·00| 7·15| 7·89| 8·03|16·60|
  |Alumina         |  -- | --  | 5·09| 4·18| 1·50| 2·20|
  |Lime            |  -- | 0·90| 0·82| 0·47| 2·31| 1·60|
  |Magnesia        |  -- | --  | 0·15| 0·23| 0·12| 3·73|
  |Water and       |     |     |     |     |     |     |
  | organic acids  |19·00|17·00| 6·22|17·81|15·87|19·90|
  +----------------+-----+-----+-----+-----+-----+-----+

(1) Limonite from Lausitz; (2) Limonite from Auer, nr. Morizburg; (3 to
6) Swedish limonite.


_Ochre_

Ochre, or yellow Terra di Siena, forms earthy-looking masses, fawn,
reddish-yellow to brownish-red in colour. Whilst not infrequent in
Nature, ochre is only found in small quantities, as pockets, and not
as extensive deposits. The discovery of a bed of good coloured ochre
is, however, always a very valuable find, bright natural ochres being
somewhat rare, and most kinds requiring special preparation before they
can be used as painters’ colours. Owing to the comparative scarcity of
good coloured ochres, they are often called after the place of origin,
such as Thuringian, Italian (Siena), English, etc., ochre.

In nearly every case, ochre is a decomposition product of various
ferruginous minerals, which has been transported by water, often in
admixture with other minerals, and finally deposited in the places
where it is now found. Most ochres consist of varying mixtures of clay,
ferric hydroxide and lime; and, as a rule, the higher the proportion of
ferric hydroxide, the deeper the colour. Thus, for example, the ferric
hydroxide may amount, in the dark grades, to 25% of the entire mass,
whilst in the lighter kinds it may be as low as 3%. It is very rare
that ochre is put on the market in its native condition, being mostly
subjected to chemical treatment enabling a definite shade of colour to
be obtained. This will be gone into more fully later.


_Yellow Earth_

Yellow earth is found in many places as compact masses, and less
frequently as schistous deposits. It has a fine earthy fracture, and is
mostly devoid of lustre, except for a faint shimmer on the surface of
fracture; slightly greasy feel; and a tendency to crumble, in water,
to a non-plastic powder. It contains silica, ferric oxide and water
in varying proportions, and the yellow earths from different deposits
always vary slightly in percentage composition. These differences are
clearly shown in the following analyses of two varieties from the
vicinity of Amberg (Bavaria):--

                      I.       II.
  Silica            33·23%    35·10%
  Alumina           14·21     14·40
  Magnesia           1·38        --
  Ferric oxide      37·76     36·80
  Water             13·24     13·60

When heated, the colour changes gradually to red, and the earth becomes
extremely hard. There are several recognised commercial grades,
the price of which varies mainly in accordance with the colour and
fineness. The Amberg variety is specially esteemed, the Hungarian and
Moravian kinds being less valuable.

The colour not being particularly good, this earth is never used for
fine work, but is largely employed as a yellow wash for houses and as
ordinary distemper. It may also be used as an oil paint.

Red Ochre is a less important, cheap variety of ochre, chiefly used in
cheap paints and for low-priced wall-papers. It occurs in the deposits
as clayey masses.


_Terra di Siena_

Terra di Siena is a very pure form of ferric hydroxide. When ground,
the light to dark brown lumps furnish a pale to dark yellow powder,
which can be transformed into a number of gradations by burning. In
spite of its handsome colour, this pigment is deficient in covering
power, in addition to which it darkens when mixed with varnish, and
dries slowly.


(C) THE RED EARTHS

Apart from the small quantities of native vermilion handsome enough
for direct use as painters’ colours when reduced to powder, the red
earths, with practically no exception, consist of ferruginous minerals,
and it is only within a recent period that red painters’ colours have
been prepared from certain chemical waste products from manufacturing
processes. In all cases, however, compounds of iron and oxygen
constitute the bulk of the red earths. In addition to ferric oxide,
which is the chief material used for making the important red colours,
are compounds of ferric oxide and water, _i. e._ ferric hydroxides. The
ferric oxide pigments are among the most important in the entire series
of earth colours, being on the one hand very cheap, and on the other
so handsome in colour that ferric oxide can be used for the finest
paintings.

Ferric oxide can also be shaded very extensively by a fairly simple
treatment, so as to furnish a whole range of very handsome shades.

In nature, ferric oxide occurs in numerous varieties of one and the
same mineral, red iron ore, which is also known as hematite, blood
stone, raddle, etc.


_Red Ironstone_

Red hematite occurs native as rhombohedral crystals, which mostly
consist solely of ferric oxide, and may be considered as pure oxide
for the purposes of the colour-maker. The difference between the
several varieties is due, not to any chemical variation, but entirely
to changes in physical structure. The varieties with a radial, fibrous
structure are known as red hematite, the colour of which ranges from
blood red to dark brown and is frequently accompanied by metallic
lustre. The scaly modification of this mineral forms micaceous iron
ore, and is usually a deep iron black. In the neighbourhood of
volcanoes it is frequently found as particularly handsome crystals.

Iron cream (frosty hematite) is the name given to a beautiful cherry
red variety, which easily rubs off, has a greasy feel and is composed
of extremely fine scales.

The so-called raddle occurs in Nature as a readily pulverulent earthy
mass of ferric oxide contaminated more or less with extraneous
substances. On account of its abundance and low market price, it is
largely used in painting.

Although mixed with numerous foreign substances, certain clay
ironstones, oölitic ironstones and siliceous ironstones may be regarded
as ferric oxide in the sense understood by the colour-maker, all these
minerals having a deep red to deep brown colour and being capable of
finding advantageous employment as pigments.

Ferric oxide is distinguished by two properties which render it
specially valuable to the colour-maker. When combined with water,
its colour is no longer red, but a handsome brown; and, on the other
hand, when heated, the colour passes through brown into a permanent
dark violet. By suitable treatment of such minerals as consist mainly
of ferric hydroxide, mixtures can be obtained which contain the oxide
and hydroxide in variable proportions and give a whole range of shades
between brown and red.

The preparation of these colours is easy when very pure red ironstone
is available. The somewhat expensive pigment, Indian red, is--when
pure--really nothing but a very pure ferric oxide of Indian origin.
Ferric oxide, however, often contains impurities which considerably
influence the colour of the product. Owing to the fact that large
quantities of ferric oxide are formed as by-products in certain
chemical processes which are carried out on a very extensive scale,
this oxide, which is very pure, can be advantageously used for making
iron pigments, especially as its application for other purposes is very
restricted, and it can therefore be had at a very low price.

The following analyses show the composition of a number of red
ironstones, Nos. 1, 2 and 3 being hematite from Froment, or Wetzlar,
No. 4 from Wetzlar, Nos. 5 and 6 hematite from Whitehaven, No. 7
from Thuringia, No. 8 from Bohemia, No. 9 from Spain, No. 10 from N.
America, and No. 11 from England.

  +-----+--------+-----------+---------+-------------+-----------+--------+
  |     | Ferric | Manganese |         |             | Alumina,  |        |
  | No. | oxide. |  oxide.   | Silica. |  P{2}O{5}.  | lime and  | Water. |
  |     |        |           |         |             | magnesia. |        |
  +-----+--------+-----------+---------+-------------+-----------+--------+
  |  1  |  94·00 |   Trace   |   2·00  |      --     |   Trace   |  2·00  |
  |  2  |  80·95 |     --    |  16·74  |    0·51     |   0·97    |  0·83  |
  |  3  |  73·77 |     --    |  23·16  |    0·45     |   1·41    |  1·21  |
  |  4  |  92·45 |     --    |   5·63  |    0·19     |   0·65    |  1·08  |
  |  5  |  96·27 |     --    |   4·20  |      --     |   0·59    |  4·40  |
  |  6  |  63·05 |   5·29    |  22·76  |      --     |   0·49    |  4·40  |
  |  7  |  85·00 |   1·601   |   3·304 |    0·457    |   8·795   |  0·633 |
  +-----+--------+-----------+---------+-------------+-----------+--------+

  +---+------+-------+-----+-----+------+-------+--------+-------+--------+
  |   |      |Man-   |Alu- |     |Mag-  |       |        |Phos-  |Loss on |
  |No.|Iron. |ganese.|mina.|Lime.|nesia.|Silica.|Sulphur.|phorus.|calcina-|
  |   |      |       |     |     |      |       |        |       |tion.   |
  +---+------+-------+-----+-----+------+-------+--------+-------+--------+
  |  8| 33·64|  0·10 | 7·58| 8·10| 0·82 | 17·80 | Trace  |  0·19 |   9·82 |
  |  9| 31·38|  0·19 | 0·06|29·95| 0·35 |  0·87 |   --   |  0·09 |  23·68 |
  | 10| 62·54|  1·93 | 1·71|  -- | 0·50 |  3·80 |  0·02  |  0·04 |   2·23 |
  | 11| 62·91| Trace | 1·39| 0·70| 0·42 |  5·89 |  0·05  |  0·11 |    --  |
  +---+------+-------+-----+-----+------+-------+--------+-------+--------+

There are certain other minerals closely allied, both chemically and
mineralogically, to red ironstone, namely, the brown hematites or
ironstones used in the manufacture of iron. Brown hematite consists
of ferric hydroxide, Fe{2}O{3}H{2}O, and occurs in a variety of forms
in Nature, the most frequent being pea (oölitic) ore, which owes its
name to the spherical shape of the grains. Some brown hematites are
decomposition products of other minerals, and contain sulphur and
phosphorus in addition to ferric hydroxide. Like the pure hydroxide,
they are brown in colour, but differ therefrom considerably in their
chemical behaviour when heated. This is particularly the case with the
so-called bog ore, which is mostly found, as spongy yellow-brown to
black masses, in swamps, and owes its origin to the decomposition of
various ferruginous minerals. It varies greatly in chemical composition
and occasionally contains up to about 50% of sand. The amount of ferric
oxide in bog ore varies between 20 and 60%, and it also contains 7–30%
of water, up to 4% of P{2}O{5}, small quantities of ferrous oxide and
manganese hydroxide, together with, in most cases, mechanically admixed
organic residues.

The phosphorus content makes bog iron a very inferior material for
smelting, the resulting iron being of low quality. Nevertheless, it can
sometimes be advantageously used in making earth colours, though the
products cannot lay much claim to beauty of colour.


_Bole_

The native earth pigments known by this name form masses of the colour
of leather to dark brown, with a conchoidal fracture and an earthy
appearance. Bole chiefly consists of iron silicate combined with
water, some varieties containing small quantities of alumina. The
composition fluctuates very considerably, most varieties containing
41–42% of silica, 20–25% of alumina, and 24–25% of water, the remainder
consisting of ferric oxide. Some kinds, such as Oravicza and Sinope
bole, contain only 31–32% of silica and 17–21% of water.

Bole is used as a paint for walls, clapboards, etc., and is only
mentioned here because of its relationship to the ferric oxide pigments.


_Alum Sludge_

Large quantities of clarification sludge are produced, in alum works,
as the sediment from the red liquors. This sludge consists mainly of
ferric oxide, with small quantities of other oxides and sulphuric acid
(basic ferric sulphate), and would be an entirely worthless by-product
except for the fact that it can be manufactured into pigments, some of
them of great beauty.

All alum makers should treat this residue and convert it into
pigments, which they could put on the market at a low rate, the cost
of preparation being small. Since the material is chiefly composed of
ferric oxide, the resulting colours are very similar to those obtained
from iron ores; and all shades, from yellow-brown, through red, to the
darkest brown, are represented.


_Mine Sludge_

The water frequently present in iron mines sometimes contains large
quantities of sediment, which consist mainly of iron ochre and can be
advantageously worked up into pigments. There is scarcely any need to
mention that all substances containing ferric oxide can be used for
making any of the pigments obtainable from the oxide itself, the only
difference between the various raw materials being their degree of
purity, so that it is not always so easy to obtain a certain desired
shade from a given material in such beauty as is furnished by another
material, the small quantities of impurities associated with the ferric
oxide having, in many instances, an important influence on the colour.


(D) BLUE EARTHS

Only two minerals are known which are capable of direct use as blue
pigments, viz. vivianite (native Prussian blue) and copper carbonate
(azurite, ultramarine), and as neither of them is particularly
handsome, they are only used for unimportant work. Lapis lazuli is no
longer employed.


_Azurite, or Ultramarine_

This mineral, which is of frequent occurrence with malachite and
other cupriferous minerals, forms small crystals of a beautiful deep
azure blue consisting of cupric oxide in combination with carbon
dioxide and water, expressed by the formula 2CuCO{3}, Cu(OH){2}, or
Cu{3}(OH){2}(CO{3}){2}, and containing 69·19% of cupric oxide, 25·58%
of CO{2} and 5·23% of water. The colour of the powdered mineral is much
paler than that of the crystals. The pigment, which is used for cheap
paints, is not particularly stable, and loses much of its beauty when
applied to plaster.


_Vivianite_

This mineral occurs in many places as crystalline masses, but also
forms earthy deposits, some of which, especially in certain bogs,
attain considerable thickness. The colour is between indigo and
blackish blue; and the freshly won mineral often has an unsightly
whitish appearance, which, however, soon changes into the pure blue.
The cause of this peculiarity is due to the fact that vivianite
originally consisted of hydrated ferrous phosphate, which is white,
this compound being transformed, under the influence of the air, into
the blue ferric phosphate.

Vivianite contains ferric oxide, phosphoric acid and water, but
in variable proportions. The original composition, expressed by
Fe{2}(PO{4}){2} + 8H{2}O{2}, corresponds to 43·03% of ferrous oxide,
28·29% of P{2}O{5} and 28·68% of water; but, in the air, part of the
ferrous phosphate is oxidised to basic ferric phosphate, so that the
content of ferrous oxide may range from 9·75 to 42·71%, and that of
ferric oxide between 1·12 and 38·20%. Vivianite is also sold as blue
ochre, and is now seldom used as a painters’ colour, owing to the
introduction of a large number of artificially prepared blues, which
are superior to vivianite in colour and are cheaply made. However, it
can still find application in localities where it is obtainable in
quantity.


(E) GREEN EARTH PIGMENTS

The green earth pigments comprise green earth (Verona green) and
malachite. Like the blue earths, they cannot lay any particular claim
to beauty, but they are very cheap, and consequently are largely used
where low price is the chief consideration.


_Green Earth_

In Nature, green occurs as an entirely non-crystalline earthy mass,
which is probably a decomposition product of augite. It has a close,
earthy fracture, a colour between seladon and olive green, and a
slightly greasy appearance. In point of chemical composition it
consists of silica, alumina, magnesia, sodium, potassium, ferrous oxide
and water, the usual representative formula being ROS{1}O{2} H{2}O, in
which RO symbolises a metallic oxide.

The colour is due to ferrous oxide; and if left exposed to the air for
a long time, or subjected to powerful calcination, the great affinity
of ferrous oxide for oxygen causes the colour to turn red and red-brown.

Green earth is found in many localities, _e. g._ Bohemia, Hungary, the
Tyrol and Cyprus, the finest, however, occurring near Verona, on which
account it is known as Veronese earth.


_Malachite_

The commercial pigment consists of powdered malachite, a mineral
which usually occurs in compact masses of a handsome emerald green
colour, though isolated lumps exhibit considerable variation in shade,
some of them being dark green and others very pale. In chemical
composition, malachite is closely allied to azurite, consisting of
cupric oxide, carbon dioxide and water, and the difference is entirely
one of percentage proportions. The formula is CuCo{3}, Cu(OH){2}, or
Cu{2}(OH){2}CO{3}, corresponding to 71·90% of cupric oxide, 19·94% of
carbon dioxide and 8·16% of water.

Powdered malachite (even the dark green varieties) is always rather
light in colour, and for this reason is not much used. Furthermore, the
mineral is rather hard (3·5), and is consequently difficult to grind;
in addition to which the mineral is fairly expensive, on account of its
employment as a source of copper, particularly fine pieces being also
used as ornaments or for making works of art. Moreover, like all copper
compounds, it is very sensitive to the action of sulphuretted hydrogen,
and liable to discoloration in course of time.


(F) BROWN EARTH PIGMENTS

Numerous minerals are adapted for the manufacture of brown pigments. On
the basis of chemical composition, they may be classed in two groups;
those consisting of ferric hydroxide, and those in which the brown
colour is due to organic substances.

The first group comprises the minerals which have already been
mentioned in connection with the red earth pigments, bole and brown
ochre (umber), Terra di Siena, Cologne earth and a number of other
earths rich in ferric hydroxide belonging to this category. The second,
or organic group, includes compounds that are very rich in carbon and
are therefore of a very dark colour, the shades ranging from light
brown to black, _e. g._ the true umbers and asphaltum.


_Umber_

As already mentioned, the term “umber” was formerly applied to brown
varieties of ochre, whereas at present it is extended to certain
masses of brown-coal character, often interspersed with iron ochre and
sometimes containing manganese. Umber generally consists of fairly
dense, earthy masses, which are dried and ground--after crushing and
levigation, if necessary.

Valuable varieties are Cappagh brown and Caledonian brown, both with a
reddish tinge.

It is thus evident that “umber” now implies two different kinds of
materials, organic masses and iron-manganese compounds, which can also
be used as oil paints. These umbers can also be extensively shaded by
burning, the final colour being particularly influenced by the amount
of manganese compounds present.

The carbonaceous umbers (Cassel brown, Carbon brown) are combustible,
and mostly leave behind a merely small residue of ash. An important
property of these umbers is their partial solubility in alkalis, a
peculiarity which is utilised for the preparation of brown wood stains.


_Asphaltum_

Asphaltum forms very friable dark brown to black masses, which, in
contact with a light, easily ignite and burn with a bright, but very
smoky, flame, disengaging a peculiar, “bituminous” smell, and leaving
only a very small quantity of ash.

Extensive deposits of asphaltum are found at the Dead Sea, the Pitch
Lake on the island of Trinidad, in Dalmatia, and many other places,
where, however, it is in an impure condition and frequently contains
large quantities of sand. In many localities the rock is impregnated
with asphaltum, which makes it dark brown to black in colour and gives
rise to a bituminous odour when rubbed.

Peat beds sometimes contain pockets of a mass with a handsome brown
colour and consisting of a mixture of humic acids and other organic
substances which may be ranked with the humin bodies that are always
formed when organic matter decomposes in presence of an insufficient
supply of oxygen. These bodies are dark coloured, mostly deep brown,
rich in carbon, and, to some extent, similar to brown coal or peat in
chemical composition.

Their high carbon content renders these substances very inert towards
chemical reagents, and therefore particularly adapted for the
preparation of painters’ colours. Genuine Vandyke brown, which is the
handsomest brown known, is an earth rich in humin compounds; and Cassel
brown also belongs to this group.


(G) BLACK EARTH

The colour of these earths is entirely due to carbon, and pure carbon,
a certain form of which occurs native, is itself used as a pigment.
Actually, there are only two minerals that require to be mentioned in
this connection: black schist and graphite.


_Black Schist_

In most cases this is a clay shale, so rich in carbon as to appear
deep black. In commerce, this mineral is also erroneously called
“black chalk”; but at present it is seldom used as a pigment or
drawing-material, black chalks being produced far more cheaply than the
expense of preparing the natural article.

Grey clay shales are used for making grey earth pigments (stone grey,
and mineral grey).


_Graphite_

This mineral is found, in a very pure state, in many localities,
celebrated deposits occurring in England, Siberia, Bohemia and Bavaria,
whilst North American graphite has lately come into prominence.

Graphite is a modification of pure carbon, and is met with in the form
of hexagonal (rhombohedral) crystals, usually occurring as hexagonal
plates with a lustrous, iron-black colour. It rubs off easily, and
readily burns away, leaving a very small amount of ash, when subjected
to a very high temperature in presence of air.

The principal uses of graphite are as an anticorrosive paint for iron,
and for making lead pencils.

As already mentioned, the term “earth colours” has been considerably
broadened of late. Whereas, formerly, it was restricted to colours
prepared exclusively from minerals by a simple treatment, limited to
crushing, levigation or calcination, it now includes the pigments
obtainable from large by-products of certain chemical processes. This
latter class is especially important as affording an opportunity of
utilising products formerly considered worthless and whose removal
often entailed heavy expense.

By drawing on these materials the industry of the earth colours has
greatly enlarged its scope. At present, many colours of this kind are
on the market, and it is to the interest of many manufacturers to
endeavour to utilise certain waste products in the same direction.
The advantage of such a course hardly needs emphasising; but, to give
only a single example, it may be mentioned that the manufacture of
fuming sulphuric acid from green vitriol, by the old process, produces
residues which were formerly looked upon as quite worthless, and sold
at very low prices, but are now worked up, in a number of factories,
into very handsome and durable pigments.




CHAPTER III

THE PREPARATION OF THE COLOUR EARTHS


The preparation of the raw materials for the purpose of making earth
colours is a very important matter, because many minerals or pigmentary
earths merely require mechanical treatment to render them at once fit
for use. The mechanical preparation differs considerably, in accordance
with the raw material under treatment, substances that are found native
in a finely powdered condition only needing, for the most part, to be
levigated.

It rarely happens, however, that the raw material occurs in condition
for use direct, an example of this kind being afforded by the finest
clays or ochres. Whilst these are found in a state of extremely
fine powder, they nearly always contain certain quantities of sandy
ingredients or even large lumps of foreign minerals, and therefore
require levigating. Sometimes they need crushing as well, the small
particles cohering so strongly that mere treatment with water
(levigation) is unable to separate them. Mechanical force is therefore
necessary, a passage through grooved rollers being generally sufficient
to crush the lumps; but in some cases stamps have to be used.

When solid materials have to be treated, mechanical appliances must
always be used, their selection depending on the materials in question.
Thus, gypsum, for example, can be crushed with ordinary rolls or mill
stones, its degree of hardness being so very low (2) that it can be
scratched with the finger-nail.

If, however, the material to be reduced is limestone, which belongs to
the third degree of the scale of hardness (can only be scratched with
an iron nail), or heavy spar (hardness 3–3·5), very powerful stamps or
edge-runners must be employed to break it down into small lumps, which
can then be further reduced, without any special difficulty, in an
ordinary mill.

It is thus evident that a great variety of mechanical appliances are
used in the manufacture of earth colours. Before going into their
construction it is necessary to point out that, whatever the mechanical
treatment employed, a considerable expenditure of mechanical force is
entailed; and more power is needed when mixtures have to be prepared.
It is therefore essential, in planning a factory for making earth
colours on a large scale, to make provision for ample motive power.

This power may be supplied by a steam engine; but it must not be
forgotten that the prime cost and running expenses of such an engine
are considerable, and form an important item in view of the low value
of most earth colours. Consequently, it is highly important to be able
to generate motive power as cheaply as possible.

Now, the cheapest and most uniform source of power is water; and
therefore, wherever the conditions allow of the erection of the colour
works near a stream or river, which can supply the power to run the
various machinery, the most favourable circumstances will have been
secured, the power being obtained at minimum cost, whilst the upkeep
of the motor cannot be very great. If there is sufficient head for the
water to be run through a trough over the top of the levigation tanks,
the conditions will be ideally favourable.

Wind power costs nothing, once the motor has been installed; but
unfortunately, one is dependent on the weather, and sometimes there
is not enough wind, for days together, to drive the sails at all, and
therefore all the operations have to be stopped, including levigation,
the water for which has to be raised by a windmill pump.

In districts where the winters are severe, water power may also fail
and work have to be stopped; and consequently, even when water power
is the prime source of energy, a steam engine must be installed as a
stand-by, being, of course, only used when the main source of power
gives out or proves insufficient.

The machines employed for preparing the raw materials in the
manufacture of earth colours may be divided into the following groups:--

Machines operating entirely by pressure: crushers; machines acting by
impact: stamps; those acting by impact and pressure: vertical mills
(edge-runners), ball mills, centrifugal mills; and, finally, machines
with a frictional action: grinding mills. Then there are the levigating
machines, which do not reduce the material but separate the coarser
particles from the finer. The construction of the foregoing machines
is a matter for the machinery manufacturer rather than the maker of
earth colours; but as the business of the latter is dependent on them,
a short description is considered necessary. The selection depends, on
the one hand, on the nature of the materials to be treated, and, on the
other, on the size of the works, since a manufacturer who has to deal
with large quantities of a given raw material will require different
machines from those used on a small scale. The sole purpose of the
following description is to indicate to the colour maker the way in
which the reduction of the raw material can be accomplished.


CRUSHING MACHINERY

_Crushers and Breakers._--Crushers usually consist of grooved iron
rollers revolving on horizontal axes. One of the rollers is fixed, the
other being adjustable by screws, in order that lumps of different
sizes may be treated in one and the same machine, which may be employed
either to turn out a roughly crushed product, or to reduce it to a
certain degree of fineness.

If several pairs of crushing rollers be mounted in series, and each
set a little closer than its predecessor, the material can be reduced
progressively from large lumps to a fairly fine powder.

Each pair of rollers is geared together by pinions, and is turned in
such a way as to draw the material in between. If the gear pinions have
the same number of teeth, the two rollers will revolve at the same
speed and will then merely crush the material into lumps of a size
depending on the distance at which the rollers are set apart.

Nevertheless, by simply altering the gear ratio of the pinions, the
crushing action of the rollers can be supplemented by a grinding
action, a much finer powder being then obtainable than otherwise, the
one roller running at a higher speed than the other.

These crushers differ in strength of construction, very strongly built
machines being required for dealing with large lumps of hard material,
whereas substances of low crushing strength, such as clay or other
earthy materials, can be treated in much lighter machines. In any
case, however, it is advisable to have the machine stronger than is
absolutely necessary for the work in view; for, although the prime
cost is thus increased, the outlay on repairs will be reduced, and the
machines can, if necessary, be used on harder material as well. The
framework supporting the rollers should always consist of a strong iron
casting; and the machine should be set up as close as possible to the
engine or motor, to minimise the loss of power in transmission through
long shafting, etc.

[Illustration: FIG. 1.]

Fig. 1 represents a breaker (made by the Badische Maschinenfabrik,
Durlach), suitable for the rough crushing of clayey materials supplied
in large lumps. It can, however, also crush shale, lime, chalk, as well
as hard, sticky masses which would clog up a stone-breaker.

The material fed into this breaker is gripped at once by the powerful
projecting teeth, which are connected together by sharp-edged ridges,
and is crushed in such a way that it can be easily reduced still
further by a succeeding pair of smooth rollers.

[Illustration: FIG. 2.]

The granulator (Fig. 2), made by the same firm, is an example of a
machine for crushing harder materials. It is similar in construction to
a stone-breaker, but differs in the movement of the jaws, and combines
the properties of breaker and grinder, inasmuch as it tears the
material as well as crushes it. The figure shows the machine adapted
for direct electric drive. If necessary, these granulators can be
fitted with classifying jig screens.

_Stamps._--Stamps or stamping-mills have been used from prehistoric
times, and were probably employed for reducing hard materials long
before the introduction of grinding-mills. The underlying principle of
the stamping-mill is very simple. The material to be reduced is placed
in a trough or mortar, and the ram or head, which is of considerable
weight, is raised by a mechanical device and then allowed to fall
freely, from a certain height, on to the material underneath, which it
crushes. The heavier the head and the greater the height of fall, the
greater the effect produced. As a rule, a large number of stamps are
mounted together, and in such a way that half of them are being lifted
while the other half are falling. Either a separate mortar or trough is
arranged under each stamp, or else the whole drop into a common trough
charged with the material under treatment. Sometimes a lateral movement
is imparted to the material in the trough, so as to bring it under the
action of all the stamps in succession.

Although the construction of stamping-mills in general appears simple,
various modifications are employed for different purposes.

As a rule, a single passage through a stamping-mill is not sufficient
to reduce the material completely to the desired fineness, the first
product always containing large and coarse fragments of various sizes,
as well as fine powder.

If the latter were left in with the larger pieces for the second
stamping it would impede the work, and the stamping-mill should
therefore be provided with means for classifying the material
discharged from the trough, to separate the fine from the coarse and
grade the latter into sizes. This is usually effected by means of a
grading-screen.

Stamping-mills are chiefly used for reducing brittle materials. A
number of stamps arranged in a row are alternately lifted, by means
of cams mounted on a common shaft, and then let fall on to the
material lying on a solid plate, or else on a grating through which
the crushings fall. Fig. 3 is a stamping-mill constructed by H. F.
Stollberg, Offenbach.

[Illustration: FIG. 3.]

These mills are very strongly built, as independent units, the
frame being of cast-iron and the rams of best wrought-iron with
interchangeable chill-cast heads. In some mills the stamps are rotated
during the up-stroke, in order to equalise the wear on the heads, and
also to economise power.

The grating or trough holding the material is perforated with holes,
the diameter of which varies with the material under treatment and the
desired degree of fineness in the product. To increase the efficiency
of the mill, the grating or trough is adapted to move while the mill is
running, in order to clean itself automatically. These mills are made
in different sizes, with 2, 4, 6, or 8 heads.

_Edge-runners._--This type of crusher is highly suitable for reducing
earth colours in large works. The special feature of the type is that
both stones are mounted vertically and turn on a common shaft in
the same way that a cart wheel does on its axle. These runners are
particularly useful for reducing clay, chalk and other earth colours,
which have to be dealt with in large quantities. They will also crush
fairly large lumps, and they can therefore be used for the further
reduction of materials roughly crushed in a breaker, etc. The material
may be treated in either the wet or dry state, only slight alteration
being needed to change from one method to the other.

There are numerous different patterns of edge-runner, but all of them
can be divided into two groups, viz.: mills with stationary troughs,
whilst the shaft carrying the runners rotates; and those in which
the trough revolves, and the stones merely turn on the stationary
horizontal shaft.

[Illustration: FIG. 4.]

Comparison of the efficiency of the two types has shown that the
revolving-trough type is the better, giving a larger output per unit
time with a reduced consumption of power. Figs. 4 and 5 show a vertical
section and plan respectively of this type of edge-runner. The trough
_G_ is turned by means of a toothed crown gearing with the bevel
pinion _O_ mounted on an overhead shaft _C_ driven by a belt pulley _N_.

[Illustration: FIG. 5.]

The bearings of the vertical shaft _J_ of the trough are situated
at _L_ and _M_. The runners _H_ are loosely mounted on the fixed
horizontal shaft _E_ and revolve in consequence of the friction between
them and the material in the trough. As the latter revolves, the
material is continuously pushed aside by the runners, and is again
brought under them by the action of scrapers.

The great advantages afforded by edge-runners, in consequence of their
simplicity, easy management and low wear in comparison with other
grinding appliances, have led to their reintroduction on a large
scale. It should, however, be borne in mind that the edge-runner
mill must be of a pattern suitable to the materials it will have to
treat. The method of drive usually depends on local conditions. The
revolving-trough type is chiefly useful for mixing, on account of the
ease with which the materials can be charged.

The capacity of edge-runner mills depends on the nature of the
material, the diameter and weight of the runners, the speed at which
they are run, and also on the rate at which the reduced material is
discharged in order to give place to fresh portions of the charge. This
is effected by means of two sets of scrapers, the individual members
of which can be adjusted in any direction. Their ploughing action also
greatly assists the mixing effect.

[Illustration: FIG. 6.]

Fig. 6 illustrates an edge-runner mill with revolving trough and
overhead drive; and Fig. 7 one with stationary trough and bottom drive;
both made by the Badische Maschinenfabrik, Durlach. The runners are of
grey cast-iron, chill-castings or cast-steel being used for crushing
hard materials. The trough in all cases is lined with detachable
chill-cast plates. Special attention is bestowed on the lubrication of
all the moving parts, and all the lubricators are easily accessible.

[Illustration: FIG. 7.]

The main shafts of the fixed-trough machines have forged cranks, and
the metal crank bearings are provided with dust caps. All the shaft
journals run in detachable metal bushes.

A special advantage attaching to this type is the automatic screening
device and the returning of the screen residue. In some cases,
complicated appliances are employed to return the coarse residue from
the screen, bucket elevators, worm conveyors, etc., all entailing
increased motive power, not inconsiderable wear, and a higher prime
cost; but in this instance the object is achieved, without extra power
or wear, by very simple means. The dust-proof shell enclosing the
runners and screen is provided with large doors and charging hoppers.

The motive power required to drive edge-runner mills depends on the
dimensions of the mill and on the class of material to be treated; the
larger the mill and the coarser the material, the more power needed to
drive it.

This type is the more suitable for raw materials that are of an earthy
character, so that all that is necessary is to destroy the cohesion
of the particles, as is the case, for example, with clay and all earthy
minerals.

[Illustration: FIG. 8.]

The wet method of crushing with edge runners is particularly suitable
as a preliminary to levigation. A machine arranged for this purpose
is shown in Fig. 8. It consists of two sets of edge runners, one
with fixed, and the other with revolving trough. The material is
introduced by hand, or by suitable charging mechanism, into the upper,
fixed-trough machine, where it is continuously sprinkled with water and
kneaded by the one runner, and is passed thence to the second roller
which forces it through the slotted bed into the bed of the lower set.
The slotted beds of the upper and lower set are offset; and the chief
function of the lower set, with rotating bed, is to secure intimate
admixture of the material which, in most cases, is already sufficiently
reduced.

[Illustration: FIG. 9.]

_Ball Mills._--Ball mills are generally used for crushing dry materials
to fine powder. The mill shown in Fig. 9 is a typical form of grinding
drum enclosed in a dust-proof casing, the latter being provided,
at the top, with an opening connected to the dust exhaust pipe. The
discharge outlet at the bottom can be closed by a slide.

[Illustration: FIG. 10.]

The drum is provided with two strong lateral shields or cheeks (Fig.
10), one of which carries the interchangeable cross-arm and the
charging hopper. Both cheeks are lined with detachable chill-cast
plates, to take up the wear. The bed is formed of heavy steel bars
(which can be turned round), between which are arranged adjustable
slits for the discharge of the reduced material. Guard sieves are
mounted all round, and close to, the bed, and interchangeable fine
screens surround these in turn. The mesh of the fine screens determines
the fineness of the product, and the residue falls down on to a plate
which returns it to the interior of the drum. The reduction of the
charge is effected by a number of very hard, forged steel balls of
various sizes.

The mill must be run in the direction marked by the arrow on the outer
shell, so that the residue on the screens can be returned to the drum
by the plate provided for that purpose; and the prescribed working
speed must be maintained. The mill must not be overloaded. The impact
of the balls should be mild, but distinctly audible. Overloading
reduces the output. Idle running causes the most wear, since the balls
then roll directly on the bed, which, of course, should be prevented
as far as possible. The feed is continuous; and, of course, only dry
material should be introduced.

When the balls have lost size and weight through wear, they must be
replaced by a fresh set.

_Pulverisers._--Pulverisers are the best form of crusher for tough and
not over-hard materials. They are simple and strong in construction,
of high capacity with comparatively small consumption of power, and
furnish a good, uniform product, the size of which ranges from fine
powder to coarse granules, according to the screens used and the class
of material treated.

[Illustration: FIG. 11.]

The crushing is effected by a cross-arm beater, composed of four to
six radial steel arms on a divided, cast-steel hub, keyed on to
the horizontal shaft. The arms are hardened, and are adjustably and
detachably mounted on the hub.

The beating action of the arms, which run at high speed, forces the
material against the studded surface of the hardened cheeks of the
machine and also against the hardened square steel bars forming the
periphery, the repeated impact of the material on itself, as well as
against the arms and bars, progressively reducing it until small enough
to fall through the screen on the under half of the casing, into a
closed receptacle below. The screen mesh varies according to the degree
of fineness required.

The peripheral bars are mounted in a very simple manner, and in such
a way that when one edge of the bars is worn, a quarter turn brings a
fresh, sharp edge into operation, so that all four edges of each bar
can be utilised.

To prevent the escape of dust, the machine is provided with an
air-circulation chamber, which maintains the flow of air in continuous
circulation, the resulting strong draught also drawing the fine
material through the screen and keeping the meshes open. By this
means the capacity of the pulveriser is considerably increased. The
interchange of the crushing organs and screens, and also the cleaning
of the machine, can be effected without difficulty or loss of time.

The charge is introduced through a feed hopper at the side, and may
vary, according to the size of the machine, from nut size to lumps
twice as large as a man’s fist. If necessary, suitable mechanical feed
devices can be applied.

[Illustration: FIG. 12.]

_Disintegrators_ (Figs. 12 and 13).--This type of machine is used for
reducing medium-hard or soft materials, especially where it is desired
to obtain a comparatively large output of a gritty product.

In the patterns shown, the main shaft is of steel, with dust- and
dirt-proof red-brass bearings with pad or ring lubrication. The spindle
case draws out to facilitate cleaning. Mechanical feeding attachments
can be provided.

According to local conditions, the disintegrator can be mounted either
on a brick foundation, with lateral discharge outlet into a storage
bin, or on a raised grating of iron joists.

If the product is to be conveyed to a distance, it is advisable to have
a hopper-shaped collector leading directly to a worm conveyor or bucket
elevator.

[Illustration: FIG. 13.]

The arrangement shown in Fig. 13, in which the disintegrator is mounted
on a dust-proof cast-iron collector, has been found very suitable
for colour works of various kinds (aniline, lead, mineral and other
colours), particularly on account of the suppression of dust; whilst
the automatic charging worm greatly increases the capacity as compared
with charging by hand.


LEVIGATION

The effect of levigation is based on the circumstance that bodies of
greater density than water remain longer in suspension in that medium
in proportion as the fineness of their particles increases. This
treatment consequently enables the finer portions of a substance to
be mechanically separated from the coarser. Levigation is extensively
practised in colour works because it furnishes powder of finer grain
than can be obtained by even the most careful grinding.

The appliances used for levigation may be of a very simple character,
consisting only of several tubs or tanks, mounted in such a way that
the liquid contained in one can be run off into the one next below.
With this primitive plant, the material to be levigated is stirred up
in the water in the uppermost tub and left to settle until the coarsest
particles may be assumed to have settled down, whereupon the turbid
water is drawn off into another tub, in which it is left to settle
completely. When the clear liquid has been carefully drawn off, a
fine sludge is left in the bottom of the tub, consisting of the fine
particles of material mixed with water.

When a particularly fine powder is required, a single levigation does
not always suffice, but the liquid in the second tub must be left to
settle for a short time only, and then run into a third for complete
subsidence.

[Illustration: FIG. 14.]

A well-designed levigator for treating large quantities of powder is
illustrated in Fig. 14. A stirrer _R_, driven by cone gearing, is
arranged in a wooden or stone vat _G_. The levigating water enters
close to the bottom of the vat, through the pipe _W_. When _G_ is half
full of water, the stirrer is set running, and the substance to be
levigated is added. After a while, the water laden with the levigated
powder begins to run off at _A_ into the long narrow trough _T_{1}
provided, at the opposite end from _A_, with a number of perforations
through which the water runs into the trough _T_{2}. From this it
escapes through the perforations into the trough _T_{3} and thence
successively into _T_{4} and _T_{5}, finally discharging into the large
tank _S_.

The coarsest and heaviest of the water-borne particles deposit in the
trough _T_{1} finer particles settling down in _T_{2}, and so on in
succession, until the water reaching the tank _S_ contains only the
very finest of all in suspension, these taking a long time to settle
down to the bottom. The deposit in the upper troughs can be returned to
the vat, whilst that in the lower ones will be fine enough to dry as it
is. The residue in the vat is discharged through _Z_ when the operation
is finished.

It will be evident that the fineness of the product depends on the
number and length of the troughs _T_, the larger these factors the more
delicate will be the particles remaining in prolonged suspension in
the liquid.

Many earth colours require no treatment beyond levigation to fit them
for use in paints. This is the case with, _e. g._, the white clays;
and certain grades of ferric oxide, which occur native in the state
of fine powder, may also be included in this category. In many cases,
however, if large quantities of a finely pulverulent mineral be stirred
up with water and left to stand, the deposited solid matter forms such
a highly coherent mass that it can only be distributed in water with
difficulty, the fine particles adhering so firmly together that it is
hardly possible to stir them up again completely in the liquid by means
of a paddle.

Nevertheless, this can be easily effected by using a special appliance
of the kind employed by starch manufacturers for a similar purpose,
viz. the levigation of starch. This apparatus is designed in such a
way that the pulpy charge of material is gradually and completely
disseminated in the introduced liquid.

[Illustration: FIG. 15.]

Fig. 15 shows a device of this kind, consisting of a circular vessel
provided with a step bearing for a vertical shaft driven by cone
pinions. The lower part of the shaft is provided with a thread, on
which a nut is adapted to travel up and down. By means of rods, this
nut is connected to a wooden cross-bar provided with stiff bristles on
its lower face. A horizontal handle is attached to the nut. The water
is admitted through the pipe on the right.

In working the apparatus, the shaft is rotated and the handle held
firmly, thus causing the nut and attached cross-bar to rise to the
limit of its travel. The levigating liquid, mixed with the material
under treatment, is then admitted, until the vessel is full, and when
the solids have completely subsided, the clear liquid is drawn off,
and the operation is repeated until a thick layer of sediment has
accumulated on the bottom of the vessel.

To levigate this, the cross-arm carrying the bristles is lowered
until it just touches the surface of the deposit, and a continuous
stream of water is admitted through the pipe at the side. The bristles
gradually disseminate the upper layers of the sediment in the water,
which becomes turbid and is then drawn off into another vessel,
cement-lined pits being used in the case of large quantities. When
the brushes no longer encounter any of the sludge, the cross-arm
is lowered sufficiently to stir up another layer; and in this way,
large quantities of solid matter can be distributed in water. If the
cross-arm is rotated at low enough speed, the coarser particles of
material keep on settling down again, and the collecting vessels will
receive only the finest particles.

In addition to the mechanical separation of coarse and fine particles,
levigation accomplishes another purpose, namely that the prolonged
contact of the treated material with water dissolves out any admixed
soluble constituents which might affect the quality of the colour, the
latter being left in a purified condition.

For successful levigation it is essential that the charge should be
in a sufficiently fine condition at the outset. Clayey raw materials
require no preliminary treatment other, perhaps, than passing them
through a disintegrator, whereas hard, crystalline substances must
first be ground in a wet mill, such as an edge-runner mill with
stationary bed, into which the materials are fed with an admixture of
water, provision being made for keeping the charge under the runners
all the time. The crushed material is screened previous to levigation.

In the levigation process a few vessels of large size are preferable
to a number of small ones. The nature of the material will determine
whether any stirrers are required or not, these being unnecessary in
the case of the pigmentary earths, which naturally remain a long time
in suspension and therefore do not require stirring up.

The pulpy levigated material is taken out of the tubs, etc., drained
(if necessary) and dried. The draining may be effected in bags, or--in
large plants--filter presses or hydro-extractors. In these latter
instances, pumps will be provided for feeding the sludge direct to
the presses, and conveyors for delivering the pressed material to the
drying-plant.


DRAINING AND DRYING

The levigated colour earths form a stiff pulp containing a large
quantity of water, which can be eliminated in various ways. Usually,
the mass is dried by spreading it out thinly on boards and leaving it
exposed to the air until it has become solid; or else it is only left
long enough to acquire the consistence of a thick paste, which is then
shaped into cones or blocks, which are allowed to dry completely in an
airy place. If the colours are to be sold in the form of powder, the
dried lumps are crushed.

To accelerate drying, the pulp may be put through a hydro-extractor, or
dried in hot-air stoves or rooms. As, however, this last method entails
special appliances and also expenditure, this acceleration is only
resorted to when rendered necessary by special conditions.

_The Hydro-extractor._--When a substance is set in rapid rotation, it
tends to fly away from the centre at which the rotational force is
applied. The centrifugal force thus coming into action increases with
the velocity of rotation and with the distance of the substance from
the axis of rotation.

The centrifugal hydro-extractor consists, therefore, of a vessel in
rapid rotation; and if a liquid be introduced into such vessel, it is
projected with considerable force against the peripheral walls. If
the peripheral surface be perforated, the liquid portion of a charge
consisting of liquid and solid matters will be ejected through the
perforations, while the solid matter remains inside. As a rule, a few
minutes’ treatment in a hydro-extractor is sufficient to separate the
water from a thin pulp so completely that the solid residue is in an
almost completely dry state. A hydro-extractor which, though of an old
pattern, is well adapted for the purposes of the colour-maker, is shown
in Fig. 16.

[Illustration: FIG. 16.]

The drum A, which revolves easily on a vertical axis, is of metal, and
is provided with a large number of fine perforations on its peripheral
surface. It can be rotated at high speed by means of the crank _f_ and
pinions _d_, _e_, or by the fast-and-loose pulley _a b_ connected with
a source of power. To prevent any of the charge from being projected
over the rim of the drum, the upper edge is turned over so as to leave
only a comparatively small opening at the top. The lower end of the
drum shaft carries a strong steel spindle, which must be carefully
machined and enable the drum to revolve as easily as possible. This
is essential, because even small machines require a comparatively
large amount of motive power--which is not surprising in view of the
high speed at which the drum has to revolve in order to perform its
functions.

The drum is enclosed in a casing of somewhat larger diameter, which may
be of any convenient material. The bottom of the casing is preferably
tapered slightly downward, and is covered, at its lowest part--below
the bearing of the drum--with a sieve communicating with a pipe through
which the ejected liquid is drained off.

When a liquid, containing solid matter, is fed into the drum, which is
already running at high speed, the liquid is thrown, by the centrifugal
force, against the peripheral surface of the drum and escapes through
the perforations, leaving the solid matter behind. Where large crystals
are in question, as for instance in sugar factories, the centrifugal
machine can be employed without any additional precautions, the liquid
being expelled and the crystals being practically dried by keeping the
machine running a short time longer. In the case of the pulp obtained
by levigating colours, however, this procedure would result in failure,
because the fine solid particles would be ejected along with the liquid
and the drum would be left quite empty.

In this case it is therefore necessary to provide means for retaining
the solid matter in the drum, and allow only the water to escape, with
which object the drum is lined with a bag of closely woven fabric,
open at the top and fitting snugly against the inner surface of the
drum. When the drum is first started, the ejected liquid is milky,
no fabric being sufficiently close to retain all the extremely fine
solid particles present. In a very short time, however, the liquid
will begin to run away perfectly clear, this occurring as soon as
the pores in the fabric have become so far obstructed by the projected
solids as to allow water alone to pass through. The milky water is
then returned to the feed tank and run slowly into the machine. The
water is very quickly expelled, and the colour remains in the drum as
a stiff paste, of sufficient consistence to be moulded into lumps of
any desired shape. The use of the hydro-extractor may be particularly
recommended when ample motive power is available and accelerated
draining is desirable.

[Illustration: FIG. 17.]

Fig. 17 illustrates a modern type of hydro-extractor with bottom
discharge and suspended drum, the shaft of which is coupled directly to
an electro-motor.

_Filter-presses._--Whereas the hydro-extractor is only used in
particular cases for the purpose of the earth-colour manufacturer, the
filter-press enjoys more extensive application. Every filter-press is
composed of a number of closely fitting press frames, held together by
the pressure of a screw. These frames, when assembled, form chambers
provided with inlet and outlet openings. Suitably shaped and stitched
filter-cloths are secured inside the chambers, and the sludge to be
filtered is run into the press from a high-level tank. The water
passes through the filter-cloths and runs off, whilst the colour earth
gradually fills the chambers. When draining is completed, the press is
taken apart and emptied. In this way the earths are obtained in the
form of more or less dry cakes, which are then put through further
treatment or dried.

[Illustration: FIG. 18.]

Fig. 18 shows a Dehne filter-press suitable for the earth-colour
manufacturer. Wood internal fittings are often used, because wood does
not affect the shade of the colours; but, wherever the nature of the
materials admits, iron presses are to be preferred on account of their
greater durability and the certainty of the joints continuing tight.
The finer the grain of the levigated colour, the more difficult the
expulsion of the water; but as a rule, a pressure of 115–195 inches,
water-gauge, will be sufficient.

If the sludge be run into the press from a tank at sufficient height,
two charges can be worked in a day, but the cakes will not be as firm
as butter of medium hardness. It is better to pump the charge into
the press by means of a special diaphragm pump. The drainage is then
incomparably quicker, the cakes will be formed in about an hour and
will also be drier. A good deal, however, depends, naturally, on the
nature of the earth colour.

If the colour contains acid, alkali or salts, the filter-cloths can be
washed by flushing the press with water under pressure. The cloths are
made of specially fine cotton fabric. The press-runnings, which are
never quite clear, are collected in a clarifying tank, where they are
treated with lime and kieserite, whereby gypsum is formed, and the mass
is put through a filter-press, which retains the solids and leaves the
effluent clear.

[Illustration: FIG. 19.]

Filter-cloths which have become choked by use are spread on a table and
scrubbed with water, or else washed in a special machine (Fig. 19),
consisting of a rotary drum, with belt drive, the rotation circulating
the water in the interior trough and enabling it to extract the dirt
from the cloths. The flow and discharge of the water are controlled
by valves, and the water may be warmed by admitting steam into the
machine. The size of the washer depends on that of the filter-cloths.

From the press, the cakes of colour are conveyed to the drying-plant,
usually by the aid of automatic machinery.

_Drying Appliances._--The stiff paste or cakes from the hydro-extractor
or filter-press can be shaped, but require to be dried before they are
put on the market. Drying is a wearisome operation, the finely divided
material taking a very long time to dry completely, even during the
summer months, whilst in winter it is almost impossible to get certain
colours--such as ferric oxide colours and levigated clay--quite dry in
the air, the inside of the lumps remaining soft and pasty after lying
for months.

The only way in which this troublesome delay in the completion of
the operation can be overcome is by artificial drying; but as the
employment of artificial heat entails expense, it is necessary to carry
on the process with the smallest possible outlay, in view of the low
commercial value of most earth colours.

Long experience has convinced the author that the arrangement of
the drying-rooms in many colour works is based on entirely wrong
principles, and that a great portion of the heat furnished by the fuel
is wasted. For this reason the description of a properly arranged
drying-room will be welcomed by a number of readers.

It is a well-known fact that hot air is lighter than cold.
Consequently, when a room is artificially heated, the highest
temperature will be found just under the roof or ceiling, and articles
placed in that part of a heated room will dry much faster than those
near the floor. If the drying-room is heated by an ordinary stove,
articles placed on a fairly low level will only dry very slowly,
because the hot air flowing from the stove tends to ascend.

In order, therefore, to utilise the entire space of the drying-room,
it is necessary to place the heating apparatus in such a position
that the whole of the room will be warmed as uniformly as possible.
The stove should therefore be situated in a chamber underneath the
drying-room proper.

[Illustration: FIG. 20.]

Because air that is already saturated with moisture cannot take up
any further quantity, care must be taken to remove the damp air
continuously from the drying-room, and to replace it by dry air. This
may be effected by suitably designed ventilation, on the lines shown
in Fig. 20, which represents a drying-room arranged in such a way
as to provide for all the above-mentioned contingencies, and ensure
continuous drying.

The heating apparatus is located in the cellar, and consists preferably
of a slow-combustion stove comprising a cast-iron cylinder, with an
air inlet _A_ (with sliding regulator _T_), for the air of combustion,
and a shoot _F_ at the top, through which the stove is fed with
fuel--preferably coke, on account of its great heating power.

The stove is surrounded by an iron or brick shell _M_, having two flues
_R_ and _R1_ leading to the chambers I and II, where they terminate in
register cowls _K_, which can be adjusted, by turning the handles _h_,
so that when the slots _o_ in _K_ coincide with corresponding slots
in the end of the pipe, the maximum amount of hot air from the stove
is delivered into the drying-chambers; and, by suitably adjusting the
cowls and the draught through the fire-door _T_, it is possible to
regulate the temperature of the chambers to within one degree of the
thermometer scale. When only one of the drying-chambers is required to
be heated, the register in the other is closed, and the whole of the
hot air is delivered to the first one. With this arrangement none of
the heat is wasted, and the contents of one chamber can be dried while
those of the other are being removed and replaced.

The moisture-laden air from the drying-chambers can be led direct into
the stove chimney. When coke is used, the flue gases consist almost
entirely of carbon dioxide. If the vent pipes are led from the top
of the drying-chambers into the chimney, the hot gases ascending the
latter induce a strong draught in the chambers and carry off the moist
air into the open. These pipes, also, are fitted with registers, which,
when suitably adjusted, assist in the maintenance of a uniform drying
temperature.

The colours to be dried are spread on trays laid on suitable racks
in the drying-chambers; and, by carefully planning out the available
space, a very large quantity of colour can be quickly and completely
dried in a comparatively small plant. The cost of the fuel is so small
as to be more than counterbalanced by the saving of time.

The heating arrangements in drying-rooms are capable of improvement
in many respects, especially where steam is at disposal; and in such
cases, it is better to substitute steam heating for a fire. It will
then be necessary to put in a good fan, or other device, to ensure the
removal of the moist air. An excessive room temperature--above, say,
50° C. (122° F.)--is not only superfluous, but in many cases injurious,
because, apart from the fact that some colours change in shade when
over-warmed, an unduly high temperature causes the surface layers to
dry very quickly and form a crust which prevents the escape of water
vapour from the interior of the material.

Another form of drying-plant for earth colours is the drying-floor,
a large room with a rammed concrete or stone floor, intersected with
brick flues (about one foot square), covered with iron or concrete
slabs and conveying hot flue gases from a furnace. These floors are
particularly suitable where there is a possibility of utilising an
existing supply of hot flue gases.

Drying-tunnels are specially adapted where large amounts of material
have to be dried. The tunnels are built of brick and provided with a
rail track on which the trucks carrying a series of trays laden with
colour are run. As the trucks move slowly forward, they are met by a
current of hot air which dries the charge. The tunnel is kept filled
with laden trucks, each fresh one introduced pushing a finished one out
at the further end.

In many cases, drying troughs are also useful. These are long,
semicircular, jacketed troughs of boiler plate, hot air or steam being
passed through the jacket space. A worm conveyor keeps the contents
moved forward, turned over and mixed to facilitate drying.

Mention may finally be made of vacuum drying-cupboards, which are
heated, air-tight chambers, for the material, in which the air is
partially exhausted, thus increasing the rate of evaporation of
the water and causing the materials to dry quickly at a much lower
temperature than otherwise.


CRUSHING AND SIFTING

The distributing and covering power of the earth colours depends--apart
from their special properties--on the fineness of their particles. For
this reason, all the means adopted for the purpose of pulverisation
are of particular interest. The most important crushing and powdering
devices have already been described, and may be referred to, all that
needs mention in addition being the fact that stone mills also are used
for fine grinding.

The ground products, however, are not entirely homogeneous, always
containing, in addition to the very finest particles, those of a
coarser nature which must be removed by sifting.

Sifting machines are essentially sieves through which the colour is
passed. The sieves are made of wire gauze or bolting-cloth, stretched
on prismatic frames which are rotated (centrifugal sieves), or
superposed on the flat and reciprocated. In centrifugal sieves, the
material is projected against the sieve, and the whole apparatus is in
a state of vibration, or else beaters are provided to keep the fine
orifices in the sieve from choking up.

Nowadays there are numerous types of sifting devices, none of which,
however, can be considered as the best for all purposes, since
each type of earth colour behaves differently and requires special
treatment. The proportion of moisture in the material, also, has an
important influence on the method of treatment required.

[Illustration: FIG. 21.]

A typical flat sifting-machine, with eccentric jig motion, is
illustrated in Fig. 21. The machine is fed through a hopper provided
with feed rollers, the rate of feed being adjustable. The screened
product is discharged through a shoot at one side of the machine, and
the residue at the opposite side, into boxes, etc., placed underneath.

For materials that give off a large amount of dust, the machine can
be enclosed in a dust-proof casing, in which event the product and
residue are delivered into drawers. The machine is easily cleaned and
the sieves quickly changed, and is well adapted for dealing with a
succession of different materials. The hopper can be fitted with a pair
of adjustable crushing rollers.

[Illustration: FIG. 22.]

Fig. 22 is a drum sifter, which is fed by means of a hopper and worm;
and the drum can be covered with wire or silk gauze. The sifted
product falls into a worm conveyor in the bottom of the casing and is
discharged at the side. This may be replaced by a series of mouths
for discharging direct into bags, or the machine can be adapted to
deliver into an elevator, worm conveyor or other means of transport to
a distance.

The screenings are discharged through a shoot at the back of the
machine, and can be handled in various ways. A beater is provided to
clear the drum and increase the output.

[Illustration: FIG. 23.]

Fig. 23 illustrates a centrifugal sifting-machine for producing very
fine powder in large quantities without any escape of dust. It contains
a screening drum, the frames of which are detachable and facilitate
changing the sieves. A beater revolving inside the drum projects the
powder against the sieves, such portions as pass through being taken up
and discharged by a worm conveyor; this, however, can be replaced by a
bagging device, etc.


CALCINING

Colour earths are sometimes calcined at a high temperature in order to
modify their structure and shade, the operation being accompanied, in
some cases, by the destruction of organic admixtures and the expulsion
of volatile constituents.

An important feature of calcining is that it improves the covering
power of many colours, especially heavy spar and certain ferric
oxide pigments. This alteration is probably due to the heat causing
the finest particles to cohere, and also to the expulsion of
chemically-combined water, etc.

The change of shade, which is often dependent on the degree and
duration of the heating, is probably also connected with cohesion; but
in many instances it is attributable to chemical modifications produced
by the treatment; ferric hydroxide, for example, losing its water of
hydration when heated and becoming transformed into ferric oxide.

The details of the calcination process vary with the nature of the
material, and will therefore be described, together with the appliances
used, when we deal with the colours which require to be put through
this treatment.


MIXING AND IMPROVING

It is very important that the maker of earth colours should always
be able to turn out his products uniform in shade, and since the raw
materials are liable to vary in character, and the composition of the
earths from one and the same deposit is not invariable, the desired
shade has to be obtained by mixing. For this purpose, standard samples
must be prepared, for comparison in matching.

[Illustration: FIG. 24.]

Mixing is a highly important operation, on the proper performance of
which oftentimes depends the sale of certain colours and the reputation
of the maker. It may be effected in various ways, such as shovelling
the ingredients together or by combining the work with grinding in
edge-runner mills, ball mills, etc. Another method is the mixing barrel
shown in Fig. 24, a strong cask mounted on an axial shaft driven
by a motor, etc. The barrel is filled about two-thirds full of the
materials to be mixed, and, after closing the feed door, is slowly
rotated, since, if run at excessive speed, the contents are merely
projected against the sides of the barrel by centrifugal force, and
it can then be turned for hours without result. The mixing effect can
be considerably increased by mounting the barrel so that the shaft
is offset from the longitudinal axis of the barrel by an angle of
about 30°, the contents being then moved from side to side at each
revolution and thus more intimately intermixed by the twofold motion.

[Illustration: FIG. 25.]

In addition to such home-made appliances, there are mixing-machines
of the type illustrated in Fig. 25, the body of which is fitted with
a distributing worm at the top, and a pair of rollers at the bottom.
Below the rollers, which are covered by plates that can be adjusted at
a convenient angle, is a worm conveyor delivering into an elevator,
outside the machine casing, which connects the two worms. One or more
discharging-doors, according to the size of the machine, are provided
under the worm conveyor at the end next the elevator. The feed hopper
can be arranged on the elevator or on top of the machine, according to
local conditions.

In working this machine, the elevator and distributing worm are started
and the full charge is fed into the hopper. When it has all passed
through the distributor and is lodged on the sloping plates and bottom
rollers, the latter and the worm conveyor are set in motion, the
material being then carried through by the rotation of the rollers
and dropping on to the conveyor, which delivers it to the elevator,
to be returned to the distributor. In this way the charge is kept in
continuous circulation, and the finely divided particles are repeatedly
intermingled, a uniform mixture being obtained. The effect is
heightened by the grinding action of the rollers as the material passes
between them.

The serial order of the various ingredients, their physical condition
(granular or powder), and their density, are all immaterial, the mixing
being effected so intimately that when, for example, a colour is shaded
with aniline dyes, the ingredients are so completely blended in less
than an hour that even the smallest sample then taken will perfectly
represent the bulk.

These machines are made in various sizes, are entirely automatic,
both in charging, discharging and mixing, and are quite dust-proof,
the consumption of power being also small. If necessary, they can be
combined with a crusher or sifter feeding direct into the hopper.

A simple means of ascertaining whether the mixing is completed, and
one that can also be employed for judging the character of ground
materials, consists in placing a sample of the product on a sheet of
white paper and spreading it out, under gentle pressure, with a steel
or horn spatula. No irregularities, streaks, spots or granules should
then be discernible either by the unaided eye or under a magnifier.

Improving, which consists in staining earth colours with other (usually
organic) colouring agents, to improve the shade, is an operation which
is generally resorted to only in case of need, because it means extra
expense, and is of no value unless light-proof colours are used. No
permanent effect can be obtained by merely mixing-in coal-tar dyes at
random. In addition to certain organic dyestuffs, artificially prepared
mineral colours and colour lakes--artificial preparations of an organic
dyestuff with an inorganic substratum--are also used for improving.

Another way of improving earth colours is by precipitating certain
coal-tar dyes on them, in presence of a fixing agent. Of course the
dyes used must not only be fast to light, but also inert towards
the substratum and to any other ingredient, such as lime, that is
subsequently added to the earth colours.

The following dyestuffs (Höchst) are suitable for direct precipitation
on siliceous colours (green earths, clay, ochres, etc.).

Auramine, conc. O, I, II; new phosphine extra; chrysoidine A cryst., B
cryst., C extra; Vesuvine (all marks); cachou brown D, G; dark brown
M, MB; safranine G, GS conc., B conc.; rhodamine O extra, B, B extra;
fuchsine (all marks); fuchsine acetate; cerise G, R; grenadine O, R,
RR; maroon O extra; new fuchsine O, P; methylene violet (all marks);
peacock blue P; Victoria blue B, R; thionine blue GO; methylene blue
(all marks); malachite green (all marks); brilliant green (all marks);
coal black O, I, II.


MOULDING

The colour pulp can be made into tablets by moulding it in dry boxes
divided into a number of compartments. The colour shrinks in drying,
and the tablets will then easily fall out of the moulds. Cones are
obtained by placing the pulp in a box, the bottom of which is
perforated with numerous holes of uniform size, the box being then
tapped against the surface of a stone table. At each stroke, a certain
amount of colour is forced, in the shape of small cones, through the
perforations, on to a sheet of paper underneath. The cones are then
dried.

Some colours are moulded into blocks by forcing the partly dried paste
into suitable moulds--preferably of metal, so that they may be engraved
with the maker’s name, or other imprint--and left to dry slowly
and without cracking. The cakes may be prevented from crumbling by
incorporating a small quantity of adhesive, such as a weak solution of
dextrin, with the water in which the colour is suspended.




CHAPTER IV

WHITE EARTH COLOURS


The white earth colours are important for the purposes of the
colour-maker, because not only are they used by themselves as paints,
but also serve in the production of light shades of other colours.

The white colours containing clay or lime are the most abundant and
important of all, and will therefore be described first. The lime
colours comprise caustic lime, carbonate of lime (chalk or powdered
limestone), gypsum and bone ash.


CAUSTIC LIME

Though this product is not used direct as a painters’ colour, it is
employed in the preparation of compounds that are so used. It is made
on a large scale for the preparation of mortar, and there is therefore
no need for the colour-maker to manufacture it himself, since it
can always be bought from a lime-burner. It must be borne in mind,
however, that lime for the colour-maker’s purposes must possess certain
properties, failing which it is of no use to him. What these properties
are and how the product is made will now be briefly described.

When carbonate of lime, _i. e._ native limestone, is exposed to strong
heat it parts with carbon dioxide and is transformed into burnt or
caustic lime.

  CaCO{3}            = CaO           + CO{2}
  Carbonate of lime.   Caustic lime.   Carbon dioxide.


The limestone is burned either in kilns of very simple construction,
or else in more complicated furnaces in which a continuous process is
maintained. The ordinary limekiln, which can be found in many parts of
the country, consists merely of four walls, with a door in the front
one for the introduction of the fuel. Kilns of this kind are usually
set up in the vicinity of the limestone deposits, and are abandoned
when they get worn out.

The limestone is broken to lumps of fairly even size, about as large
as a man’s head, and these are piled up in a domed heap in the kiln,
sufficient space being left between the lumps for the passage of the
flame. A fire is then lighted under the pile, pine wood being mostly
used for this purpose on account of its high content of resin, which
gives a very strong flame. The fire is kept up until the top of the
pile has become white hot, and only a blue, smokeless flame is visible.
The appearance of this denotes that the burning is ended, the fire
being then allowed to die out and the lumps left until cool enough to
be taken out of the kiln.

This operation is performed with great care, particular importance
being attached to preserving the lumps as intact as possible and
preventing the formation of dust, which is of little value. The lime
made in this way is endowed with properties that render it valuable
for the purposes of the colour-manufacturer; but, on the other hand,
possesses certain disadvantages.

Owing to the use of wood as fuel, the caustic lime obtained in this
way is usually a very pure white, because the burning is continued
until the whole mass is glowing and the firewood has been completely
consumed. If this is not the case, the burnt lime is greyish in colour,
from the finely divided particles of carbon, which, of course, spoils
the lime for colour-making. The defects existing in lime burned in the
above type of kiln originate in the irregular character of the product.
It will be evident that the limestone lumps that are nearest the fire
will be far more strongly heated than those in the upper part of the
dome; and when calcined lime is kept incandescent for a long time, it
becomes so compact in texture that it quenches with great difficulty
when brought in contact with water. This condition is known as “dead
burnt,” and such lime is of little value.

The lumps at the top of the pile are least exposed to the heat, and
very often still contain carbonate, as is shown by the effervescence
produced on treatment with an acid. Such lime is imperfectly burnt,
and the lumps frequently still exhibit the crystalline structure of
limestone when broken. They quench rapidly, but when mixed with a
little extra water, the mass is no longer of the buttery consistency
typical of caustic lime, but contains gritty portions consisting of
unaltered limestone.

Owing to the defects of dead burning on the one hand and insufficient
calcining on the other, colour-makers now prefer lime that has been
burned in continuous kilns, because, when properly made, such lime is
very uniform in character, and is also cheaper than that burned with
such an expensive fuel as wood. In consequence of the greater capacity
of the continuous kiln, and the more uniform character of the product,
the old-fashioned kilns are more and more falling into disuse.

The arrangement of the continuous kiln is very simple. The kiln
consists of a fairly high shaft, open at the top, and provided at the
bottom with a small hole for the removal of the burnt lime. A coal fire
is lighted, and as soon as the kiln is heated up, alternate layers of
limestone and sufficient coal for burning it are introduced. The burnt
lime sinks to the bottom of the shaft and is pulled out, with iron
hooks, from time to time.

Given the right proportions of coal and limestone, the lime made in
these kilns is burnt to just the right degree, and is excellent for
builders’ use. In many cases, however, it is less valuable to the
colour-maker, and in some quite useless. For example, when the coal
is not completely consumed, carbon, even though only a very small
quantity, is deposited on the lime, and the burnt lime, instead of
being a brilliant white, as it should be, is grey; and colour made
therefrom is also greyish white and will spoil the shade of other
colours with which it is mixed.

The chemical composition of the original limestone also has an
influence on the character of the burnt lime. Limestone consisting
entirely of carbon dioxide and lime is so rare that sufficient is never
available for making burnt lime on a large scale. Even the purest
limestone found native in large quantities--namely marble--is not pure
carbonate of lime, but contains a certain proportion of extraneous
substances. At the same time it is too expensive to use for technical
purposes.

The ordinary impurities present in limestone are ferrous oxide, ferric
oxide, magnesia and organic matter. The presence of ferrous oxide can
usually be detected by the greenish tinge of the raw limestone, and
the reddish cast of the burnt product. Ferric oxide is revealed by its
reddish colour, in both the limestone and burnt lime.

Magnesia, which is present, for example, in dolomitic limestone, cannot
be detected by the colour, either before or after burning, this oxide
being itself perfectly white; but its presence is a drawback because if
in large quantity it makes the lime very difficult to quench, and such
lime is never of a fatty character.

Organic matter betrays itself by the colour, the lime being dark
tinted, varying from grey to black. Black limestones usually contain
carbon in an extremely fine state of division, and are quite useless
to the colour-maker owing to the impossibility of completely burning
off this contained carbon, which always imparts a greyish tinge to
the burnt lime. The behaviour of limestones in this respect varies,
however, considerably, and can only be ascertained with certainty by a
trial burning. Many that are rather dark in colour will, nevertheless,
burn perfectly white, whereas others, much lighter in shade, always
give a product that is not quite pure in tone. This divergent behaviour
seems to have some connection with the chemical composition of the
organic matter in question. If it consists of coal, or substances
analogous thereto, no really pure white lime can be obtained from
a light grey limestone, it being impossible to burn off the finely
divided carbon completely.

In addition to making a trial burning with a fairly large sample of
material, the behaviour of a limestone towards hydrochloric acid will
afford some information as to the nature of the grey colouring matter.
If the limestone dissolves completely when suffused with the acid, the
indications are favourable for its usefulness to the colour-maker.
If, on the contrary, a black residue is left, the coloration is due
to finely divided carbon, and there is then little prospect of the
material furnishing a suitable product. In any event, a trial burning
is the most reliable guide. In addition to carbon, the presence of any
large proportion of ferric or ferrous oxide is objectionable, since,
in either case, the product will be tinged red with ferric oxide, into
which the ferrous oxide is transformed at calcination temperature.

In addition to comparing the colour of the product with a standard
sample, the suitability of a burnt lime for colour-making can be tested
by quenching. If a lump about the size of the fist be placed in a large
porcelain basin and suffused with a small quantity of water, preferably
poured in a thin stream, the lime, if properly burned, will continue
to absorb the water for a considerable time, like a sponge, and will
very soon give evidence of a brisk reaction by increasing in bulk and
generating such an amount of heat as to cause the immediate evaporation
of a few drops of water allowed to fall on the surface of the mass.
Finally, the entire lump will crumble down to a very delicate,
voluminous powder, consisting of slaked lime (calcium hydroxide).

This chemical reaction is expressed by the equation:--

  CaO  + H{2}O = Ca(OH){2}
  Lime   Water   Calcium hydroxide.

When the amount of water added to burnt lime is no more than
sufficient to effect its transformation into hydroxide, this latter,
as already stated, forms a delicate white powder. The addition of more
water results in the formation of a homogeneous pulp, of a peculiar
fatty character. Since this fatty appearance is only possessed by pure
lime, it is a criterion of high quality in burnt lime, and contrasts
strongly with that of the less valued poor (or lean) lime.

Calcium hydroxide acts as an extremely powerful base, and therefore
must not be mixed with colours that are sensitive to the action of
strong bases. As a matter of fact, its direct use in painting is very
small. Of course, a thin milk of lime is used for whitewashing walls,
etc.; and if any colouring ingredients are added they must be such--_e.
g._ ochres--as are not affected by the lime. Nevertheless, quick
and slaked lime are very important in colour-making, as forming the
originating material for the preparation of a number of colours.

When slaked lime is mixed with sufficient water to form a stiff pulp,
and is left exposed to the air for some time, a change will be observed
to take place, the mass solidifying gradually (commencing on the
outside) and finally crumbling to a soft white powder. This change is
due to chemical action, the lime having a great affinity for carbon
dioxide, which it readily takes up from the atmosphere--a fact which
explains the solidification mentioned. It would be erroneous to assume
that the lime is again completely converted into calcium carbonate in
this way; for, though such conversion does ultimately take place, it
requires a very long time for completion.

The resulting compound is, actually, a double compound of calcium
oxide and carbonate. Although this compound has fairly strong basic
properties, they are, nevertheless, far weaker than those of caustic
lime, being partly neutralised by the carbon dioxide absorbed. If the
superficial area of the slaked lime be increased by spreading it out
thinly, so as to offer greater opportunity for the action of carbon
dioxide, the formation of the double compound in question will be
greatly accelerated.

This double compound is prepared artificially in special works, and
the resulting colours are put on the market under various names. They,
too, must not be mixed with colours that are sensitive to alkali, and
on this account they cannot be used in fine paints. If applied as a
white priming to the walls of rooms, care must be taken to cover the
coating with some substance that will protect the topping colour from
the action of the lime. For this purpose, painters use a wash of milk,
soap and water, etc.

An important property of lime is its behaviour towards casein, the
substance forming the curd of milk. With this body it combines to
form a mass which sets hard and is highly resistant, viz. calcium
caseate, and is formed when limewash is stirred up with milk or freshly
precipitated casein. Weatherproof distempers for outside use are
prepared in this manner.


PEARL WHITE

The preparation frequently met with in commerce under this name is
nothing more than a burnt lime of great purity. It is prepared in the
coastal districts by burning oyster shells, the resulting burnt lime
being easily transformed into a fine powder, the pure white colour of
which is due to the absence of iron. It is used in the same way as pure
burnt lime, and is mainly of interest in seaside towns where oyster
shells are often accumulated. It may be pointed out that the name pearl
white is often applied also to pure white grades of white lead.


VIENNA WHITE

This colour is prepared from any kind of burnt lime that is
sufficiently pure; that is, free from ferric oxide. The method of
preparation is simple, requiring no special apparatus, and can
therefore be carried out wherever suitable lime is available.

Operations are commenced by carefully slaking well-burnt lime with
water, a sufficient excess of which is added to produce a fairly thick
pulp. To accelerate the absorption of carbon dioxide, the mass is
exposed to the air in thin layers, by spreading it out on boards, so as
to present a large surface to the air. As soon as the pulpy character
has disappeared, the mass is detached from the boards, and is pressed
and kneaded, with wooden paddles, into prismatic cakes which are left
exposed to the air--being, of course, protected from the wet--until
the absorption of carbon dioxide is complete--a condition that can be
recognised by the earthy character of the product. The cakes are then
dried, an operation entailing great care, since lightness is a sign of
good quality, whereas a damp product is very heavy.

In forming the cakes they must not be touched by the bare hands,
because the lime is so caustic that it would soon destroy the skin. The
foregoing method of manufacture is capable of many improvements, which
can be introduced without adding much to the cost of production.

If the lime is formed into large blocks, it will evidently take a
long time for the mass to acquire, all through, the earthy character
indicating combination with carbon dioxide. This drawback can be easily
remedied by forming the mass into small cakes, which will become ripe,
owing to their larger surface, much sooner than the bigger blocks.

A very good plan to adopt in moulding is to form the burnt lime into
a stiff paste with water, preferably by adding enough water to make a
viscous mass, and leaving this in a lime-pit for several weeks, the
prolonged storage enabling the lime to acquire the already mentioned
fatty character, and at the same time to become highly plastic.
Lime treated in this way can be forced through a nozzle, forming a
cylindrical rope, which can be cut by a knife into convenient lengths
and left on boards for a few days until they have become firm enough to
stand up without breaking. Cylinders made in this manner, with a length
of about four inches and a diameter of two inches, will absorb carbon
dioxide very quickly.

The absorption can be still further accelerated by setting up the
cylinders in an atmosphere highly charged with the gas, for instance
in the vicinity of a manure pit, as they will then avidly take up the
carbon dioxide abundantly liberated from the rotting manure. Similar
acceleration will take place if the boards carrying the cylinders are
placed in a stable, or in a room where wash for making spirits is
fermenting, because large quantities of carbon dioxide are liberated in
both places.

Working the caustic mass by hand is accompanied by so many
inconveniences that it seems highly desirable to employ some mechanical
moulding device which will render contact with the wet lime entirely
superfluous. It may be pointed out that such a device can also be
advantageously used for moulding all earth colours in paste or pulp
form, and in particular for shaping ferric oxide colours into rods or
small cylinders.

[Illustration: FIG. 26.]

Such a machine (Fig. 26) is composed of a rectangular box with
semi-cylindrical bottom, a detachable shaft carrying a sheet-metal
worm being arranged in the box so that the worm is in contact with
the rounded bottom and is continued into the cylindrical extension of
the box. This extension terminates in a hollow cone, to the mouth of
which nozzles of varying aperture (square, rectangular or round) can
be attached. A knife, operated by hand or mechanical means, enables
the extruded soft mass to be cut into convenient lengths, which drop
on to a series of easy running rollers in front of the nozzle, and are
thereby delivered to an endless-belt conveyor from which they can be
transferred to the drying-boards.

When the box has been charged with the lime pulp and the worm is
rotated, the latter forces the soft mass into the cone and extrudes it
through the nozzle, so that, as long as there is any material in the
box, it is discharged as a continuous rope, of square, rectangular or
cylindrical section, on to the guide-rollers, where it can be cut off
into lengths by the knife.

A fundamental condition for the preparation of a good Vienna white is
the employment of pure raw material, which must be free from ferric
oxide or earthy impurities, and fully burned. An excellent material
for this purpose is calcined mussel shells, which furnish a loose,
and at the same time very pure, lime, and are very largely used for
lime-burning in places such as Holland, where they are available in
large quantities.

Vienna white is not much used as a paint colour, owing to its powerful
alkaline properties which have a destructive effect on many colours.
It is, however, largely employed as a polishing agent, for which
purpose it is powdered and is put on the market--mostly in bottles--as
Vienna lime. Its very handsome white colour and low price render it
particularly suitable for coarse painting, for example as a prime
coating for painted interior walls. To guard against the danger of
the painted decoration being destroyed by the alkaline nature of the
white, it is advisable to coat the dried ground with alum solution, the
alumina of which combines with the lime to form an insoluble compound
to which organic colours adhere well. The sulphuric acid also enters
into combination with the lime, the resulting gypsum having no effect
on the paints subsequently applied.


CHALK

The name chalk is used for a number of commercial substances which
differ considerably in both the mineralogical and chemical sense.
French chalk, for instance, is a mineral belonging to the steatite
group and, apart from its name, has nothing in common with true chalk,
except the white colour, and even this differs altogether from that of
chalk properly so called. It is therefore necessary, in the interests
of proper nomenclature, to differentiate the various kinds of chalk,
commencing with the mineral known by that name to the chemist and
mineralogist.

In chemical composition, true chalk is calcium carbonate, but of a
fossil character, for if chalk dust be examined under a high-power
microscope, it will be seen to consist of the shells of minute animals,
and is therefore to be regarded as fossil. The organic matter of the
animals has long disappeared, leaving the inorganic material, a very
pure calcium carbonate, behind.

[Illustration: FIG. 27.]

Such progress has been made that the zoological status of the animals
which inhabited the shells--many thousands of which are present in a
lump of chalk--has been identified; and it is known that these animals
were of marine type. Fig. 27 shows the appearance of the animal remains
in Meudon chalk when highly magnified, the upper half being viewed by
transmitted light and the lower by reflected light.

Notwithstanding the extremely minute dimensions of the chalk
animalculæ, their remains form rocks of great thickness in all parts of
the world. In Europe we find, for example, extensive chalk formations
in England, whose Latin name Albion was bestowed on account of the
white chalk cliffs occupying long stretches of the coast. The hills of
Champagne consist almost entirely of chalk; and Rügen, together with
many other islands, is nearly all chalk cliffs.

It is only in very rare cases, however, that chalk occurs in
sufficient purity to be immediately suitable for use as a pigment
or writing-material. For the most part it contains other minerals,
or large fossils, from which it has to be separated by mechanical
treatment. Nodular flints are often met with in chalk, and many
deposits contain such large numbers of the petrified shells of the sea
urchin that the chalk really cannot be used as a pigment at all, by
reason of the high cost of purification. The only places where chalk
can be advantageously worked for the preparation of pigment is where
the mineral is in a high state of purity, and also contains only very
few sandy particles. Such chalk deposits are worked on a mining scale,
and, as a rule, in the state in which the chalk comes from the quarry;
it is in the form of a soft mass, easily scratched with the finger-nail
and of fairly high density, owing to the considerable quantity of water
with which it is ordinarily impregnated.

In order to convert this crude chalk into a product that can be
used as a pigment, it is first left to dry until the lumps can be
easily broken, and then crushed into small pieces, from which all the
extraneous minerals, which occur as large lumps, are sorted out and
removed. This picking process is important, especially when the chalk
contains flints, because these latter are very hard and would injure
the millstones in the subsequent grinding.

The lumps of chalk are reduced by mechanical means, such as a
stamp-mill, or, more frequently, in a mill of the same type as for
grinding flour, since it is impossible to get the lumps so dry as to
produce the degree of brittleness necessary for a thorough reduction
in a stamp-mill. The millstones are enclosed in a wooden casing, and
the chalk is ground in admixture with water, the ground mass escaping,
through an opening in the casing, as a thick pulp which is stored for a
considerable time in large tanks.

Experience has shown that this method of prolonged storage in contact
with water greatly improves the colour. The only explanation of this
fact is that the chalk still contains a very small amount of organic
matter, which gradually decomposes in presence of water. The evidence
in favour of this is the peculiar smell given off during storage.

Even with the most careful grinding, chalk cannot be transformed into
such a fine powder that is directly fit for all purposes; and the only
way to obtain the requisite fineness is by levigation. Owing to the
large quantities that are usually handled in this process, the milky
liquid coming from the mill is mostly run into large brick tanks,
where it is left to settle until all the chalk has deposited and
the supernatant water is perfectly clear. Tapping-off being usually
impracticable, the water is generally drawn off by careful syphoning,
so as not to disturb the fine sludge at the bottom of the tank.

The deposit in the settling-tanks is shovelled into wooden boxes,
perforated at the sides to enable the water to drain away, the chalk
being prevented from escaping by lining the boxes with linen cloths.
The pulp soon loses its liquid character and shrinks considerably,
the boxes being then filled up with more sludge, and so on until the
contents have ceased to shrink. When the mass is so far dry that it
will no longer run when lifted, the boxes are covered with boards and
inverted, discharging the contents on to the boards, on which the mass
is left to become quite dry. Filter-presses are also used.

Large prismatic masses of chalk never dry so uniformly as to prevent
the formation of cracks, and if the chalk is to be sold in this form
the cracks are plastered up with thick pulp; this operation, however,
being superfluous when the chalk is to be sold as powder.

In order to obtain a more compact product and accelerate the drying of
the moulded lumps, some makers use presses, in which the fairly dry
chalk is subjected to progressive heavy pressure.

Owing to the fineness of the component particles of chalk, they adhere
so firmly together, without any bind, that a fair amount of force is
necessary to break down a piece of perfectly dry levigated chalk.
Sometimes, however, chalk exhibits the unpleasant property of losing
its cohesion almost completely when dry, and in such cases it can only
be shaped into prisms with great trouble. This peculiarity is specially
accentuated when the chalk contains magnesia; and in order to mould
chalk of this kind into blocks, a binding agent, such as ordinary
glue, must be added to the water used in grinding, care being taken
not to use too much, or the chalk will become too hard, when dry, for
certain purposes, _e. g._ as drawing or writing chalk.

For some purposes, chalk is sold in powder form, and very high purity
is not then essential, an admixture of magnesia or clay being harmless.
Gilders, for instance, use large quantities of chalk for priming
picture frames, and stir the chalk up with a certain amount of bind
(mostly size), to give the particles the desired cohesion.

The chief requirement exacted of a good quality chalk is a handsome
white colour; and this depends entirely on the quality of the raw
material, not on the method of preparation. It is known that a
substance quite devoid of colour will furnish a perfectly white powder,
because the colourless particles reflect the light in all directions
without breaking it up into its constituent yellow, red and blue rays.
Chalk, too, is in reality a colourless substance, and reflects light
with greater uniformity in proportion as the fineness of the particles
increases. Consequently, when one has a chalk that is not perfectly
white, it can, nevertheless, be made to furnish a very handsome product
by bestowing great care on grinding and levigation. Properly prepared
chalk should be as fine as the finest flour.

When the colour of the best grades of chalk are compared with what may
be termed pure white--such as that of white lead, zinc white, permanent
white--a skilled eye will always detect a greyish or yellowish tinge in
the former, even if obtained from the whitest Carrara marble.

The grey tinge is due to the presence of organic matter, which cannot
be eliminated by any known means, but which can be shown to exist by
the fact that when such chalk is heated to incandescence in the air
for a short time, the resulting burnt lime is pure white, the organic
matter having been burned off. A yellow tinge is caused by minute
traces of ferric oxide, which--as also ferrous oxide--almost invariably
accompanies calcium carbonate; and limestone free from determinable
quantities of these oxides is of rare occurrence. Ferrous oxide does
not reveal its presence in limestone unless in large proportion, its
pale green colour being of low tinctorial power, whereas ferric oxide,
which is a very strong colouring agent, can be more readily detected.

To those who are engaged in the manufacture of white earth colours,
however, it is quite immaterial whether a limestone or chalk contains
ferrous oxide, because that oxide quickly changes into ferric oxide in
the finely divided product, and a chalk which was originally pure white
will become decidedly yellow in a short time.

Fortunately, such a yellow-tinged product can be rendered perfectly
white by simple means and at small cost, all that is necessary being to
add a suitable quantity of a blue colouring matter. When this has been
done, the chalk will seem pure white to even the most skilled eye.

This result of adding a blue pigment is based on the well-known
physical fact that certain kinds of coloured light produce white
light when combined, the colours that give this effect being termed
“complementary.” A pure blue is complementary to a yellow with a
reddish cast--_e. g._ ferric oxide--and therefore a chalk that is
tinged yellow by a small quantity of ferric oxide can be changed into
a seemingly pure white substance by the addition of a blue pigment.

The only pigments of use in this connection to the colour-maker are
such as have very intensive colouring power and at the same time
are low enough in price. Such substances are ultramarine, smalt and
coal-tar dyes. Smalt is the best because its colour is unalterable.
In point of chemical composition, this substance is a very hard glass
coloured blue by cobaltous oxide. For improving the colour of chalk
or any other white, the smalt must be in an extreme state of fine
division, and levigated to an impalpable powder. Ultramarine can be
used for the same purpose, but is not so permanent.

To ascertain the correct proportion of blue pigment, it is advisable
to make a systematic experiment, which is easily performed. Exactly
90 parts of the chalk in question are triturated with 10 parts of
blue pigment in a mortar until the entire mass has become a perfectly
uniform pale blue powder, which contains 10% of the blue ingredient.

Several samples, each representing one hundred parts of the white
pigment to be corrected are carefully weighed out, 1 part of the blue
powder being added to the first sample, 2 parts to the second, 3 to the
third, and so on, and the mixtures are compared with a standard white
substance, such as best white lead or zinc white, to see which most
nearly approaches the standard colour. It is then easy to calculate
how much of the blue requires to be added to 100 or 1000 lb. of the
material to be corrected.

The correction can be effected in several ways; for instance, by
grinding the blue pigment directly with the bulk, by adding it at the
levigation stage, or mixing it with the dry, finished product. The
first two methods are attended with certain drawbacks which render
it difficult to obtain a perfectly uniform product, owing to the
specific gravity of the blue pigments being higher than that of the
whites. Consequently, when the two are mixed in presence of water--as
is always the case in grinding and levigation--the heavier blue
pigment settles down more quickly, and several strata can be clearly
distinguished in the sediment. The upper layers will still have a
decided yellow tinge--the proportion of blue being too small for proper
correction--whilst the next in order will be pure white--accurately
corrected--and those at the very bottom will be decidedly blue, because
they contain the largest proportion of the blue substance.

The most satisfactory results are obtained by dry mixing; and this can
be successfully practised when the colour-maker has a cheap source of
power (such as water power) available. Where, however, costly power
plant has to be provided, only the finest grades of white pigments can
be improved in this way, the expense of labour being too high for cheap
materials.

As a pigment, chalk possesses many valuable properties. The organic
structure of chalk gives it high covering power as a wash, a thin layer
applied to a surface sufficing to mask the colour of the underlying
ground completely. The lime in chalk being combined with carbonic acid,
its basic properties are so extensively weakened that chalk can be
mixed with even the most delicate colours without fear of their shade
being affected. A coating of pure chalk paint on any surface will never
change colour in the air; and on this account, chalk is extensively
used both as an indoor wash and by wall-paper manufacturers.


PRECIPITATED CHALK

Many chemical processes furnish soluble salts of lime that constitute a
by-product of little value. These salts, however, can be advantageously
utilised for the preparation of an artificial chalk which is preferable
to the native article in many respects. For instance, where large
quantities of calcium chloride solution are available, and soda can
be purchased at a sufficiently cheap rate, they can be converted into
artificial chalk, because these two substances react on each other,
forming, on the one hand, calcium carbonate, which is precipitated as a
very delicate, insoluble powder, and on the other, sodium chloride, or
common salt, which remains in solution, according to the equation:--

  CaCl{2} + Na{2}CO{3} = CaCO{3} + NaCl.

If, however, these solutions were mixed together in a crude state, the
resulting product would be of only low value as a pigment, being of a
yellow tinge and never pure white. This is due to the fact that the
impure lime salts, being waste products from chemical works, frequently
contain fairly large amounts of ferric oxide, and the soda also is
often so high in that impurity that the colour of the precipitated
chalk is considerably impaired.

Fortunately, there is no difficulty in eliminating this ferric oxide
by chemical means, and obtaining a product of superior colour to
the best native chalk. This is effected by treating the perfectly
neutral lime-salt solution with calcium carbonate, which causes the
precipitation of the iron, a corresponding amount of lime passing into
solution.

In order to eliminate the ferric oxide from the lime-salt solution so
completely that not even the most delicate chemical test known will be
able to reveal any trace remaining, the solution is placed in a vat
and stirred up with finely powdered chalk. If the solution contains
any free acid, effervescence, due to the liberation of carbon dioxide,
will take place; and in such event the addition of chalk is continued
until the free acid is all neutralised, and the added chalk sinks to
the bottom undissolved. The chalk should be in slight excess, so that a
decided sediment is visible at the bottom of the liquid when at rest.

This deposit is stirred up again at intervals with the liquid for
several days. When ferric oxide is present, the colour of the deposit
will gradually change to a yellowish brown, through the precipitation
of ferric hydroxide by the chalk; and in this way the final traces of
iron can be removed.

The liquid is then carefully drawn off, without disturbing the
sediment, and the soda solution is run in so long as a precipitate of
calcium carbonate continues to form. The completion of the reaction
can be ascertained by pouring a small quantity of the liquid into a
tall, narrow glass, leaving it to clarify, adding a little more soda
solution and observing whether any further precipitate is produced.
On the other hand, it may be that an excess of soda has already been
added in the precipitating tank; and this can be determined by testing
a sample with turmeric paper--blotting-paper soaked in a solution
of the colouring-matter of turmeric root--which is turned brown by
alkaline reagents. Even in very dilute solution, soda will give this
colour change, and the test is therefore very accurate. The complete
precipitation of the lime in the solution can be ascertained by passing
a small quantity through blotting-paper and treating it with a little
acid potassium oxalate solution, which, if lime be present, will at
once produce a strong crystalline precipitate of calcium oxalate, which
is only very sparingly soluble in water. If the oxalate gives merely
a slight turbidity, the residual amount of lime is so small that the
process may be regarded as complete.

Since carbonate of soda is usually much dearer than the lime-salt
liquor, it is preferable to leave a small quantity of the lime
unprecipitated. Given sufficient care in effecting the precipitation,
and especially when fairly strong solutions are used, a brilliant white
precipitate of calcium carbonate is obtained, which is in such a finely
divided state that the minute constituent crystals can only be detected
under a high magnifying power.

This precipitated chalk being already in an extremely fine condition
needs no further preparation, and, when washed, is ready for immediate
use, forming a handsome pigment with excellent covering power.

When precipitation is ended, the deposit is allowed to settle down,
and the clear supernatant liquid is carefully drawn off so as not to
disturb the delicate sediment, which is then stirred up thoroughly with
clean water, left to subside, washed again, and then spread out to dry
on cloths which are suspended by the four sides. The surplus water
drains away and the residue gradually assumes the consistency of paste,
in which condition it can easily be moulded to any desired shape. If
left long enough to dry completely, it forms a very delicate powder,
furnishing a pigment of excellent quality.

If this precipitated chalk be moulded into prisms for sale, the blocks
are laid on one of their broad sides until firm enough to turn over
on to one of the narrow faces, slabs of gypsum being used as the
supporting material, in order to ensure uniform drying. The gypsum
absorbs water with avidity and thus dries the prisms evenly.

A defect of these prisms is their great fragility; but their strength
may be improved by mixing a little very weak solution of dextrin to
the mass after the last washing-water has been completely removed. In
drying, the dextrin binds the material of the prisms sufficiently to
keep them from breaking except under the influence of a fair degree of
force.


CALCAREOUS MARL

As already mentioned, calcium carbonate rarely occurs in a perfectly
pure condition in Nature; and chalk, also, is frequently contaminated
by other minerals. A variety of limestone occurring as extensive
deposits in many places is that in which calcium carbonate is
associated with clay. Sometimes the clay predominates, and the mineral
is then known as marl, being really a clay contaminated with chalk. If,
on the other hand, the chalk forms the chief constituent, the mineral
is termed calcareous marl.

Calcareous marls are used in much the same way as limestone, some
modification, however, being necessitated by the presence of the clay.
Although limestone containing a certain amount of clay can be burned
in the kiln, it yields an inferior lime that is of little use to the
builder owing to its low binding power. Marl of a certain composition
finds an important application in the manufacture of hydraulic lime or
cement.

The only kind of marl suitable for pigment is that containing clay
with very little colour; and this is of somewhat rare occurrence,
because most marls contain sufficient ferric oxide to give them a
yellow shade. Marl that is fairly free from ferric oxide, however, can
very well be used as pigment; and many white pigments sold as “chalk”
are really finely ground marl. In accordance with the general practice,
in the colour industry, of giving colours a great variety of names, and
suppressing the real names, which, so far as the artificially prepared
colours are concerned, should bear some reference to their chemical
composition, numerous white earth colours bear fancy names, though
really consisting of chalk, lime (generally marl), or white clay.

In France, where both chalk and clay are of frequent occurrence--the
soil of Champagne, for instance, being all chalky--the manufacture of
the white earth colours is extensively practised, and a large number
are put on the market, usually named after the place of origin, and
consisting of either calcium carbonate or marl.

The trade names of the white earth colours include Cologne chalk,
Bologna chalk, Briançon chalk, Champagne chalk, Blanc de Bougival,
Blanc de Meudon, Spanish white, Blanc d’Orleans, Blanc de Troyes, etc.
All are either more or less pure chalk, marl, or a fairly white clay,
pipeclay--which is also used for making clay pipes and for removing
grease spots.


GYPSUM

The mineral known as gypsum, or alabaster, consists of calcium
sulphate, or sulphate of lime, its composition being expressed by
CaSO{4} + 2H{2}O. In gypsum the crystalline structure is just
discernible, whilst other varieties, such as the so-called “marine
glass,” occur in considerable quantities as large, perfectly
transparent masses. “Russian glass” consists of large, transparent
lumps possessing the specific property of gypsum, viz. that of cleaving
in two directions, in a high degree. Alabaster is composed of finely
granular masses, which are either quite white, or else yellowish, or
traversed by grey veins. This variety of gypsum is very abundant in
central Italy, and the best blocks are employed for the production of
works of art.

Ordinary gypsum, which frequently occurs in the vicinity of dolomitic
limestones, is found in a great variety of colours, bluish-grey,
yellowish or reddish tints being the most common. Pure white lumps,
which are plentiful in some deposits, can be used as white pigment,
the method of preparation being simple, viz. merely reducing the mass
to powder. This is easily effected, the specific hardness of gypsum
being only 2; and in many cases it is soft enough to scratch with the
finger-nail.

If the original gypsum is white, the powder forms a dazzling white
flour which, notwithstanding, is of comparatively little value as
a pigment, on account of its low covering power. For this reason,
powdered gypsum is chiefly used for making plaster of Paris (calcined
gypsum) for plaster casts and stucco. Gypsum may also be employed to
advantage for lightening various colours, since it is inert towards
even the most delicate.


KAOLIN, PIPECLAY

Large areas of the earth’s surface are covered with clay, which often
attains a considerable thickness. Nevertheless, the kind of clay that
is suitable for use as pigment is comparatively scarce. The principal
requirement for this purpose is a pure white colour, but by far the
great majority of clays are either yellow or of a shade between blue
and grey (for example the clay of the Vienna basin).

The character of clay is just as varied as its colour. In some places,
large deposits of extremely fine clay are found, the material, when
mixed with water, forming a highly plastic mass which, when dried and
subjected to slight pressure, furnishes a very soft powder. On the
other hand, some clays are so interspersed with large quantities of
sand, large stones and the debris of mussels, that they cannot be used
until they have been put through very careful mechanical treatment.

This great divergence in the physical character of clays is due to
their method of formation. Clay originated in the weathering of
felspar, which chiefly consists of a double salt, a compound of the
silicates of alumina and potash. Under the influence of air and water,
this compound is decomposed, the potassium silicate passing into
solution, whilst the aluminium silicate, being insoluble in water, is
carried away by that medium. When the water can no longer carry the
particles of aluminium silicate in suspension--for example when it
reaches a sea or lake--the silicate settles down to the bottom, and a
deposit of clay is formed.

If the original felspar was very pure, and in particular very low
in iron, the resulting clay will be of a handsome white colour. An
example of this is afforded by kaolin, or porcelain earth, which is
preferably used for making china. If, however, the felspar contained a
considerable proportion of ferric oxide, the resulting clay is yellow;
and if stones or mussel shells became incorporated with the clay prior
to deposition, these bodies will be found as inclusions in the deposit,
and such clay will require much troublesome preparation--grinding and
levigation--before it is fit for use.

For the purposes of the colour-maker, the most suitable clay is one
that is pure white, free from inclusions, and does not change colour
when exposed, in a finely divided state, to the action of the air.
Many clays that were originally white gradually assume a yellow tinge
on prolonged exposure to air and moisture, because the clay contained
ferrous oxide, which changes, in the air, to the stronger pigment,
ferric oxide.

Many kinds of clay merely require a simple levigation to fit them for
use as pigment. The lumps of freshly dug clay are placed in large
tanks, etc., filled with water and stirred up continuously in order
that, instead of forming a plastic mass which is very difficult to
distribute in water, the particles detached from the lumps may pass at
once into suspension. This turbid water is then transferred to another
tank, etc., where the minute particles of clay are allowed to settle
down, and the water becomes quite clear.

Where this work is carried on on a large scale, it is advisable to put
the freshly won clay into large pits close to the clay deposit, and
to leave it there, covered with water, during the winter season. The
freezing of the water breaks down the larger lumps of clay, by the
resulting expansion, and this facilitates the subsequent levigation,
the cohesion between the particles being destroyed.

If the clay contains larger proportions of lime or magnesia, a little
experience will enable their presence to be detected at once by the
way the clay behaves on being placed in contact with water. Pure clay
quickly forms a fatty and extremely plastic paste, and sticks closely
to the tongue when applied in the dry state. On the other hand, clay
containing much lime or magnesia is far less plastic when mixed with
water, and the dry clay hardly adheres to the tongue at all.

These latter clays are classed as poor or lean, in contrast to the fat,
plastic kinds. For certain purposes for which clay is used as pigment,
these admixtures are not harmful; whereas others, especially quartz
sand and mica, not infrequently present in white clays, constitute a
serious drawback.

As already mentioned, clay is formed by the weathering of felspar,
which is a constituent of granite and gneiss, both rocks composed
of quartz, mica and felspar. When the clay has been derived from
the weathering of such rocks, it is easy to understand that it may
contain admixtures of quartz and mica, which are frequently visible
to the naked eye, or at any rate under the microscope. Whereas clay
forms a white, amorphous mass, the grains of quartz sand are decidedly
crystalline, transparent and of vitreous lustre; the scales of mica,
on the other hand, appearing as thin tabular crystals, mostly of a
green or brown colour and exhibiting, when viewed at certain angles, a
brilliant metallic sheen.

Quartz sand can be eliminated from clay without any special difficulty,
quartz being of higher specific gravity and therefore settling down
quickly, leaving the delicate particles of clay in suspension in the
liquid. The scales of mica are harder to get rid of, their tabular form
retarding deposition from the suspending liquid; and on this account,
several washings are often required to separate them completely.

In all cases where clay is to be used as a white distemper, the
presence or absence of lime is immaterial; but where it is to be
employed for removing grease, lime is a drawback. This is also
sometimes the case when the clay is wanted for the purposes of the
colour manufacturer. The author has found, by experience, that
perfectly pure, white clay forms a good paint, in a vehicle of oil or
varnish--a purpose to which it has, so far, been seldom applied, if at
all. Such paint is of good covering power, and possesses the valuable
property of remaining quite unaffected by atmospheric influences.

If, however, the clay contains even but a small quantity of lime, it
cannot possibly be used as an oil or varnish paint, for though the
freshly made paint has a very good appearance, its character soon
changes, turning viscous and suffering a considerable diminution of
covering power. Thinning with turps or boiled oil results in the
formation of small lumps, so that it is quite impossible to obtain a
uniform coating on even a small surface.

This behaviour is apparently due to the presence of the lime, the
explanation being that the fatty acids always present in the oils and
varnishes used for the paint combine with the lime to form compounds
which, from the standpoint of the chemist, must be regarded as soaps.
The small lumps already mentioned really consist of lime soap, and
the formation of these colourless compounds accounts for the lessened
covering power.

Given a fine white clay, otherwise capable of forming a valuable
pigment, it is sometimes possible, by simple means, to eliminate
accompanying lime, provided the amount of the latter is not too great,
and also provided that very cheap hydrochloric or acetic acid is
available. The acid need not be pure, and the impure but very strong
pyroligneous acid, which is very cheap on account of its empyreumatic
smell, may be used.

To eliminate lime from the clay, the still moist levigated mass is
introduced, in small quantities, into a vat containing the requisite
quantity (see later) of hydrochloric or acetic acid, the addition being
continued until the liquid gives only a faintly acid reaction with
blue litmus paper. When the clay is run in, effervescence is produced
by the liberation of the carbon dioxide displaced by the stronger acid
employed.

The amount of lime present in a clay may be determined by very simple
means. A small sample of the clay is dried by artificial heat, until
of constant weight, and exactly 100 parts by weight of the dry mass
are placed in a glass and suffused with hydrochloric acid, sufficient
of the latter being used to make the liquid still strongly acid after
effervescence has ceased.

The contents of the glass are transferred to a filter, and washed
with pure water so long as the washings continue to redden blue
litmus paper. The residue is then dried until of constant weight, and
the difference between the initial and final weights will give the
percentage of substances soluble in hydrochloric acid.

After performing this simple test on a clay, it is easy to calculate
the quantity of acid needed to extract all the soluble constituents
from a given weight of the material. All that is necessary is to
measure the volume of acid required to extract a small quantity of
the clay completely. Thus, if one pint of the acid at disposal is
sufficient to treat one pound of the clay, the amount needed for a
given quantity of clay is a simple matter of calculation.

Since, on account of the cost of pure hydrochloric acid, crude acid
will always be used, it will be necessary to remember that this crude
acid always contains ferric oxide in solution--this being the cause of
its yellow colour. If the amount of acid taken is barely sufficient to
combine the whole of the lime, leaving the latter slightly in excess,
the ferric oxide--which would otherwise tinge the clay yellow--will be
precipitated.

If, on the other hand, the acid is in excess, the clay is obtained free
from all constituents soluble in the acid. The purified clay must then
be freed from the calcium chloride, formed by dissolving the lime, by
a thorough washing, since the clay would otherwise always remain moist
on account of the hygroscopic properties of the chloride in question.
Moreover, any small residuum of free acid would constitute a drawback
on the clay being mixed with other colours.

Calcium chloride is very soluble in water, and therefore can be
completely removed from the clay by washing. The purified clay is left
to settle down as completely as possible, and after drawing the liquid
off from the sediment, the latter is suffused with pure water and left
to settle once more. As a rule, two such washings will cleanse the clay
of calcium chloride and free acid sufficiently to render the product
suitable for any purpose.

When large quantities of clay have to be treated in this manner,
considerable amounts of calcium chloride solution will be obtained,
which can be advantageously utilised for the production of
precipitated chalk, all that is necessary being to collect the liquor
in a large tank and treat it with a small quantity of slaked lime, to
transform the surplus free acid into calcium chloride and precipitate
the ferric oxide present in solution. At the end of a few days the
liquor in the tank will consist of a very pure solution of calcium
chloride which will furnish an excellent precipitated chalk when
treated in the manner already described under that heading.


BARYTES, OR HEAVY SPAR

This mineral--chemically, barium sulphate, BaSO{4}--occurs native, as
extensive deposits, in many places--England, Bohemia, Saxony, Styria,
etc. It sometimes forms handsome tabular crystals, but more frequently
compact masses, which may be pure white, grey yellow, etc., in colour,
and are distinguished by high specific gravity (usually 4·3–4·7),
to which the mineral owes its name. This high density also limits
the application of the mineral, and it cannot be used as a pigment,
in the true sense of the term, being only suitable as an adjunct to
artificially prepared colours.

The employment of barytes in the colour industry is often regarded as
adulteration, which, however, it is not when the case is considered
from the right point of view. For instance, the only preparation which
can properly be termed white lead consists of basic lead carbonate.
This, when pure, is a rather expensive pigment, whereas, for certain
purposes, the consumer requires a product that can be obtained at a low
price. In order to satisfy this demand, the only course open to the
colour-maker is to mix the white lead with a cheap white substance,
which enables him to turn out different grades of white lead, which,
although low in price, are far inferior to the pure article in covering
power. Pure white lead being itself a very heavy substance, the only
bodies suitable as adjuncts are such as are also of high specific
gravity; and of all the cheap pigments known, heavy spar is the only
one endowed with this property. Consequently, this substance is
extensively used in making the cheaper grades of white lead and the
pale kinds of chrome yellow.

The only cases in which the addition of heavy spar to a colour can be
regarded as an intentional fraud on the consumer is when he is sold,
as pure white lead, chrome yellow, etc., a product really composed
of a mixture of such colour and barytes. Moreover, the presence of
barytes in white lead can be easily detected by a simple examination,
pure white lead readily dissolving, with considerable effervescence,
in strong nitric or acetic acid, whereas barytes is insoluble in all
acids, and therefore remains, as a heavy white powder, at the bottom
of the vessel. In this way both the presence and amount of barytes
contained in a sample of white lead or chrome yellow can easily be
ascertained.

The preparation of barytes for the purposes of the colour-maker is
entirely a mechanical operation. The barytes, which though fairly hard
is easily reduced, is crushed with stamps, ground in a mill and finally
levigated, it being impossible to obtain a sufficiently fine powder
even by repeated grinding.

Native barytes must not be confounded with the artificial barium
sulphate sold as permanent white or blanc fixe, which is an extremely
finely divided barium sulphate obtained by precipitating a solution
of a barium salt with sulphuric acid or a soluble sulphate, and is
a painters’ colour that is highly prized for certain purposes. Both
the native sulphate and the artificial variety have the property
of remaining completely unaltered by exposure to air, and they can
therefore be mixed with any kind of pigment without fear of the colour
deteriorating.

As a rule, barytes is first roughly crushed in edge-runner mills or
stamps, and then ground to the extreme degree of fineness obtainable in
ordinary mills. Even with the greatest care, however, it is impossible
by this means to obtain sufficient fineness of division for mixing with
fine colours, the only way in which this can be accomplished being by
levigation.

Given a fairly pure white barytes to begin with, levigation furnishes a
handsome white pigment that can be mixed with colours of any kind; but
when used by itself in association with oil or varnish, its covering
power is very low and the colour never perfectly white. Native barytes
is therefore unsuitable, as such, for paint.

Varieties that are not pure white are sometimes corrected with
ultramarine, added in the grinding-mill. If the yellow tinge is due
to iron compounds, this can often be remedied by treating the finely
ground material with hydrochloric acid, which dissolves them out, this
treatment being followed by a thorough washing with pure water.

As already mentioned, white lead is most frequently mixed with barytes,
this being usually added when the white lead is being ground, by
feeding the two materials to the mill and grinding them together.

The crudeness of mechanical methods of reduction is clearly
exemplified by comparing the most carefully ground and levigated
barytes with that obtained by artificial means. The permanent white
largely used in the production of wall-paper, and quite unalterable
in air, is, chemically speaking, identical with native barytes, viz.
barium sulphate. The two also seem to be identical in crystalline
habit, as is usual in the case of one and the same mineral, whether
native or prepared by artificial means. Artificial barytes is obtained
by treating a soluble salt of barium with sulphuric acid, or a solution
of sodium sulphate (Glauber salt), so long as a precipitate continues
to form.

This precipitate is barium sulphate, which subsides completely on
account of its extreme insolubility, this being greater than that of
any other salt known. The rapid rate of deposition results in the
formation of extremely small crystals, which, being colourless and
reflecting the light completely, appear to be perfectly white. Even
when permanent white is applied in very thin layers to any surface, its
covering power is very considerable, by reason of the extremely fine
subdivision of the material.

This behaviour of artificial barytes in comparison with that of the
natural product, affords an important hint in connection with the
preparation of earth colours, namely, that in order to obtain products
of specially good quality, the endeavour should be to reduce the raw
materials to the finest condition possible. This result is accomplished
most securely by bestowing the greatest care on grinding and
levigation; and it is therefore highly important that the manufacturer
should select, from the various apparatus used in reducing the
materials, those that are best adapted for the purpose.


CARBONATE OF MAGNESIA

Although carbonate of magnesia is seldom used alone as a pigment, it
can be advantageously employed as such when circumstances permit.
It is met with not infrequently, in Nature, in a crystalline form,
as magnesite or bitter spar, the latter name arising from the fact
that the soluble salts of magnesia have a bitter taste. Still more
frequently, magnesia occurs in association with calcium carbonate, in
the mineral dolomite, which contains up to 20% of magnesia.

A less abundant native mineral is hydromagnesite, which consists
of basic magnesium hydrocarbonate. Hydromagnesite is a very light,
chalk-white mass, with a non-greasy feel, which, when reduced to a soft
powder, forms an excellent material for paint. It is highly inert, in
a chemical sense, and can therefore be mixed with the most delicate
colours, having no other effect thereon than to render them lighter in
shade.

This product can also be prepared artificially, by treating a dissolved
magnesium salt with a solution of carbonate of soda, the result being
the formation of a pure white precipitate, which is very brilliant when
dry, and is characterised by unusually low specific gravity. In some
places, conditions are such that this preparation can be made on a
large scale at very low cost. For instance, there is a spring at Bilin,
in Bohemia, the water of which contains large quantities of alkali
carbonates in solution; whilst in the vicinity of Saidschütz is a
spring fairly rich in magnesia salts. The waters from these two springs
are concentrated by evaporation, and mixed in large tanks; and when a
sufficient deposit of the resulting basic carbonate of magnesia has
accumulated, it is taken out of the tanks, placed on linen filters and
washed with water. The residue is dried slowly, without the employment
of a high temperature, and then forms a white powder, which is very
light and can be used for a number of purposes, chiefly medicinal,
though it is also well adapted as a material for paint.

For this latter purpose it is, however, far too expensive; but since
the conditions obtaining at Bilin are certain to occur elsewhere, we
have included carbonate of magnesia among the earth colours.

On account of its specific lightness, carbonate of magnesia is
specially adapted for making pale shades of certain delicate lake
colours, which, if toned with even perfectly pure chalk, would undergo
alteration in course of time. Carmine, for instance, can be graded,
by the addition of carbonate of magnesia, into every possible variety
of shades between the pure red of carmine itself and the palest pink;
and the resulting colours are quite permanent whether mixed with gum
solution or any other vehicle.


TALC

Although this mineral is not used as a pigment by itself, it must be
mentioned here because it is not infrequently employed for mixing with
other colours, and is also used in the wall-paper industry. It also
serves to distribute certain pigments in a state of fine division, the
“rouge végétal” of the perfumer, for example, usually consisting of
talc and a small quantity of very fine carmine.

In commerce the name talc is sometimes applied to two separate
minerals, true talc and steatite or soapstone. The former is rarely
met with native as well-defined crystals, mostly occurring as scaly
masses in primitive rocks. Thin pieces exhibit a certain degree of
flexibility. The hardness of this mineral is so small that it can
be scratched with the finger-nail; and its sp. gr. is 2·9–2·8. Talc
is easily scraped, and the powder remains sticking to the knife, a
property which renders the substance difficult to reduce to powder,
because it balls together and takes a very long time to convert into
a fine flour. The process is facilitated by calcining the talc and
quenching it in cold water, this treatment increasing the hardness
and at the same time making it more brittle, and thus more easy to
pulverise.

A characteristic feature of all the talc minerals is their peculiar
greasy appearance and feel. The colour varies, white pieces alone being
of any use to the colour manufacturer. The yellow- or green-tinged
varieties owe their shade to the presence of ferric and ferrous
oxides. In chemical composition, talc consists of a combination of
magnesium silicate with hydrated silica, the supposed formula being:
4MgO . SiO{2} + H{2}O . SiO{2}, and the percentage composition: silica,
62·6%; magnesia, 32·9%; water, 4·9%.


STEATITE OR SOAPSTONE

Steatite so closely resembles talc in most of its properties, that
the two minerals were long regarded as identical. Whereas, however,
talc is scarcely acted upon at all by the strongest acids, steatite is
completely decomposed by prolonged boiling therewith, although both
minerals have exactly the same composition.

As a pigment, steatite is far more important than talc, and, as French
chalk, is largely used for drawing or writing. To prepare it for this
purpose pure white steatite requires no preliminary treatment, beyond
cutting the large lumps up into quadrangular prisms, which are mounted
in wood, like lead pencil, and used for writing on the blackboard. The
powder produced in the cutting process is made up into pastel crayons.
With this object, the powder is mixed with a sufficient quantity of
some mineral pigment to produce a mass of the desired shade, and is
kneaded to a stiff paste with water containing an adhesive such as gum,
glue or tragacanth mucilage. The mass is shaped into prisms, which,
when dry, are cut into pencils and mounted in wood. Steatite being like
talc, without action on even the most delicate colours, can be used as
a diluent in the preparation of light shades.




CHAPTER V

YELLOW EARTH COLOURS


All the yellow earth colours, without exception, have ferric oxide as
their colouring principle, the differences in shade being entirely
due to the varying proportion in which that oxide is present. The
various names under which they are known date back to a period when
the chemical nature of these colours was still unknown, and have been
mostly derived from the locality of origin.

The yellow earths can therefore be divided into two groups, according
to their chemical character. The first group, in which the ferric oxide
is present as hydroxide, comprises all the ochres, Siena earth, and
a number of others which are obtained from native ochre by special
treatment. In the colours of the second group, ferric oxide is still
the colouring principle, but is combined with other substances in place
of water.

It is, as a matter of fact, incorrect to rank the ochres in general as
yellow earths, because they can be made to yield nearly every variety
of colour from the palest yellow to the deepest red, brown and violet.
These colours merit the particular attention of the colour-maker and
the painter, being distinguished by very low cost of production,
unusual permanence and beauty of tone. In the interests of that highly
important matter to the artist, namely the production of colours of
unlimited permanence, it is desirable that colour manufacturers should
bestow greater care on the manufacture of these colours than has
hitherto been the case. An extremely favourable point about nearly all
these pigments is that they can be very cheaply prepared by artificial
means, so that the manufacturer is in a position to turn out a large
number of the handsomest and most durable colours with a small amount
of expense and labour.


THE OCHRES

Ochres are found in many localities, most frequently in stratified
rock and rubble. The deposits are rarely extensive, mostly occurring
in pockets or beds. Wherever found, ochre may be termed a secondary
product, that is to say, one that has been formed through the
destruction of other minerals. The analysis of ochres from different
deposits shows great divergence in composition; and some consist almost
entirely of pure ferric hydroxide, that has already undergone natural
levigation and can be used as a pigment as soon as dug.

Such a form is, however, rare, and most ochres are intermixed with
smaller or larger amounts of extraneous minerals, the contamination
being sometimes so great as to preclude the use of the ochre as pigment
by reason of the high outlay required for extracting the colouring
constituents.

Occasionally, the ferric hydroxide is associated with a certain
proportion of clay, and as this increases, the ochre passes over into
ferruginous clay. This class can also be used as pigment, in certain
circumstances, that is to say when it is sufficiently rich in ferric
oxide to furnish a deep red mass on calcination. When, however, the
proportion of ferric oxide is low, its pigmentary power is no longer
sufficient, and the clay has not the requisite beauty of colour. The
ordinary earth used for making tiles is an example of this class, its
colour in the raw state being an ugly brownish-yellow, but turning a
dull “brick” red when fired.

In some deposits the ferric oxide is accompanied by lime. Unless the
latter exceeds a certain proportion, such ochres, too, are suitable as
pigments, the lime being easily removed by simple levigation; but when
the amount of lime is high, it is difficult to obtain certain highly
coloured shades of ochre from such material. These shades entail the
calcination of the ochre, and the temperature required is oftentimes
insufficient to transform the lime into the caustic state. Moreover,
the presence of caustic lime would be a drawback in some cases, it
being then impossible to mix the ochre with other colours without
endangering the shade through the action of the lime on these latter.

The following analyses will show the percentage composition of ochres
from various deposits:

                      Ochre from--
  +-------------+---------+----------+--------------+
  |             | Comoal  | Vierzen. | St. Georges. |
  |             |(Savoy). |          |              |
  +-------------+---------+----------+--------------+
  |Ferric oxide |   19    |   23·5   |      25      |
  |Lime         |   2     |    --    |      --      |
  |Alumina      |   20    |   69·5   |      70      |
  |Magnesia     |    1    |   69·5   |      70      |
  |Silica       |   44    |   69·5   |      70      |
  |Water        |    7    |    7     |       5      |
  +-------------+---------+----------+--------------+

In the majority of cases the mineralogical characteristics of an ochre
enable conclusions to be formed as to its suitability as pigment. Good
ochre is more or less yellow to dark brown in colour, and can easily
be crushed between the fingers to a soft, fine powder which feels like
powdered steatite and does not produce a sensation of grittiness, this
latter indicating the presence of fine grains of sand in the ferric
oxide. The behaviour of the ochre in presence of water is specially
important. If it adheres firmly to the tongue, and forms a fairly
plastic paste when mixed with a little water, the mineral contains a
large percentage of ferric oxide, and as a rule will yield ochre of
good colour.

In general it may be said that the value of an ochre varies directly
with its content of ferric hydroxide or oxide, because when this is
large the ochre will furnish a wide range of colours under suitable
treatment.

A simple test for quality consists in weighing out an exact small
quantity (10 grms.), and heating it to a temperature not exceeding 110°
C., until the weight remains constant. A simple calculation then gives
the amount of uncombined water in the sample. Since the proportion of
such water varies in different parts of one and the same deposit, the
test must be repeated, in order to obtain accurate results, on samples
taken from different points, or, preferably, on a properly prepared
average sample.

Even drying changes the colour of ochre considerably. To ascertain the
behaviour of an ochre on calcination, a large sample is dried at 110°
C. until the weight is constant, and divided up into a number of small
samples weighing, say, 10 grms. each. The samples are then heated to
different temperatures, one to the melting-point of lead, another to
that of zinc, and so on.

The higher the temperature employed, the more will the colour of the
ochre approximate to red; and specimens very rich in ferric oxide will
give bright red colours. Beyond this range, a further increase in
temperature will give violet shades, varying with the temperature and
the duration of heating. After this preliminary test, it is desirable
to make another on a larger scale, with quantities up to about 1 lb.
For this test, the different kinds of ochre frequently found in the
same deposit should be mixed together, in order to obtain an idea of
what the mean product, obtained in working on the large scale, will be
like.

On the whole, the results of this second test will be the same as in
the first series, the only object of the second test being to gain
information which may be particularly valuable in practical work. The
bottles in which the calcined samples are stored should be marked with
the temperature and length of heating, so that, when it is subsequently
desired to obtain an ochre corresponding to a particular sample, all
that is necessary will be to heat it to the same degree from the same
length of time. The performance of this simple test will be of great
assistance in standardising the work with a minimum loss of time.

When it is desired to ascertain the composition of an ochre
superficially its behaviour towards hydrochloric acid may be noted. A
weighed quantity of the freshly dug (undried) ochre is treated with
pure acid, free from iron, which will dissolve out the ferric oxide
and lime, leaving clay and quartz sand behind. The presence of lime is
indicated by effervescence on contact with the acid; and if there is no
effervescence, lime is absent. At the end of several hours the acid is
carefully decanted from the undissolved residue which is then stirred
up with water, left to subside, and weighed when dry. This method will
give the amount of substances, other than ferric oxide and lime, in
the sample. These substances usually consist of clay or sand.

For a quantitative determination, a small quantity--usually 1 grm.--is
weighed out, treated with a corresponding amount of hydrochloric acid,
and the solution filtered into a glass. The residue on the filter is
washed with distilled water, the washings being united to the acid
solution.

This solution is treated with ammonia so long as a precipitate of
ferric hydroxide continues to form, this being collected on a tared
filter and dried at 110° C. The precipitate may be regarded as pure
ferric hydroxide, and its weight will indicate the proportion of
hydroxide in the ochre with sufficient accuracy for technical purposes.

In reality, however, it is not pure ferric hydroxide, but contains in
addition all the oxides that are precipitable by ammonia, lime being
always carried down as well. It is therefore desirable to dissolve the
precipitate with a little hydrochloric acid, and reprecipitate with
ammonia.


CALCINING (BURNING) OCHRE

In many places ochre is only put through a very simple mechanical
preparation before being sold for pigment, namely left to dry in the
air so that most of the uncombined water evaporates. No matter how this
drying process is protracted, however, it is impossible to get rid
of all the water in this way, a certain proportion being retained by
the hygroscopic action of the ferric hydroxide, and to expel this the
mass must be heated to above 100° C. Drying is usually succeeded by
pulverising and sifting the loose earthy mass, which is then ready for
sale.

When the ochre contains sand or stones, this treatment is not
sufficient, and levigation is necessary. No particular trouble is
involved, the mineral being fairly heavy as the result of its content
of ferric hydroxide. A simple method of treatment suffices to improve
the value of the ochre considerably, and enables a grade that is not
particularly bright-coloured in its natural condition to be converted
into products of very handsome tone and various shades. This treatment
consists in heating the raw ochre to a definite temperature, during
which process the colour changes progressively, and any desired tone
can be obtained by suddenly cooling the hot mass.

The reason for this phenomenon is that the higher the temperature, the
larger the amount of water driven off from the ferric hydroxide, until
finally, when a very high temperature has been reached, the whole of
the water is expelled, and the ferric hydroxide is transformed into
ferric oxide. The hydroxide is brown, whereas the oxide, provided the
temperature has not been raised too high, exhibits the characteristic
colour known as “iron red.”

Consequently, the colour of moderately calcined ochre ranges through
a whole scale from brown to red; and the higher the temperature
employed, the redder the tone. If the heating be protracted after all
the hydroxide has become oxide, the latter undergoes molecular change,
increasing considerably in density and altering in colour; and after
very prolonged heating, the colour finally becomes violet.

The calcination, or burning, of ochre is ordinarily performed in a very
crude manner. The mineral is crushed to the size of peas, and spread
out on an iron plate which is made red-hot. As soon as the ochre has
reached the desired shade of colour, it is dropped into a tub of water
and then crushed to powder. The calcination requires great experience
on the part of the operator, because so long as the product is hot, it
has quite a different colour from that assumed on complete cooling.
Since only comparatively small quantities of ochre can be treated in
this way, and the operation unnecessarily increases the cost of the
product, owing to the large consumption of fuel, it is highly desirable
to employ a simple calcining apparatus capable of treating large
quantities.

Such an apparatus may consist of an iron drum, mounted with a gentle
slope inside a furnace, from which it projects at both ends. A shaft
carrying a sheet metal worm is rotated inside the drum; and the whole
apparatus is very similar to an Archimedean screw.

When the iron drum is raised to a strong red heat, and small quantities
of ochre are fed continuously into the upper end of the drum, the
rotation of the worm will push the material forward, and contact with
the glowing sides of the drum will produce the necessary calcination,
the degree of which can be modified by altering the speed at which the
worm is turned. The calcined product is discharged at the lower end of
the drum, either into a vessel of water, or, if only moderate heating
has been applied, direct into a collector.

[Illustration: FIG. 28.]

Fig. 28 represents an apparatus designed by Halliday for the dry
distillation of wood waste; but, with slight structural modifications,
it can also be used for calcining ochre. The material to be heated is
introduced, in small pieces, into the feed hopper _B_, and is carried
downward, by the worm _C_, into the red-hot drum _A_, through which
it is propelled by the worm _D_ until it drops out, at _F_, into the
tank _G_. The length of time the material is subjected to calcination
depends on the speed at which the worm _D_ is run. The pipe _E_ carries
off the water vapour expelled from the charge.

In order to obtain a uniform product when ochre is calcined in an
apparatus constructed on this principle, it is necessary that the
material introduced should be fairly regular in size, a condition which
is easily fulfilled by squeezing the freshly dug ochre between fluted
rollers, and then passing it over a series of screens, each grade being
then calcined separately.

Moreover, the apparatus is only suitable for calcining at medium
temperatures; and when highly calcined products are in question,
the operation is best performed in fire-clay cylinders, or in thick
cast-iron drums, similar to gas retorts, built into a furnace.

Other devices for calcining ochre will be described later.


OCHRES FROM VARIOUS DEPOSITS

As previously stated, ochres are frequently met with in Nature, both
in the immediate vicinity of iron ore, and also at considerable
distances from such deposits. In the latter case, the ochre must be
assumed to be the decomposition products of ferruginous minerals and
to have been carried off by water until the latter became stagnant and
allowed the ochre to settle down. In their method of deposition these
ochres are therefore analogous to clay, and they, too, often contain
large quantities of extraneous minerals, which have given rise to the
diversified substances grouped under the name of ochre.

Although ochres are so widespread in Nature, only certain kinds, found
in certain localities, have acquired a high reputation. For the most
part, these ochres are such as have already been prepared in a high
degree, by Nature, for the purpose for which they are employed.

Thus, we find that all the ochres which have acquired a high repute
among painters for particular beauty of tone and permanence, are
distinguished by two properties: a high content of ferric hydroxide and
great purity.

The former of these properties imparts brightness of colour; and such
products will furnish, on calcination, a wide range of colour shades.
When, as is the case with the finer qualities of ochre, the mineral
contains only a very small proportion of impurities, there is no
difficulty in bringing it, by simple grinding or levigation, into a
condition in which it is at once fit for use as a pigment.

The Italian ochres have, for long ages, enjoyed a high reputation
for their beauty of colour and permanence. This category includes,
for example, the renowned Siena earth, Roman earth, Italian umber,
and other ochre colours. This high renown is probably due less to
the inherent properties of the mineral than to the circumstance that
the art of painting attained a high state of development at an early
period, and that the artists paid special attention to the use of
bright and permanent colours for their work.

Although, at present, many deposits of ochre are known that are
quite able to compete, on the score of beauty, with the best Italian
products, the good name of these latter has nevertheless been
maintained. It is true that the name of Italian ochre is often merely
borrowed, for application to a product originating in some other
country, varieties of terra di Siena, for instance, being put on the
market that have actually been derived from deposits in Germany.

As a result of this custom, certain names, such as terra di Siena,
umbra di Roma, have become generic terms, and their use denotes, not
an intention to suggest that the earth colours in question really come
from Siena or the vicinity of Rome, but that the properties of the
article are equal to those of the old-established colours of Siena or
Rome.

It would occupy too much space to go into an exhaustive description of
all the native varieties of ochre, and would inevitably lead to a good
deal of repetition. It will therefore be sufficient, for our purpose,
to deal with only a few of them.

The best-known ochres are those of Rome and Siena, the latter being
frequently called, in commerce, by its Italian name, terra di Siena.

Roman ochre forms yellowish-brown masses, of fairly fine texture and
composed of ferric hydroxide and clay. They are put on the market both
in the raw and calcined state. On calcination, the colour soon changes
to red, and if carefully performed, the resulting colours have a very
warm, fiery tone.

Closely approaching Roman earth is the English ochre, which is worked
more particularly in Surrey, and is not infrequently sold as Roman. In
many deposits this English ochre occurs in such a high state of purity
that the best pieces are picked out and sold without being even crushed
or ground. The pieces of lower quality are very carefully ground and
levigated, for the purpose of being calcined for the production of
different shades, and then furnish highly prized colours.

In point of chemical composition, the ochre family also includes terra
di Siena, bole, umber and Cassel brown. These minerals, however, are
not yellow like ochre, but brown, and will therefore be dealt with
along with the brown earth colours.


ARTIFICIAL OCHRES

Products very similar, both in chemical composition and colour,
to the native ochres can also be very simply and cheaply made by
artificial means. Their preparation may be particularly recommended to
colour-makers who desire to turn out a wider range of iron pigments,
but are not in a position to obtain natural ochres at a low price.

In the manufacture of artificial ochre, an endeavour is made to imitate
the natural processes which have led to the formation of ochre,
and, of course, to avoid anything likely to hinder the production
of a suitable colour earth, for example the presence of sand or a
considerable admixture of extraneous minerals.

As already mentioned, the chief impurities in natural ochres are clay
and sand, both of which can be easily excluded during the manufacture
of artificial ochre, or their amount controlled in such a manner that
paler or darker products can be obtained at will, and the tone varied,
in any desired manner, by calcination, as in the case of the native
article.

The raw material for artificial ochre is always a ferrous salt,
which can be purchased in large quantities and at very low prices,
namely green vitriol, which, in the pure state, consists of ferrous
sulphate, FeSO{4} + 7H{2}O. This substance forms sea-green crystals,
which are readily soluble in water and impart an objectionable inky
flavour thereto. On exposure to the air, green vitriol turns an ugly
brown colour, and is no longer completely soluble in water, passing
gradually into the condition of basic ferrous sulphate. This is because
ferrous oxide is a highly unstable substance, which attracts oxidation
and changes into ferric oxide. This latter, however, requires for
the production of soluble salts a larger quantity of acids than does
ferrous oxide, and therefore the oxidation of ferrous sulphate in the
air leads only to the formation of salts that are imperfectly saturated
with acid, namely basic salts.

When a solution of green vitriol is left exposed to the air, basic
ferric sulphate is also formed, which settles down to the bottom of
the vessel as a rusty powder. If, however, a corresponding quantity of
sulphuric acid be added to the solution at the outset, the resulting
ferric sulphate remains in solution.

On treating the green vitriol solution with one of caustic potash,
caustic soda or quick lime, the ferrous oxide is thrown down as
the corresponding hydroxide, forming a voluminous greyish-green
precipitate. This hydroxide still possesses a great affinity for
oxygen, and when the precipitate is brought into contact with air, its
colour rapidly changes to a rusty red, through the transformation of
the ferrous hydroxide into ferric oxide. The ferrous hydroxide can also
be precipitated by alkali carbonates, the deposits behaving in exactly
the same manner as that thrown down by the caustic alkalis.

Various methods can be adopted in the preparation of artificial ochre,
the selection depending on the properties desired in the finished
product. To obtain an ochre with particularly good covering power, the
method must be different from that employed to furnish a cheap product,
in which low price is more important than covering power.

In the former case, the ferrous hydroxide is mixed with substances
which, in themselves, possess fairly high covering power, such as chalk
or white clay; in the second, gypsum, which is of low covering power,
is used.

The preparation of the cheapest kinds of artificial ochre will be
described first, followed by that of the higher grades which belong to
the most valued artists’ colours.

For cheap artificial ochres, the ferrous hydroxide is thrown down by
caustic lime from a solution of green vitriol. According as a lighter
or darker shade is required, two to three parts of ferrous sulphate
are dissolved in water, care being taken to select crystals of a pure
green colour, since those that have a rusty look are only imperfectly
soluble, because they contain basic ferric sulphate.

The solution will always be cloudy, owing to the partial precipitation
of the hydroxide by the lime in the water; but this is immaterial.
For the precipitation, a milk of lime is prepared by slaking one to
two parts of quicklime (according to the quantity of ferrous sulphate
to be treated) in water, and stirring this up in enough water to make
a thin milk. Care must be taken to exclude any large particles of
lime, since these would find their way into the finished product and
make the colour uneven. On this account, the milk of lime should be
carefully strained through a loosely woven cloth or fine sieve, into
the precipitation vessel.

The ferrous sulphate solution is then poured in, the mixture being kept
stirred, and an ugly, grey-green precipitate is produced, consisting of
a mixture of ferrous hydroxide and calcium sulphate, the reaction being
explained by the equation:--

               FeSO{4} + Ca(OH){2} = Fe(OH){2} + CaSO{4}.

The larger the amount of ferrous sulphate solution added to the milk
of lime, the darker the resulting ochre. As soon as all the ferrous
sulphate is in, the stirring is suspended, and the liquid is left until
quite clear. The water is drawn off through tapholes in the side of
the vessel, care being taken not to disturb the fine precipitate, and
fresh water is added, in which the deposit is stirred up and again left
to settle down. This operation, which is once or twice repeated, is to
wash the precipitate.

When this object has been sufficiently accomplished, the mass is
shovelled out of the vessel and spread thinly on boards, where it is
left until the desired shade of colour has been attained, the colour
changing quickly on exposure to air, owing to the oxidation of the
ferrous hydroxide into ferric hydroxide. To ascertain whether oxidation
is complete, a large lump of the mass is broken across; and if it is
of a uniform yellow-brown colour throughout, without being darker on
the outside than in the middle, all the ferrous hydroxide will have
been transformed into the ferric state. The product can now be dried at
once, and when ground will be ready for sale.

To obtain different varieties from the product, it is carefully heated
(in a finely powdered condition) in shallow pans; but the operation
needs caution, or the water in the gypsum present will be expelled,
giving rise to drawbacks that are manifested when the colour is used.

For instance, in mixing such a colour with water, the gypsum would
again absorb water and cause the whole mass to set as a useless
solid lump. Since gypsum parts with its water at a comparatively low
temperature, it is better not to heat these cheap ochres at all, but
to obtain the various shades by modifying the proportion of ferrous
sulphate employed.

Another defect of the ochres prepared by this method resides in the
excess of lime present, it being impracticable to measure out the
quantity of lime used with such accuracy that only just enough is taken
to precipitate the ferrous hydroxide, there being always a slight
excess. This lime is transformed into calcium carbonate on the mass
being exposed to the air, just as in the preparation of Vienna white;
but as the saturation with carbon dioxide takes a considerable time,
some of the lime remains in the caustic state and is liable to affect
other colours that may be mixed with the ochre.

An artificial ochre uniting in itself all the qualities of the natural
product, and also capable of being shaded by burning, can be prepared
in the following manner. An accurately weighed quantity of pure
crystallised ferrous sulphate is dissolved in a definite amount of
water, and the solution is treated with successive small portions of
crude nitric acid, until all the ferrous oxide has been changed into
the ferric state. The change can be detected by a very decisive test.
If a liquid containing ferric oxide in solution is brought into contact
with a solution of red prussiate of potash (potassium ferricyanide), no
precipitate is formed in the absence of ferrous oxide, but only a brown
coloration; whereas, if ferrous oxide is present, a beautiful blue
precipitate is formed at once, the colour of which is so intense that
very small quantities of ferrous oxide can be detected by this means.

For the purpose now under consideration, the presence of small amounts
of ferrous oxide in the solution is immaterial, because they are soon
changed into ferric oxide on exposure to the air. It might, therefore,
be asked, why take the trouble to oxidise the ferrous oxide by means
of an agent involving expense, which could be saved by allowing the
oxidation to take place in the air?

The advantage, however, of the direct employment of a solution of
ferric oxide is that it gives at once a colour that can be dried
straight away; whilst at the same time the colour undergoes no change
in drying, whereas it does when ferrous oxide solution is used.

The method of producing ochres from this ferric solution varies
according as the product is to be used without any further treatment
than drying, or is to be modified by firing.

In the former event, caustic lime is again used as the precipitant, but
in only just sufficient quantity to throw down all the ferric oxide in
the solution. This amount can be calculated exactly, 36·84 parts by
weight of pure burnt lime being required for every 100 parts of pure
ferrous sulphate taken. The actual quantity, whether larger or smaller,
will depend on the relative purity of the sulphate and lime; and this
can readily be ascertained by a simple trial.

The lime is used in the form of milk of lime, as already described. If
lime alone is employed, the precipitate will consist of pure ferric
hydroxide and the calcium sulphate thrown down at the same time. The
resulting colour, when dried, will be an intensely brown mass, which
can be used in place of the very dark natural ochres.

In order to obviate entirely the disadvantages resulting from the
presence of a large amount of caustic lime in the precipitate, fine
levigated chalk or white clay is added in the preparation of the
lighter shades of ochre, the addition being made as soon as the
two ingredients have been brought into contact; and the mixture is
thoroughly stirred, to ensure uniform admixture with the ferric
hydroxide. The colour of the settled deposit will be lighter or darker
in proportion to the amount of chalk or clay employed; and in this
way the whole range of shades from pale yellow to bright brown can be
obtained without the application of heat.

Ochre that has been made with chalk is unsuitable for toning by heat,
because this treatment would causticise the lime, and the ochre could
not be mixed with other colours, since these would be affected by that
substance. On the other hand, when white clay is used in preparing
the ochre, the latter can be more easily toned by firing, provided
care be exercised in the process. The ochre must be dried completely
in the air, and either spread out in thin layers on iron plates, for
the burning process, or else put into a drum, of the kind already
described, in which the mass is moved onward by a worm.

The clay remains unaltered in firing, but the gypsum parts with its
water of crystallisation. In order to restore the latter, the ochre
issuing from the drum is discharged direct into a vessel of water, in
which it can be kept in constant motion by a stirrer. The water is soon
warmed by the heat of the mass, and absorption by the gypsum proceeds
at a rapid rate. When the whole charge has been fired and collected in
the vessel of water, the stirrer is stopped and the precipitate dried,
being then ready for use.

In certain circumstances, ochre can be made by other methods. In large
towns, ammonium salts are sometimes obtainable at a moderate price,
being manufactured in large quantities as a by-product in gasworks. For
our purpose, crude gas liquor might be used, since it contains ammonia
for the precipitation of the ferric hydroxide. In most cases, however,
this gas liquor contains only very small quantities of ammonia, and,
therefore, in a works of any size, very large vessels would be needed
for the production of a comparatively small quantity of ochre. On this
account, preference is given to crude carbonate of ammonia, which is
also obtainable at low prices.

On bringing a solution of this salt into contact with one of ferric
oxide, ferric hydroxide is precipitated, and the sulphate of ammonia
resulting from the reaction remains in solution. By stirring white clay
into the liquid at the same time, the ochre can be correspondingly
lightened in shade.

The precipitates obtained in this way can be dried at once, and
converted into any shade obtainable with natural ochre, from brown
to red, by strong firing. The sulphate of ammonia still remaining in
the air-dried product is completely volatilised by the heat, and the
resulting ochres are even superior to the natural varieties in beauty
and permanence.


OCHRES AS BY-PRODUCTS

In the manufacture of certain chemicals, substances of divergent
composition are obtained which are sold under the name of ochre and are
used as painters’ colours. Whereas ochre, properly so-called, consists
of either ferric hydroxide or ferric oxide in association with clay,
lime, etc., the products now under consideration are basic ferric salts
composed of varying quantities of ferric oxide in combination with
certain proportions of sulphuric acid.

These ochres are obtained as by-products in the manufacture of green
vitriol from pyrites, and in alum manufacture; and, according to their
origin, they are classed as vitriol ochre, so-called alum sludge, and
pit ochre. All the basic ferric sulphates of which they are composed
form fairly large crystals, and, therefore, in most cases, the covering
power is small. On this account the products are of low grade and are
put on the market at low prices, for which reason they are largely used
in making cheap paints.

_Vitriol Ochre._--Commercial green vitriol is, for the most part,
manufactured from native sulphides of iron. When many of these
sulphides are piled in heaps and left to the action of the air, oxygen
is gradually absorbed and green vitriol is formed which is dissolved
out by rain and is collected in large clarifying tanks.

In the case of pyrites, however, the mineral must first be roasted in
a current of air, since otherwise its conversion into green vitriol
would only proceed in a very sluggish manner. In any event, the aqueous
solution of ferrous sulphate has to be concentrated, by evaporation, to
the point at which the green vitriol crystallises out.

Both in the clarifying-tanks and--still more so--in the
evaporating-pans, a rusty-looking sediment forms at the bottom,
consisting of basic ferric sulphate. This originates in the partial
oxidation of the ferrous oxide (first formed) while the pyrites is
exposed to the air, and since the quantity of sulphuric acid present is
insufficient to saturate all the ferric oxide, basic salts are produced.

The yellow-brown sludge deposited in the pans during the concentration
of crude green vitriol liquor, constitutes the product termed vitriol
ochre, which contains varying amounts of ferric oxide, sulphuric acid
and water, according to the quantity of ferric oxide resulting from the
oxidation of the pyrites and the character of the latter, _e. g._:--

  Ferric oxide           65–70%
  Sulphuric acid         14–16%
  Water                  13–16%

Although the colour of these ochres is not particularly handsome, they
can be transformed, by firing, into colours of fairly good quality. As
this subject will be more thoroughly gone into when dealing with the
preparation of the red iron pigments, the applicability of these ochres
will only be casually referred to here. During the burning process,
these ochres, of course, part with the whole of their contained water;
and by protracted, high calcination, the whole of the sulphuric acid
can also be expelled, so that finally nothing but pure ferric oxide is
left.

_Alum Sludge._--Solutions of crude alum always contain a certain amount
of ferric oxide which settles down at the bottom of the pans during
concentration. This sludge, too, consists of basic ferric sulphate,
but is inferior in covering power to vitriol ochre, the crystals being
of coarser grain. On the other hand, the ochreous sediment from the
alum concentrating-pans has the valuable property of being readily
transformable into red-brown to pure red tones by burning. For this
reason, particular attention has been devoted to this sludge in a
number of alum works.

Since the products are only of value when burned, and the shades
thereby obtained are always red, they will be dealt with more fully
along with the red earth colours.

_Pit Ochre._--Springs containing small quantities of ferrous sulphate
and other salts are met with in many iron mines, but, in most
cases, the amounts are too small for their recovery by artificial
concentration to be contemplated. If, however, the conditions allow of
the springs being easily diverted, they may often be utilised for the
preparation of low-grade ochre.

The chemical composition of these pit ochres varies considerably, and
depends on the geological character of the locality. Water can only
dissolve such minerals as occur in the form of fairly readily soluble
compounds; and for this reason pit waters are always solutions of the
metals which are found in the mine.

The variety of compounds that may be present in an ochre can be seen
from the subjoined analyses of ochres deposited from pit waters at
Rammelsberg. As elsewhere, two distinct classes of ochre are met with,
having a conchoid and an earthy fracture respectively. The latter
usually contain rather more ferric oxide, and, in particular, a higher
content of foreign substances, the most important of which is quartz
sand. In the Table, the ochres with conchoid fracture are marked A, and
those with an earthy fracture, B.

                        A.       B.
  Ferric oxide        68·75    63·85
  Zinc oxide           1·29     1·23
  Copper oxide         0·50     0·88
  Sulphuric acid       9·80    13·59
  Water               15·52    18·45
  Clay and Quartz      4·14     2·00

The preparation of the ochre is a simple matter, consisting in
collecting the mass and sorting out the loose, earthy portions of a
pure yellow colour from the denser and darker parts. The former are
dealt with separately, usually by a simple process of levigation, for
the sole purpose of getting rid of the earthy matter, quartz sand in
particular.

The denser varieties require much more work, but yield a far superior
product, which, by suitable treatment, can be converted into the
finest grades of ochre. The first operation consists in a very careful
crushing, and as the pieces are often very hard, they are treated
in ordinary or stamp-mills, edge-runners being also employed with
advantage.

The product reduced by any of these means is passed through a number
of sieves, to separate the fine particles from the coarse; and the
finest dust is burnt. This last treatment causes a considerable loss
in weight, both the accompanying water and most of the sulphuric acid
being volatilised. However, since, as already stated, all varieties of
ochre can be obtained, the process is consequently very remunerative
notwithstanding the loss in weight it involves.

_Yellow Earth._--From the particulars given in the general description
of the earth colours, yellow earth may also be regarded, to some
extent, as an ochre, but one containing a large proportion of foreign
substances. It might, however, be more accurately termed a clay
contaminated with a considerable amount of quartz sand and a certain
proportion of ferric oxide. The method of preparation is on the
same lines as for ochre, but burning is never practised, nor is the
treatment so careful as for the better grades of ochre, the low price
of the colour making this unremunerative.




CHAPTER VI

RED EARTH COLOURS


The number of minerals that can be directly used as red earth pigments
is comparatively small, and by far the greater proportion consist of
ferruginous colours, a few of which are obtained by the mechanical
treatment of native iron ores or clays coloured red by ferric oxide,
the majority, however, being formed by burning certain materials of
another colour. To these belong nearly all the materials mentioned in
connection with the ochres and the brown iron colours, together with a
few by-products of the chemical industry.

In addition to the foregoing, which have ferric oxide for their
pigmentary principle, is the native mercury sulphide, occurring, as
scarlet, crystalline masses, under the name of cinnabar (vermilion).
The only reason for including natural vermilion with the earth colours
is to make the list complete, the largest proportion of this pigment
being prepared by artificial methods. The product sold as “Chinese”
vermilion may, in former times, have really been introduced from
China into Europe, and prepared there by grinding and levigating the
best-coloured lumps of the natural cinnabar; but, at the present
time, all the vermilion made--in Europe at least--is from sulphur and
mercury, by artificial processes, and the name Chinese vermilion is
merely retained to designate a particularly fine grade.

On the basis of occurrence and chemical properties, the red earths
can be classified into several groups. The first comprises natural
products requiring only mechanical preparation, such as the minerals
known as hematite, micaceous iron ore, Elbaite, etc., and the special
modification of red ironstone termed raddle. All these minerals consist
almost entirely of ferric oxide in a pure state. The mineral, bole (red
chalk, terra sigillata, Lemnos earth), is chemically allied to the
ochres, being, like them, composed of alumina, frequently accompanied
by lime and small quantities of magnesia, but differing in that ferric
oxide is always present in bole, whereas the ochres always contain
ferric hydroxide.

The second group consists of the artificial reds obtained by burning
or calcining raw materials, whose ferric hydroxide is more or less
transformed by heat into ferric oxide, such as vitriol ochre, pit ochre
and alum sludge.

Of late years the artificial earth colours have attained a high degree
of importance. They are obtained in large quantities in the manufacture
of sulphuric acid from green vitriol. Formerly, it is true, they were
also used as pigments under the name of caput mortuum or colcothar, but
were not held in much esteem; and it is only within recent times that
it has been discovered that these inferior by-products can be converted
into very handsome and brilliant colours, which now form important
articles of commerce.


BOLE

Bole, Lemnos earth, terra sigillata, etc., is, for the most part, a
product of the decomposition of highly ferruginous minerals, and
occurs, in the form of lumps, having a conchoidal fracture, in pockets
or detritus. The lumps have a sp. gr. of 2·2–2·5, are Isabella brown to
dark brown in colour, and give a slightly greasy-looking streak. There
are two distinct varieties of bole: the one adhering firmly to the
tongue, whilst the other lacks this property and, when placed in water,
crumbles down to powder in emitting a peculiar noise.

The composition of the boles varies, but all of them may be regarded
as alumino ferric silicates combined with water. Most of the specimens
examined from different deposits contain 24–25% of water, 41–42% of
silica, and 20–25% of alumina, the remainder consisting of ferric oxide
with small traces of manganese oxide.

Some varieties, however, are exceptional and contain only 30–31% of
silica and 17–21% of water, _e. g._ those from Orawitza and Sinope.
Lemnos earth, the true terra sigillata, is mostly silica (66%) with 8%
of water, and contains a smaller percentage of ferric oxide than the
others. It is also of a distinct colour, lighter than the true boles
and having a greyish or yellowish tinge.

The behaviour of the different kinds on burning is just as diverse as
their chemical composition. Whilst some kinds are infusible at even the
highest temperatures, and merely change into hard, red masses; others,
again, fuse at a moderate heat. This difference is due to chemical
composition, those high in silica being generally less refractory than
those in which alumina preponderates.

In order to render the boles suitable for painting, they are put
through a somewhat different treatment than the other earth colours.
The freshly dug material is first sorted, the uniformly coloured lumps
of fine texture being set apart and suffused with water, with which
they form a pasty mass of low plasticity, which is kneaded by hand
to make it homogeneous, and is then stirred up with more water. When
the lumps have distributed in the water, the latter is drawn off into
a second tub, and the residue is stirred up with fresh water, the
treatment being repeated until the effluent no longer shows any signs
of colour.

The liquid in which the finely divided bole is suspended is left to
settle, and the bole subsides as a fine powder, which is dried to the
condition of paste, pressed into moulds and dried completely.

Owing to its low content of ferric oxide, the colour of bole is not
particularly bright, but is very permanent--a property equally shared
by all the other ferric oxide pigments.


NATIVE FERRIC OXIDE AS A PIGMENT

In nature, ferric oxide forms extensive deposits, which, by reason of
the light red colour characteristic of certain varieties of ferric
oxide, are largely employed in painting. These colours may be classed
among the oldest known to mankind, ferric oxide pigments having been
used frequently in the most ancient paintings.

The most important varieties of ferric oxide for our purpose are: iron
glance, with its modifications, micaceous iron ore and frothy hematite;
red hematite, and raddle.


IRON GLANCE

This substance forms handsome black crystals of very high lustre,
which, when small and scaly, constitute micaceous iron ore. Both,
when rubbed down, furnish a dark red powder of no particular beauty.
Micaceous iron ore forms the transition stage into frothy hematite, or
iron cream, the sole difference being that the crystals of the latter
are much smaller, and the scales finer, the iron-black colour passing
gradually into cherry red. At the same time, the lustre, though still
high, loses most of the metallic sheen exhibited by micaceous iron ore.


HEMATITE

The variety known as hematite or bloodstone, sometimes occurring as
shiny nodules, is distinguished by its handsome red colour. Some of
the lumps are composed of long, thin crystals grouped about a common
centre so as to form a globular mass. Despite its bright colour, the
hardness of hematite (between 3 and 5) prevents it from being used as a
pigment, the value of the product not being commensurate with the cost
of reduction.


RADDLE

There are numerous deposits of red ironstone, in the state of fine
earth, where the operations of grinding and levigation have, to a
considerable degree, already been carried out by Nature. These deposits
form the mineral which, under the name of raddle, is often used as a
pigment for ordinary paints. It may be considered to have originated
in the transformation of red ironstone, by the natural forces that can
everywhere be seen disintegrating rocks, namely water and frost, into a
fine powder, which has been transported, often over long distances, by
water, and has finally settled down.

In places where the process has been carried out in this manner, the
raddle will be in a condition, as regards fineness of division and
beauty of colour, that leaves nothing to be desired, and the material
itself is ready for use as a very valuable pigment. Large deposits of
this kind, however, are of rare occurrence; but there are plenty in
which the ferric oxide is associated with varying quantities of clay,
sand, and sometimes lime.

The conditions here are on all fours with those of clay, which, too,
has been formed in a similar way. Pure clay, the so-called kaolin, is
a highly valuable material, whereas ordinary loam--highly contaminated
clay--is only of low value. In judging the quality of raddle as a
pigment, the presence of impurities is of less account than their
nature; and in some cases a very highly contaminated raddle may be
worth far more, as a pigment, than one containing only very small
admixtures of extraneous substances.

As stated above, the ordinary impurities in raddle are clay, lime and
quartz sand. An admixture of clay, even if fairly large, is no great
drawback, since the material can be used in its natural state, and also
be toned by burning. Lime is less favourable, for though a calcareous
raddle can be used as it is, the lime parts with its carbon dioxide
on calcination, becoming changed into caustic lime and imparting to
the product qualities which preclude its employment for a number of
purposes, especially for mixing with delicate organic colours.

The presence of quartz sand is immaterial when the raddle is to
be burned, inasmuch as sand is unaltered by calcination. But it
constitutes a drawback because it makes the fine raddle gritty and
unsuitable for fine paint work. The only way to eliminate this impurity
is by levigation--an expensive operation which should, as far as
possible, be avoided for these native ferric oxides, because they must
be sold very cheaply, and have to compete with the large quantities of
oxide obtained as a by-product of the chemical industry.

The suitability of a given specimen of raddle for use as a pigment may
be easily ascertained by weighing out exactly 100 grams and heating
to about 120° C. The loss of weight will give the amount of water in
mechanical retention. The residue is suffused with strong vinegar,
and left for several days, being stirred at frequent intervals. The
carbonates of lime and magnesia present will dissolve in the acid,
the ferric oxide remaining untouched. The liquid is decanted, and the
residue washed several times with water and dried, the diminution in
weight being a measure of the carbonates in the sample. If the vinegar
has turned a yellow colour, the presence of ferric hydroxide in the
mineral is indicated, this hydroxide being readily soluble in acetic
acid. If the residue feels gritty, it contains quartz sand, the amount
of which can be found with sufficient accuracy by levigating the mass
and weighing the sandy residue after drying.

Deposits occur, in many places, of a mineral similar to raddle, but
formed under peculiar conditions. Thus, there are found, in the
vicinity of brown-coal deposits that are rich in pyrites, earthy
masses which are occasionally of a handsome red colour and consist of
a variety of minerals admixed with a considerable proportion of ferric
oxide.

These masses probably originated in fires in the coal seams, whereby
the pyrites became transformed into ferric oxide and basic ferric
sulphate; and where the deposits are of sufficient size, they may be
advantageously utilised in the production of cheap reds. In most cases,
however, the minerals must be levigated, owing to the frequency with
which they contain large proportions of extraneous minerals in a gritty
condition.


BURNT FERRIC OXIDE AND OCHRES

It has already been stated, in dealing with the yellow ochres, that
these colours can be toned by burning, part of the ferric hydroxide
losing its water and changing into red ferric oxide. The more severe
the burning, the larger the amount of ferric oxide formed and the
nearer the colour of the product approximates to red. According,
however, as the original ochre was yellow or brown, the tone of the
burnt colour will lie between orange and brownish red. If the heating
be pushed so far as to transform all the ferric hydroxide into oxide,
the red will come more and more into prominence in proportion to the
amount of hydroxide in the original material. If the product consists
entirely of ferric oxide, as is the case with that obtained, as a
by-product, in the manufacture of English sulphuric acid, a pure
red ferric oxide (caput mortuum, colcothar, English red, etc.) will
be obtained. If the heating be increased above a certain point, the
pure ferric oxide will change colour, assuming a brown to violet tone
according to the temperature employed.


(_a_) _Burning in the Muffle_

Since, as a rule, the quantity of material treated in the preparation
of these brown, violet to black ferric oxide pigments for the
purposes of the painter on porcelain is not large, the same kind of
muffle furnace (Fig. 29) as serves for making enamels can be used. The
fire-clay muffle _M_ is inserted in a reverberatory furnace _O_, with
a good draught, and is raised to a white heat. The finely powdered
material to be burned is spread out evenly on plates of sheet-iron or
fire-clay, and introduced into the white-hot muffle, where it is left
for a period corresponding to the colour desired. To save time, the
plates may be pre-heated in a second muffle arranged above the first.

[Illustration: FIG. 29.]

By this means a large range of tones can be obtained from one and
the same material, by heating it to different temperatures; and the
colours so produced are distinguished, not only by their warmth of
tone, but also by very high stability. In fact, they may be regarded
as permanent, because very strongly calcined ferric oxide only passes
very slowly into solution even under prolonged boiling in the strongest
acids. Owing to this excellent property, which is equalled by very
few other pigments, and the low cost of preparation, these colours
deserve the most careful consideration by all manufacturers who are in
a position to obtain suitable material in sufficient quantities.


(_b_) _Caput Mortuum, Colcothar_

Previous to the English method of making sulphuric acid by the
oxidation of sulphur dioxide with nitric acid, this acid was
manufactured by heating dehydrated ferrous sulphate (green vitriol);
and even now, fuming sulphuric acid--oil of vitriol, or Nordhausen
sulphuric acid--is largely obtained by the same process.

When anhydrous ferrous sulphate, FeSO{4}, is exposed to a very high
temperature--strong white heat--it is decomposed into sulphur trioxide,
SO{3}, sulphur dioxide, SO{2}, and a residue, mainly composed of ferric
oxide and a little basic ferric sulphate, which remains behind in
the heating-pan. In fact, even at the highest possible temperatures
obtainable in the furnaces used for the distillation of the green
vitriol, it is impossible to recover the whole of the sulphuric acid, a
small portion being tenaciously retained by the iron.

This red residue is sold under various names--colcothar, caput mortuum,
English red, Indian red, etc.--and is used as a low-grade pigment,
and also as a polishing agent. The name caput mortuum is a survival
from the time of the alchemists, and was probably applied to indicate
a dead-burned product, from which all the active ingredients had been
removed.

Although, in former ages, this substance was held in low estimation as
a pigment, attempts have been made in recent times to convert it, by
suitable treatment, into a more valuable product; and these attempts
have been crowned with success, affording another instance of how a
high commercial value can be imparted to a waste product by proper
manipulation.

(_c_) _Calcining Ferric Oxide_

In order to obtain a series of tones of colcothar, it is subjected to
repeated calcination, but not by itself, since it would require an
extremely large quantity of fuel to effect any change of tone in view
of the very high temperature the material has already been exposed
to in the sulphuric acid plant. If, however, salt be added, then a
variety of tones can be obtained without recourse to any particularly
high temperature. It is frequently stated that the only effect of
the presence of salt is to keep the calcining temperature uniform,
inasmuch as the salt volatilises at a strong red heat, and when that
temperature is reached, the whole mass cannot get any hotter until the
whole of the salt has passed off, all the heat applied being consumed
in transforming the salt into the state of vapour.

As a rule, however, the amount of salt added does not exceed 6% of the
weight of the charge to be calcined; and this quantity does not seem
to be sufficient to keep the temperature at a uniform level through
the several hours required for the calcining process. The author is
therefore of opinion that the salt also has a chemical action on the
material during the calcination.

As already mentioned, colcothar is by no means pure ferric oxide, but
always contains basic ferric sulphate. Now, it is feasible that some
reaction may take place between the basic sulphate and the sodium
chloride at calcination temperature, with the formation of caustic
soda, which, being a far more powerful base than ferric oxide, deprives
the latter of sulphuric acid, sodium sulphate being formed. The
chlorine of the salt combines with the iron to form ferric chloride,
which volatilises at a glowing heat.

According to this hypothesis, therefore, the addition of common salt
in the calcination of colcothar is less for the purpose of maintaining
a uniform temperature within certain limits than for decomposing the
basic ferric sulphate present and inducing the formation of a product
consisting entirely of pure ferric oxide. The various tones obtained
are due to the varying length of exposure to the heat.

The following method is pursued in the conversion of colcothar into
iron pigments on a manufacturing scale. The crude colcothar from the
sulphuric acid plant is ground, as finely as possible, in ordinary
mills, and the resulting soft powder is intimately mixed with salt,
2, 4 or 6% being the usual proportions added. The calcination is
ordinarily continued for six hours in the case of the mixture
containing the largest amount of salt; but only two hours, or even one,
for the other mixtures.

The operation is carried on in earthenware pipes, a large number of
which (up to sixty) are built into a furnace. The latter is fired very
carefully, the temperature being raised only very gradually, since
experience has shown that much better coloured products are obtained in
this way than by raising the mass quickly to a high temperature.

When incandescent ferric oxide is allowed to cool down with
unrestricted access of air, the colour is not nearly so bright as when
air is excluded during the cooling. Since air has no action on ferric
oxide, this remarkable phenomenon cannot be due to the presence of
the air, but probably to the influence exerted by the rapid change
of temperature on the arrangement of the finest particles of the
oxide. Nevertheless, some manufacturers hold that rapid cooling, with
restricted access of air, improves the colour.

To exclude air from the ferric oxide during calcination, the open ends
of the pipes are flanged and covered with close-fitting plates, which
are luted with clay. The expansion of the internal air as it grows hot
would burst the pipes unless a means of escape were provided, which
consists in leaving small vent holes in the cover plates.

As previously mentioned, calcined ferric oxide is very inert,
chemically, so that, when the calcination has been strong, prolonged
boiling with the most powerful acids is needed to bring the oxide into
solution. If the heating has been continued up to the strongest white
heat, and the ferric oxide maintained in that condition for several
hours, even hot sulphuric acid will have only a slight effect on the
oxide, and the only way to make it more readily soluble is by fusion
with potassium bisulphate.

Now indifference to chemical action is just the property required of a
pigment for fine work; and in this respect, the ferric oxide colours
are superior to all others. The gradations of tone that can be obtained
from ferric oxide by varying the calcination are very numerous,
comprising all between iron red, red-brown and pure violet.

The author has tried heating ferric oxide for a considerable time at
a very high temperature, equivalent to the strongest white heat, and
obtained a product which was no longer pure violet, but had a decidedly
blackish colour. Perhaps, by greatly prolonging the heating, it might
be possible to get a pure black; but, even if this were so, the matter
would be of no special interest, because black pigments for paints can
be prepared in a much cheaper manner. All that would be accomplished
would be the proof that ferric oxide actually undergoes an extensive
molecular modification when heated.


FERRIC OXIDE PIGMENTS FROM ALUM SLUDGE

Alum is manufactured from alum shale and alum earth, the former being
a carbonaceous clay shale interspersed with pyrites, and the latter
a clay charged with pyrites and bitumen. The raw materials are left
in heaps for several years, the pyrites being thereby oxidised with
formation of free sulphuric acid and ferrous sulphate. This free acid
reacts further on the clay, which it transforms into sulphate of
alumina; and by leaching the heaps with water, a solution is obtained
which contains the sulphate of alumina and the ferrous sulphate. On the
liquor being concentrated, a basic ferric sulphate is deposited, which
is worked up into red pigment.

For this purpose it is first levigated in a special manner, the sludge
from the pans being placed in a large vat, suffused with water, and
kept in slow circulation by stirrers, which distribute the particles
in the water, forming a turbid liquid. This liquid is conducted into a
gently sloping shute, the sides of which are perforated with openings
at certain intervals, to allow part of the liquid to run off into
large collecting vessels underneath.

The heaviest of the suspended particles settle down first and are
flushed out by the water escaping through the first opening. The finer
the particles, the longer they remain in suspension, so that the liquid
escaping through the last holes carries off only a very fine powder.
The liquid collected in the different vessels is allowed to subside and
is then drawn off from the firm deposit. The operation is repeated with
fresh quantities of sludge until sufficient sludge has been collected
for further treatment. The collecting vessel furthest away from the
intake of the shute contains the finest levigated material, and this is
used for making the best ochres.

The levigated mass is dried in a very simple manner, being usually
spread out on boards, which are exposed to the air in open sheds,
covered with a roof to keep out the rain. Here the sludge is left until
it forms a pasty or earthy mass, and is then calcined.

The best calcining furnace is of the type used for colcothar; but the
pipes must be connected to an exhaust pipe for carrying off the vapours
disengaged during calcination.

[Illustration: FIG. 30.]

However, since alum manufacturers do not usually go in for making the
highest-grade pigments, simpler calcining furnaces are used, consisting
of reverberatory furnaces in which the heating-gases are allowed to
act directly on the materials of the charge. A front elevation and
section of such a furnace are shown in Figs. 30 and 31. The furnace
is constructed with several arches, one above another, marked _c_,
_k_, _d_. The charge is introduced through the openings _b_ and _b’_.
The furnace chamber is at _a_, and the ashpit at _g_. The gases of
combustion flow over the charge on the hearths of the several arches
and escape, at the top, into the stack, along with the acid vapours
liberated from the glowing mass.

[Illustration: FIG. 31.]

The further the hot gases get away from the fire, the cooler they
become, and therefore the less strongly heated the charge on the upper
hearths. Consequently, the resulting product (ferric oxide) from
the different stages of the furnace differs in colour; and a number
of gradations can be obtained by blending. The ferric oxide pigments
prepared in this way are not pure oxide, but also contain small
quantities of sulphuric acid and metallic oxides which were present in
the original crude sludge. However, by reason of the simple process of
preparation employed, these pigments are usually sold at lower prices
than those from colcothar; and for less fine work they are excellent.




CHAPTER VII

BROWN EARTH COLOURS


In point of chemical composition, the majority of the brown earth
colours are closely allied to the reds, both kinds containing ferric
oxide. The main difference consists in that, in the brown earths, the
ferric oxide is combined with water to form ferric hydroxide.

Many of the brown earth colours, however, are of entirely different
chemical composition, and either consist mainly of organic matter
derived from the decomposition of plants--and therefore very similar
to brown-coal or peat--or else contain varying quantities of inorganic
substances mixed with these dark-coloured organic decomposition
products.

The brown earth colours form a highly important group, some of the
members of which are used in the finest paintings, and, for certain
purposes, cannot be replaced by other pigments. Those containing ferric
hydroxide are found--though not very frequently--in natural deposits,
the most celebrated being the terra di Siena, occurring in the vicinity
of that city.


TERRA DI SIENA

This highly renowned pigmentary earth is found in deposits, and, in
the crude state, forms dark brown masses which are devoid of lustre,
crumble readily between the fingers, have a smooth conchoidal fracture
and absorb water with avidity, in consequence of which property they
adhere to the tongue. Their chief chemical constituent is ferric
hydroxide, with which, however, variable quantities of sand, clay
and ferric oxide are admixed. These admixtures cause a considerable
divergence in the colour of the earth, ranging from pure brown to
reddish-brown, and, in the case of very impure lumps, to an unsightly
yellow-brown.

Mineralogically, terra di Siena is often regarded as a distinct species
which, according to the results of analysis, must be considered, not
as ferric hydroxide, but as ferric silicate combined with water.
Sometimes, a portion of the ferric oxide is replaced by alumina, so
that the percentage composition of the mineral becomes approximately:
ferric oxide, 66%; silica, 11%; alumina, 10%; and water, 13%. The
hardness of this mineral is 2·5, and the sp. gr. 3·46.

The method of formation of terra di Siena was probably on the same
lines as that already described in the case of ochre, namely by the
breaking down of minerals--in this case brown ironstone--and natural
levigation, the powder being deposited in places where the water
containing the ferric hydroxide in suspension came to rest and allowed
the solid particles to settle down.

The best lumps of terra di Siena in point of purity and colour can be
used as pigments without any preparation; but in most cases the earth
is lightly calcined, in order to improve the colour. This treatment
enables a whole series of tones, from pure brown to the brightest red,
to be obtained. The stronger the heating, the more water expelled from
the hydroxide, and consequently the closer the approximation of the
colour to that of ferric oxide.

The pigments met with in commerce as terra di Siena can also be
prepared artificially, by making ferric hydroxide and heating this,
when dried, until the requisite tone is attained. For this purpose,
ferrous oxide is precipitated from green vitriol and exposed to
the air, under which conditions it is rapidly transformed into
ferric oxide, and the greyish-green colour of the mass changes to
brown. Lighter tones can be obtained by the addition of inert white
substances; and, in other respects, the method of preparation is the
same as that of artificial ochre.

These pigments are sold under various names, the dark shades, between
pure brown and red brown, being usually called terra di Siena or
mahogany brown, whilst the paler sorts are sold as satinober--more
correctly satin ochre, golden ochre, etc. Other pigments, chemically
allied to the ferric oxide or ochre pigments, are sometimes found on
the market under various and entirely arbitrary names.

It may be pointed out that the greatest confusion exists in the
nomenclature of pigments, to such an extent that, in many cases,
neither the chemist nor the manufacturer knows precisely what pigment
is implied by a given name. The confusion is still further increased by
the use of names taken from different languages.


TRUE UMBER

Umber, properly so called--also known as Turkish, Cyprian or Sicilian
umber, from the country of origin--derives its name, according to
some authorities, from the province of Umbria (Italy), where a brown
earth is found, though others ascribe it to the Latin “umbra” (shade)
because of the pigment being used for painting shadows.

True umber is an earthy mass of fine texture and liver-brown colour,
merging into chestnut in some of the lumps. Chemically, it consists of
a double silicate of iron and manganese combined with water, a portion
of these metals being usually replaced by alumina. The greater hardness
(1·5) and higher specific gravity (2·2) of true umber in comparison
with Cologne earth (which is quite arbitrarily termed “umber”), form a
ready means of differentiation between the two.

According to Viktor Merz, the umber found in Cyprus consists of: ferric
oxide, 52%; manganese oxide, 14·5%; and alumina, 3%; and is, possibly,
merely a mixture of clay with hydroxide of iron or manganese. An umber
examined by Klaproth contained 13% of silica, 5% of alumina, 48% of
ferric oxide, 20% of manganese oxide and 14% of water.

The tone of umber can be modified, in the direction of red, by
calcination, but this process is seldom employed, the dark brown shade
of this colour being the one most appreciated.

In some parts of northern Germany, Thuringia in particular, the iron
mines contain smaller or larger pockets of ferric hydroxide, of a
fine earthy texture, from which umber is prepared, by levigation and
calcination. The product is sold under various names: chestnut brown,
wood brown, mahogany brown, bistre flea brown, roe brown, according to
the shade of the calcined pigment.

A mineral (“siderosilicate,” according to Von Walterhausen) composed of
ferric silicate, and approximating in this respect to terra di Siena,
is found in the neighbourhood of Passaro (Sicily) in deposits of tuff.
It forms masses which are transparent at the edges and are usually
liver-brown to chestnut in colour. The hardness of the mineral is 2·5,
the sp. gr. 2·713, and the average chemical composition: silica, 34%;
ferric oxide, 48·5%; alumina, 7·5%; and water, 10%.

The foregoing are only a few examples of brown or red-brown earth
colours. In all these minerals the pigmentary principle is iron, in
combination either with oxygen alone (ferric oxide), with oxygen and
water (ferric hydroxide), or silica compounds (ferric silicate), and
always associated with certain quantities of other metallic oxides,
especially alumina and manganese oxide. Although but few of these
minerals have gained any special reputation as pigments, there is
no doubt that similar minerals, which are certain to occur in or
near many deposits of iron ores, could equally well be used for that
purpose. There is no need to emphasise that the discovery of such a
mineral would be a very valuable find, and that the products obtainable
therefrom could be utilised to great advantage.

The testing of a mineral for its suitability as pigment is a very
simple matter, all that is required being to subject a small quantity
to the same treatment that is applied to the earth colours on a large
scale. For this purpose a few pounds of the mineral are levigated, and
the residue is dried. To ascertain the tones obtainable by calcination,
small samples--of about 100 grms.--are placed in crucibles, and
gradually heated in a furnace. When the masses have attained a
sufficient temperature, the samples are taken out of the furnace,
at intervals of ten minutes, and left to cool. It will then not be
difficult to decide whether the mineral is at all suitable for the
purposes of the colour-maker; and if so, these tests afford at once an
indication of the temperature and time the mineral must be heated in
order to obtain pigments of definite tones.


COLOGNE EARTH (COLOGNE UMBER)

The application of the term “umber” to this earth can only have been
based on a certain similarity in colour to true umber. In chemical
composition, however, the two are quite different, Cologne earth really
consisting of a mixture of humic substances. It is well known that the
rotted wood found in the interior of decaying trees is often a handsome
brown colour; and all woody matter, after lying a very long time,
finally acquires this colour, owing to the transformation of the wood
into dark-coloured compounds richer in carbon. This effect can be seen
on the large scale, in Nature, in the case of coal, brown coal and peat.

Now Cologne earth consists of a brown-coal mould, dark brown in colour,
of earthy character and of such low cohesive power that it crumbles
with ease. Owing to this character, Cologne earth can be easily ignited
by the flame of a candle, and then burns with a strong, smoky flame,
leaving very little ash and disseminating the peculiar bituminous smell
given off when brown coal is burned.

The geological characteristics of Cologne earth enable one to conclude
that, where similar conditions prevail, materials of analogous nature
may be discovered. This earth is found embedded in a deposit of brown
coal, in which it forms pockets, and occasionally large bodies. Now,
brown-coal deposits of enormous extent occur in very many localities,
as for instance in Upper Austria and in Bohemia; and many of these
mines are sure to contain pockets of brown-coal mould, which have
perhaps been overlooked, but might very well be utilised in the
preparation of colours of very similar character to Cologne earth.

The preparation of this material is very simple. The earth coming from
the deposits is put through a simple levigation treatment which leaves,
as residue, lumps of semi-decomposed wood, mineral admixtures, sand,
etc. The levigated earth is sold in the form of cubes.

Cologne earth comes into the market under various other names, such as:
umber, Cassel brown, Spanish brown, etc.

The fiery brown which was so greatly preferred by the famous painter
Van Dyck, and named Vandyke brown after him, was of very similar
composition to Cologne earth, and is said to have been obtained from a
deep brown peat earth. The Vandyke brown of the present day, however,
is almost invariably a ferric oxide pigment, toned to the proper shade
by suitable calcination.


ASPHALTUM BROWN (BITUMEN)

As a natural product, which can be used as a painters’ colour without
any special preparation, asphaltum (bistre, bitumen) may also be
classed among the earth colours. Chemically, it is composed of
hydrocarbons of various kinds, and is thus similar to tar; in fact,
asphaltum may also be regarded as a natural tar resulting from the
decomposition of various organic substances. Many deposits of this
mineral are known, and two of them are particularly celebrated: those
of the Dead Sea, in Syria, and the Lake of Asphalt, in Trinidad. Both
deposits consist of craters filled with water on which the asphaltum
floats in large cakes.

Several kinds of asphaltum are met with in commerce, ranging in colour
from brown to black. The preparation of the material as a pigment
is confined to grinding the mass, which is always of a low degree
of hardness. Being readily soluble in oil of turpentine and then
furnishing the most beautiful brown tones when laid on thinly, the
pigment is usually sold in this condition, although it is also ground
in oil for the same purpose.

Finally, it may be mentioned that various useless materials can be
transformed, by suitable treatment, into brown pigments closely
resembling Cologne earth and applicable to the same uses. Such pigments
can be prepared from brown-coal slack (from inferior brown coal) or
bituminised wood--a variety of brown coal looking like charred wood--by
treating these materials with a lye made from wood ashes and lime, and
washing and drying the residue.




CHAPTER VIII

GREEN EARTH COLOURS


Although the number of green-coloured minerals is large, but few
of them are suitable for painters’ colours, because they occur so
rarely in Nature that their employment for this purpose is out of the
question, more especially since a very large number of green pigments
can be obtained by artificial means. The most important of the earth
colours in this category are Celadon green, or green earth, and
malachite green--the latter, however, less so, because the substance of
which it is composed can be prepared artificially.


GREEN EARTH, OR CELADON GREEN

This mineral is of a peculiar green colour, and the name “Celadon
green” has been universally adopted in the nomenclature of colour
shades. Green earth occurs native in many places, being the
decomposition product of an extensively distributed mineral, augite,
crystals of which are found in many of the deposits. The green earth of
Monte Valdo, on Lake Garda (Upper Italy) has been used for a very long
time as a pigment. It is chiefly prepared in Verona for distribution
in commerce, and from this circumstance has acquired the name “Verona
green,” or “Verona earth.” The earth is also found in Cyprus and
Bohemia, where it frequently occurs as the decomposition product of
basaltic tuff. However, whether obtained from Monte Valdo or elsewhere,
the product is always placed on the market as Verona earth.

Native green earth is always tough, mostly occurring in amygdalous
lumps, but occasionally in the crystalline form of augite. It has
a fine-grained fracture, a hardness between 1 and 2, and a sp. gr.
between 2·8 and 2·9. The colour is not always quite uniform, pure lumps
having the characteristic Celadon green appearance, whilst impure
lumps are olive green to blackish green. In chemical composition it is
chiefly ferrous silicate, and this compound must be regarded as the
actual pigmentary principle of green earth. In addition, it contains
varying quantities of other compounds which influence the depth of
shade of the product.

Verona earth chiefly consists of ferrous oxide in combination with
silica; alumina, magnesia, potash, soda and water being also present.
Analysis shows it to contain: ferrous oxide, 21%; silica, 51%;
magnesia, 6%; potash, 6%; soda, 2%; and water, 7%.

The green earths from Gösen, Atschau and Männelsdorf, near Kaaden
(Bohemia) and the Giant’s Causeway (Ireland) have the following
composition:

  +---------------+----------+----------+
  |               | Kaaden.  | Giant’s  |
  |               |          | Causeway.|
  +---------------+----------+----------+
  |Silica         |    41    |   56·4   |
  |Alumina        |     3    |    2·1   |
  |Ferrous oxide  |    23    |    5·1   |
  |Ferric oxide   |    --    |   14·1   |
  |Lime           |     8    |    --    |
  |Magnesia       |     2    |    5·9   |
  |Potash         |     3    |    8·8   |
  |Carbon dioxide |    19    |    --    |
  |Water          |    --    |    6·8%  |
  +---------------+----------+----------+

On account of the large quantity of mechanically associated water,
freshly dug green earth is greasy in character, like wet clay. In
partial drying, most of this water evaporates, the mass becoming
earthy and adherent to the tongue. Sometimes the colour is an ugly
brownish-green, owing to the presence of a considerable amount of
ferric oxide formed as the result of changes set up by exposing the
mineral to the air. Ferrous oxide is a very unstable compound, having
an energetic tendency to combine with more oxygen and thus undergo
transformation into ferric oxide; so that when green earth is left in
the air for a long time, a considerable proportion of its ferrous oxide
is oxidised to ferric oxide, the mass thereby assuming the brown tone
in question.

Such an unsightly product can, however, be converted, by simple
treatment, into one of very bright and handsome appearance; and it is
this possibility that first enabled green earth to attain importance as
a painters’ colour. Formerly it was only used as a material for common
work, being added to whitewash or employed for indoor paints.

When the crude green earth is treated with very diluted hydrochloric
acid, the compound of ferrous oxide and silica is left intact, but most
of the extraneous admixtures are removed. Ferric oxide, in particular,
passes into solution, and the calcium carbonate largely present in
some kinds of green earth is also dissolved. After prolonged contact
with the crude earth, the acid liquor takes on a brownish coloration
from the dissolved ferric oxide. Since the presence of iron salts
has no influence on the purification of the green earth, the most
impure, highly ferruginous hydrochloric acid can be used, and the
liquor can afterwards be employed in the preparation of artificial
ochre by leaving it in prolonged contact with any strongly ferruginous
mineral, such as brown ironstone, which neutralises the surplus acid.
This liquor is then precipitated by lime, alkali, etc., the resulting
deposit consisting mainly of ferric hydroxide, the further treatment of
which is conducted exactly as described in dealing with the preparation
of artificial ochre.

The treatment of the crude earth is best carried on in the same vessels
as are to be used in the subsequent levigation process. After the acid
liquor has been drawn off, the earth is brought into contact with
water, stirred up well, and the water run off, by opening tapholes in
the side of the vessel, into settling-tanks, where it is left until all
the suspended matter has completely subsided.

The colour of green earth can also be toned by the addition of yellow
ochre, thus producing a range of greens with a yellowish tinge. These
lighter shades, however, are seldom met with in commerce, the trade
judging the quality of green earth more particularly on the depth of
colour.

Green earth is a valuable pigment for all kinds of painting, on account
of its extreme permanence. It may be applied directly over lime without
suffering any change, whereas most of the cheap green colours are
destroyed in like circumstances. This behaviour renders green earth
specially valuable in fresco work, although it is also largely used as
an oil colour.

Augite is of frequent occurrence in volcanic districts; and in such
localities, deposits of green earth are certain to be found. The test
for the suitability of a green earth consists mainly in treatment
with dilute hydrochloric acid. If the mineral assumes a handsome
green tone, it will generally form a useful pigment. The test may be
supplemented by applying the colour to a fresh coating of whitewash,
under which conditions it should remain unaltered.


ARTIFICIAL GREEN EARTH (GREEN OCHRE)

A product sometimes put on the market as green earth or green ochre has
nothing beyond its name in common with green earth properly so called,
except a certain similarity in colour. This pigment is prepared by
mixing yellow ochre to a thin pulp with water and adding about 2% (of
the weight of ochre) of hydrochloric acid. After a few days, a solution
of 2 parts of yellow prussiate of potash is added, and if the liquor
still gives a precipitate when tested with ferrous sulphate, this
last-named salt is added so long as such a precipitate continues to
form.

The deposit is washed, and dried in the ordinary way. When the right
proportions have been taken, a pigment is obtained that coincides
fairly in point of tone with true Verona earth. It is, however,
inferior in point of permanence, the Berlin blue present being somewhat
unstable and decomposing very quickly when brought into contact
with lime. The reaction taking place in the production of so-called
“artificial Celadon green” is that the hydrochloric acid used dissolves
ferric oxide from the ochre, the addition of the yellow prussiate of
potash then forming a blue precipitate of Berlin (Paris) blue which,
in conjunction with the yellow of the ochre, gives a green-coloured
mixture.


MALACHITE GREEN

Although the pigment sold under this name is nearly always an
artificial product, it cannot be omitted from a work dealing with the
earth colours, because, in former times, it was prepared exclusively
from the mineral malachite. Owing to the fact that artificial malachite
green is one of the cheapest of colours, the troublesome work involved
in the mechanical preparation of the native pigment has been almost
entirely abandoned, and the malachite itself is now utilised to greater
advantage as a source of copper.

Malachite green (or mountain green) is found in nearly every case where
copper ores exist, and is still--though very rarely indeed--prepared,
in a few places, from the mineral, the dark-coloured lumps being picked
out because the lighter-coloured ones would furnish much too pale a
colour.

The treatment of malachite for the preparation of pigment presents
certain difficulties owing to the comparative hardness (3·5–4) of the
mineral, which is also rather heavy (sp. gr. 3·6–4·0). On the large
scale, the selected mineral is first put through a stamping-mill, and
then ground, very hard stones being required for this purpose. The fine
product from this (usually wet) process is levigated and dried.

The pit water of some copper mines contains certain quantities of blue
vitriol (copper sulphate) in solution; and such pit water is generally
treated for the recovery of a very pure form of copper, the so-called
cementation copper. The liquor might also be worked up into malachite
green, by collecting it in large tanks and precipitating the dissolved
copper oxide with milk of lime, the bluish-green deposit separating
in association with gypsum being transformed into a light malachite
green by washing and drying. A darker green, free from gypsum, could be
prepared by using a solution of carbonate of soda as precipitant.

Neither the native nor the artificial malachite green is particularly
handsome in colour; and both possess, in addition, the unpleasant
property of gradually going off colour in the air, all the copper
compounds being quite as sensitive to sulphuretted hydrogen as those of
lead, and finally turning quite black under the influence of that gas.




CHAPTER IX

BLUE EARTH COLOURS


Only three minerals are known to be suitable as pigments; and indeed,
at present, only two, the third, lapis lazuli, being now of merely
historical interest. Nowadays, no one would think of using this rare
and expensive mineral as a pigment, since ultramarine, which has the
same pigmentary properties, is extremely cheap, whereas the pigment
from lapis lazuli was worth its weight in gold. The only two blue earth
colours of any interest at present are malachite (copper) blue, and the
blue iron earth Vivianite; and even these, though by no means rare, are
little used, since artificial blues are now made which are far superior
in beauty and can be obtained so cheaply that the natural pigments are
put out of competition.


MALACHITE BLUE (LAZULITE)

Lazulite and malachite (mountain blue) are of frequent occurrence in
copper mines, and the former is distinguished by its beautiful azure
blue colour, which, however, suffers considerably when the crystals
are reduced to powder. Both minerals are very similar in chemical
composition, and consist of cupric carbonate. The formula of malachite
is 2CuOCO{2} + H{2}O, that of lazulite being 3CuO(CO{2}){2} + H{2}O,
so that the only difference between them is that of the relative
proportions of the substances in combination. Lazulite is also rather
hard (3·5–4·0), but owing to the small size and brittle character
of the crystals it is not very difficult to pulverise. In the air,
malachite blue behaves in much the same way as malachite green, turning
black in presence of sulphuretted hydrogen.

Malachite blue is chiefly used for indoor work, and also as a water
colour; but it is always rather pale and dull.


VIVIANITE

This mineral--also termed blue ochre--is a transformation product of
various iron ores, and occurs native as fairly extensive deposits in
some places, especially in peat bogs. It forms ill-defined crystals,
which are of a low degree of hardness (2·0) and vary in specific
gravity between 2·6 and 2·7. The colour of the freshly won mineral is
whitish or pale blue, but soon changes to a dark blue in the air, owing
to the oxidation of the ferrous phosphate, originally present, into
ferric phosphate.

Vivianite can be transformed into a pigment by a simple process of
crushing and levigation; but the product is never very handsome,
and, at best, is only suitable for quite common paint work, though
characterised by considerable stability.




CHAPTER X

BLACK EARTH COLOURS


Only two minerals are known that can be used as black earth colours,
namely black chalk or shale black, and blacklead or graphite. Whereas
the former of these is of merely subordinate importance, most of the
black chalks being prepared artificially, graphite is all the more so
because it is employed, not only as the sole material for lead pencils,
but also for making graphite crucibles, as blacklead stove polish, as
a lubricant, etc. One of its numerous applications is in connection
with the electro deposition of metals, its high electrical conductivity
causing it to be used for coating the interior of the moulds in which
this deposition is effected.


GRAPHITE

Graphite, also known as plumbago or blacklead, consists of carbon. It
is usually spoken of as pure carbon, but from a very large number of
carefully conducted analyses, it would appear that native graphite is
never quite pure, even the finest grades of the mineral containing
96·8% of carbon at the most. The accompanying substances--which in
some cases form nearly 50% of the whole--are of divergent composition
and consist of iron, silica, lime, magnesia and alkalis. Even the
combustible constituent of graphite is not pure carbon, but always
contains a certain--though small--proportion of volatile substances.
These slight traces of volatile matter are of considerable importance
in connection with the hypothesis on the origin of the mineral.

Contrary to the old idea, it is now almost universally considered that,
instead of being of volcanic origin, graphite consists of the remains
of long-dead organisms, and in this respect is closely related to
coal. This hypothesis, however, fails to explain one point, namely the
crystalline nature of graphite; for even anthracites, which form the
oldest coals known to have had their origin in the decomposition of
organic substances, do not reveal the faintest traces of crystalline
structure. The upholders of the theory that graphite was formed by
the action of plutonic forces adduce, in support, the fact that
graphite can actually be produced, in certain chemical processes, at
high temperature. Molten cast-iron in cooling causes the separation
of carbon in the form of graphite; and the same substance is also
formed, in large quantities, in gas retorts, through the decomposition
of certain carbonaceous compounds when brought into contact with
the glowing walls of the retorts. Recent investigations, however,
have shown that the temperature necessary for the transformation of
non-crystalline carbon into crystalline graphite is by no means so
high as was formerly supposed; and it is now known that the change
takes place at as low as red heat. Possibly the two theories could be
reconciled by the assumption of a very old coal--such as is found, for
instance, as anthracite in many parts of the world--being so strongly
heated, by plutonic action, as to change into graphite.

Native graphite crystallises in the form of hexagons, mostly tabular;
but really well-developed crystals are of extremely rare occurrence,
and by far the greatest quantities of this mineral are found in the
condition of dense lumps, in which only the crystalline structure, and
not any decided crystals, can be discerned. The hardness of the mineral
fluctuates within fairly wide limits, ranging from 0·5 to 1·0. The
sp. gr. averages 1·8018–1·844, but, in the case of impure lumps may
increase to 1·9–2·2.

The following analyses will give some idea of the considerable
divergence existing between graphites from different deposits:--


SIBERIAN GRAPHITE

                                   1        2
  Carbon                         94·28    40·55
  Ash                             5·72    56·56
  Water                           --       2·80


PORTUGUESE GRAPHITE

  Carbon                                  42·69
  Water (chemically combined)              3·96
  Ash                                     53·35


BOHEMIAN GRAPHITE

                                   1        2
  Carbon                         61·01    69·04
  Alumina                         7·80     6·86
  Silica                         17·34    14·18
  Magnesia                        1·03     0·53
  Lime                            2·56     0·80
  Ferric oxide                    5·54     4·00
  Potash                          0·87     0·91
  Water and volatiles             3·24     2·89
  Sulphur                         0·51     0·62


GRAPHITES FROM UPPER STYRIA

                                   1        2        3
  Carbon                         85·00    87·16    82·21
  Ash                            14·89    12·66    17·92

                                   4        5        6
  Carbon                         82·40    81·10    55·50
  Silica                         12·38    11·61    21·00
  Alumina                         3·90     5·60    14·56
  Ferric oxide                    0·53
  Manganese protoperoxide         0·62     2·00     4·84
  Lime                            0·02     2·00     4·84
  Alkalis                         Trace   Trace     0·62
  Sulphur                           --      --      0·30
  Loss on incineration              --      --      2·43

Of these Styrian specimens, Nos. 1–4 are crude kinds, of sp. gr.
2·1443; No. 5 was levigated in the laboratory, and No. 6 was levigated
from an inferior quality at the mine.

According to the character of the crystalline structure, the colour of
graphite varies, but is mostly deep black. Very pure specimens, such
as the beautiful graphite blocks (from the renowned Alibert graphite
mines in Siberia) which, as a rule, are only to be seen in exhibitions
and mineralogical collections, have the appearance of unpolished steel
or white pig iron (spiegeleisen). The most important property of native
graphite is its low hardness and cohesion, in consequence of which it
leaves a streak when drawn over the surface of paper.

Graphite seems to be of frequent occurrence all over the world, though
only few deposits are known which yield a product that is suitable for
all the purposes to which graphite is applied.

In European countries, Austria is particularly rich in graphite; and
very large deposits of this mineral are found in Bohemia. Considerable
deposits also occur in Bavaria, where they have long been worked.
English graphite is celebrated for its excellent quality. All these
European deposits, however, are surpassed, both in extent and in the
quality of their products, by those discovered in Siberia, the largest
being that producing the aforesaid Alibert graphite and situated, near
the Chinese frontier, in eastern Siberia. At one time, America imported
all her blacklead pencils from Europe, having, at that period, no known
graphite deposits furnishing a suitable product. At present, however,
deposits of this kind have been found in California, and there can be
little doubt but that many others of this valuable mineral remain to be
discovered in that enormous continent, the geological investigation of
which is still far from being complete.

The graphite of some deposits is so highly contaminated by extraneous
minerals that it cannot be utilised, since the cost of purification
would exceed the value of the product. On the other hand, the purer
kinds, when suitably refined, yield a graphite that is fully adapted to
all requirements.

The refining process may be either chemical or mechanical, the choice
of methods depending entirely on the character of the associated
minerals. If these mainly consist of coarse, stony fragments,
preference should be given to mechanical treatment; but if they are of
such a character that they cannot be eliminated in this way, chemical
methods must be employed. Sometimes the two systems are combined, by
first subjecting the graphite to a rough mechanical purification, and
then completing the operation with chemical reagents.

The mechanical treatment consists in first removing as many of the
impurities as possible by hand-picking, and grinding the remainder in
edge-runner mills, along with water. The turbid liquid, containing the
powdered graphite and extraneous minerals in suspension, is led through
long launders, the sides of which are notched at intervals to allow
the water to overflow into large pits. The graphite settling in the
first of these pits contains numerous particles of the heavy associated
minerals; but that remaining suspended in the water and carried on to
the further pits constitutes the bulk. The water is left to clarify
completely in the pits, and is then drawn off, the pasty residue being
shaped into prisms, which are compressed under heavy pressure, to
increase their density, when partially dry.

Although levigation will remove most of the accompanying extraneous
minerals, it cannot eliminate the ash constituents of the graphite.
Experiments made in this direction have demonstrated that the ash
content of the levigated graphite is exactly the same as that of the
crude material. Whilst these ash constituents do not affect the quality
of graphite for certain of its uses, they nevertheless impair its
beautiful black colour to a considerable degree. The chemical treatment
necessary to eliminate these constituents is attended with many
difficulties, the chief of which resides in the fact that the ferric
oxide present is in a form that is not readily accessible to the action
of chemicals. For this reason, attempts to purify graphite with crude
hydrochloric acid are hardly likely to prove successful, since both
the ferric oxide and the accompanying silicates obstinately resist the
action of this acid.

In order to obtain graphite of a high state of purity, the attempt must
be made to bring this ferric oxide and the silicates into a soluble
condition. This can be accomplished in various ways, and the choice
of the method will depend on the purpose for which the graphite
is intended. For example, the operations may either be confined to
purification, or else include the attainment of a maximum condition
of subdivision. When foliaceous graphite has to be treated--and this
kind of graphite cannot, in its original condition, be used for making
lead pencils--it is preferable to employ a method which will produce
both the above results. The purification may consist in crushing the
graphite to powder, and fusing this with a mixture of sulphur and
carbonate of soda, whereby the silicates present are converted into
soluble compounds, and the ferric oxide into ferric sulphide. On
extracting the melt with water, a portion of the contained salts pass
into solution and is carried off. The residue is then treated with
dilute hydrochloric acid, which dissolves out the ferric sulphide, with
liberation of sulphuretted hydrogen, and leaves the graphite in a very
pure condition after washing.

In order to render foliaceous graphite suitable for lead pencils, a
different method is pursued, but should only be employed in special
circumstances, on account of the expense entailed.

According to the process recommended by Brodie, the graphite, ground to
coarse powder, is mixed with about one-fourteenth of its own weight of
chlorate of potash, this mixture being heated, with two parts by weight
of sulphuric to each part of graphite, in a water bath so long as fumes
of hypochlorous acid continue to be disengaged. The heating must be
performed in stoneware or porcelain vessels, those made of any other
materials being strongly corroded by the chlorine compounds formed.

When the evolution of fumes ceases, the mass is allowed to cool,
and is carefully washed with a large volume of water, the residue
being then dried and heated to redness. During this calcination the
graphite undergoes a peculiar change, increasing considerably in bulk
and forming an exceedingly soft powder which, after another washing,
consists almost entirely of chemically pure carbon.

Graphite purified in this way can be used for any purpose for which
this material is employed, and may be made into the finest lead
pencils. However, as already mentioned, this process is usually too
expensive for general application.

The use of graphite for writing is more ancient than is usually
supposed, having been tentatively employed between 1540 and 1560. It
was during this period that the graphite mines in Cumberland were
discovered; and the extremely pure graphite found there soon began to
be used as a writing material.

Up to the close of the eighteenth century, lead pencils were made by
selecting pure lumps of graphite and sawing them into thin rods, which
were then encased in wooden sticks. Apart from their high price, these
pencils exhibited various defects, one of the chief being that a stick
of such pencil was seldom of uniform hardness throughout its length,
most of them being so soft in parts as to make a deep black, smeary
mark, whilst other parts would hardly give any mark at all.

The defects inherent in native graphite are completely removed by the
method now generally employed in making lead pencils; and on this
account the old process of sawing the lumps has been abandoned.

Graphite with a fine earthy texture alone is suitable for lead pencils,
scaly varieties being useless for this purpose, unless specially
prepared, since they will not give a solid black streak. By means
of the Brodie process, however, even the most highly crystalline
kinds can be rendered suitable for this purpose. Siberian graphite
is distinguished by extremely high covering power, and is specially
preferred for the manufacture of pencils. Excellent varieties for this
purpose are also found in many parts of Europe; and indeed, a large
proportion of all the lead pencils used throughout the world are made
from Bohemian, Styrian and Bavarian graphite.

At present, all pencils are made from ground graphite, the extremely
finely ground and levigated material being kneaded into a paste with
clay. This operation fulfils a twofold purpose, the plasticity of the
clay increasing the cohesion of the individual particles of graphite,
whilst the amount of clay used determines the hardness of the pencil.

The larger the proportion of clay, the harder the pencil when baked,
and therefore the paler the mark the pencil will make on paper. In the
pencil factories, the clay is incorporated in special machines; and the
operation requires extreme care, since only a perfectly uniform mixture
will give a composition of regular character in all cases.

The intimately mixed material is formed into thin rods, which are
dried and then baked, the heat driving out the water in the clay and
transforming it into a solid mass.

An addition to this main application of graphite, the mineral is also
used for making crucibles, chiefly for melting the noble metals.
Crucibles of this kind are largely manufactured near Passau, Bavaria,
and similar crucibles are made in England from Ceylon graphite.

Another important use for graphite is as a coating for iron articles to
protect them from rust. For this purpose, however, only the inferior
kinds are employed; and these can also be made up into excellent
cements capable, in particular, of offering considerable resistance to
the action of heat and chemicals.

To complete the tale of the applications of graphite, its employment
as a lubricating agent for machinery, especially for reducing friction
in machines made of wood, may be mentioned. Latterly also, the finest
levigated graphite has come into use, in admixture with solid fats or
mineral oils, for lubricating large engines, for which purpose it is
excellently adapted.


BLACK CHALK

Black chalk, slate black, Spanish chalk, crayon, etc., is not a chalk
at all, in the mineralogical sense, but consists of clay shale of
varying colour. Some kinds of this shale are pure black, almost velvet
black, and these are considered the best. Others have a more greyish or
bluish tinge and are of low value as pigments.

The purer the black, the finer the grain of the material, and therefore
the greater its value to the colour-maker. The variety obtained from
Spain is generally admitted to be the best, and for this reason the
name of Spanish chalk has been applied to all similar minerals.

In all cases the black colour of Spanish chalk is due to carbon;
but the particular modification of carbon present has not yet been
accurately identified. According to some, it is chiefly graphite,
whereas others ascribe the colour to amorphous carbon. Apparently, the
material found in different deposits contains either one or the other
of these modifications of carbon.

Deposits of black chalk are fairly plentiful, but in many of them the
material is so contaminated with extraneous minerals that a somewhat
troublesome method of preparation is needed to fit them for the purpose
of the draughtsman. With this object, the native product must be ground
extremely fine, and the powder levigated; and owing to the expense of
these processes, they are now seldom used, it being possible to obtain
a good black chalk far more cheaply than by levigating the natural
material.

This artificial black chalk is prepared by mixing ordinary white chalk,
or white clay, with a black colouring matter, shaping the mass into
prisms, and sawing these into suitable pieces when dry. The white
pigment may either be mixed with some very deep black substance,
such as lampblack, or stained with an organic dyestuff, which is, in
reality, not black, but either very dark blue or green.

The usual colouring matter used with white chalk is lampblack, mixed
to a uniform paste with thin glue, a suitable amount of clay or chalk
being incorporated with the mass. The production of a perfectly
homogeneous mixture entails subjecting the paste to a somewhat
protracted mechanical treatment. When the mass has become perfectly
uniform throughout, it is shaped into prisms, which are exposed to the
air to dry and are then cut up with a saw. Instead of prisms, the mass
can be shaped into thin sticks, which dry more quickly.

A very handsome black chalk can be made, with comparatively little
trouble, by treating chalk with a suitable quantity of logwood
decoction previously mixed with sufficient green vitriol solution to
render the liquid a deep black. This liquid is added to the dry chalk,
intimately mixed therewith, and the pasty mass shaped into sticks.
The colouring agent may be replaced by a solution of logwood extract
blackened by the addition of a small quantity of chromate of potash; or
black dyestuffs may be used.




CHAPTER XI

THE COMMERCIAL NOMENCLATURE OF THE EARTH COLOURS


Mention has already been made of the great confusion prevailing in the
nomenclature of pigments, and that many of these are put on the market
under a variety of names taken from different languages.

Although the number of the earth colours is far smaller than that of
the artificial colouring matters, the nomenclature is in a no less
confused condition.

Most frequently, earth colours are named after the localities where
they are either discovered or prepared, in combination with the word
indicating the colour of the product--for example: Cologne white,
Vienna white--or the term “earth” (Verona earth, Veronese green, etc.).
Whilst these names give, to some extent, an indication of the nature of
the pigment, others have no reference to it at all; such as colcothar,
bole, umber, etc. Finally, a number of other names in use are
calculated to produce the impression that the earth colours in question
are of an entirely different nature to their real one. As an example,
we may cite the name “French chalk,” which is not a chalk at all, but
consists of the mineral talc. Black chalk, again, is not chalk (calcium
carbonate), but a black shale; and graphite is often termed blacklead,
although it contains no lead at all, and the name is merely a survival
from the time when pencils of metallic lead were used for drawing.

In order to bring some kind of order into the various names which
are applied to the earth colours, a list of those in current use is
appended. Many of these names, it may be stated, have been selected in
a purely arbitrary manner, some manufacturers, for instance, selling
ordinary chalk under a variety of foreign names, for the purpose of
thereby obtaining higher prices. These borrowed names would seem to be
superfluous, to say the least. Pure and properly levigated chalk is
the same article everywhere, whether prepared from English, French or
German limestone; and in all cases the simple name, “chalk,” with an
explanatory “single,” “double,” or “triple” levigated, should be quite
sufficient.

In the case of earth colours that are really obtained of special
quality in certain localities, such as terra di Siena, green earth
from Verona, or the like, the corresponding name might be retained,
even if the pigment did not originate from the locality in question,
as a generic term for a pigment possessing certain properties and of a
certain composition.

In the following classification, the names of the earth colours are
given in accordance with their colour and chemical composition.


WHITE EARTH COLOURS


_Carbonate of Lime_:

Chalk; levigated chalk; Vienna white; Spanish white; marble white;
artists’ white; Bougival white; Champagne chalk; Paris chalk; Cologne
chalk; Mountain chalk; craie; blanc minéral; Blanc de Champagne; Blanc
de Meudon; Blanc de Bougival; Blanc de Troyes; Blanc d’Orleans; Blanc
de Rouen; Blanc de Briançon.


_Basic Carbonate of Lime_:

Vienna white; Vienna lime; pearl white; whiting; Blanc de chaux; Blanc
de Vienne.

_Note._--The calcareous marls, consisting of carbonate of lime and
clay, are also frequently sold under the above names, the same being
the case with gypsum.


_Silicate of Alumina_:

White earth; pipeclay; Dutch white; Cologne earth; terre d’Argile;
Argile blanc; Terre blanche.


_Silicate of Magnesia_ (mineralogically, talc and soapstone):

Talc; Venetian earth; French chalk; Venetian white; glossy white;
feather white; shale white; face-powder white; Blanc de Venise; Blanc
d’Espagne; Blanc de fard.

_Note._--Fine grades of white lead are also sold as Venetian white,
Spanish white and shale white; but can easily be recognised by their
weight. The term “prepared” white, frequently applied to earth colours
in the trade, usually indicates that the material in question has been
either levigated, ground or burnt--in short, put through some kind of
preparatory treatment--and is therefore in frequent use for all the
colours.


_Barium sulphate_:

Heavy spar; barytes; heavy earth; mineral white.


_Precipitated colours_:

Permanent white; blanc fixe.


YELLOW EARTH COLOURS

_Ferric hydroxide_, with admixtures of ferric oxide, clay, lime, ferric
silicate, basic ferric sulphate, etc.

Ochre; iron ochre; golden ochre; satin ochre (satinober); pit ochre;
vitriol ochre; Mars yellow; Chinese yellow; Imperial yellow; permanent
yellow; terra di Siena; umber; Italian earth; Roman earth; bronze
ochre; oxide yellow, etc.

Yellow ochre; Jaune de Mars; Terre d’Italie.


_Ferric Silicate_:

Yellow earth; Argile jaune; yellow wash.


RED EARTH COLOURS

_Ferric oxide_ (with alumina and silica).

Bolus; bole; Terra sigillata; Lemnos earth; red chalk; raddle; Striegau
earth.


_Ferric oxide_:

Colcothar; English red; angel red; Pompeii red; Persian red; Indian
red; Berlin red; Naples red; Nuremberg red; crocus; chemical red;
Crocus Martis iron saffron; caput mortuum; raddle; rouge de fer; Rouge
de Perse; Rouge des Indes; Rouge de Mars; Rouge d’Angleterre.


BROWN EARTH COLOURS


_Ferric oxide_:

Ferric hydroxide; Ferric silicate (conf. Yellow Earth Colours, which
are often sold under the same names as the browns. The paler kinds are
usually called “pale” or “golden,” such as pale ochre, golden ochre,
etc.). Terra di Siena; burnt Siena; satinober; mahogany brown; Vandyke
brown.


_Ferric silicate, Clay_:

Umber; umber brown; Roman earth; Roman umber; Turkish brown; Sicilian
brown; Cyprus earth; chestnut brown; burnt umber; ombre; Terre d’ombre;
Ombre brulée.


_Organic decomposition products_:

Cologne umber; Cologne earth; Cassel brown; Spanish brown; mahogany
brown; Vandyke brown; brown carmine; Terre brun de Cologne; Brun de
Cologne; Brun d’Espagne; Ombre de Cologne; Brun de Cassel; Terre
d’Ombre; Cologne brown.


_Asphaltum_ (_mineral rosin_):

Asphaltum brown; bistre; earth brown; bitumen; pitch brown; Asphalte;
Brun de bitume; Bitume.


GREEN EARTH COLOURS


_Ferrous oxide with silica, alumina, lime, etc._:

Green earth; Verona green; Celadon green; Verona earth; Italian green;
stone green; Bohemian earth; Cyprus earth; Tyrol green; permanent
green; green ochre; Terre verte; Terre de Vérone; Vert d’Italie.


_Cupric carbonate_:

Malachite green; mountain green; Hungarian green; copper green; mineral
green; Tyrolese green; shale green; Vert de montagne; Vert d’Hongrie.


BLUE EARTH COLOURS


_Cupric carbonate_:

Malachite blue; mountain blue; lazulite blue; azure blue; mineral blue;
copper blue; Hamburg blue; English blue; Cendres bleues; Bleu d’azure;
Bleu de cuivre; Vert-de-gris bleu; blue verditer.


GREY EARTH COLOURS


_Grey clay shale_:

Mineral grey; silver grey; stone grey; slate grey.


BLACK EARTH COLOURS


_Carbon_:

Graphite; blacklead; plumbago; iron black.


_Clay shale_:

Black chalk; slate black; Spanish black; Spanish chalk; oil black;
Schiste noir; Noir d’Espagne.




INDEX


  Alabaster. _See_ Gypsum.

  Alumina, silicate of, 21, 22

  Aluminium-potassium silicate, 21

  Alum sludge, 32

  ----, artificial ochre from, 148

  ----, ferric oxide pigments from, 164–167

  Ammonium salts, artificial ochre from, 145, 146

  Anhydrite, 19

  Anthracolite, 13

  Arragonite, 13

  Asphaltum, 37, 38.
    _See also_ Bitumen.

  ----, brown, 174, 175

  Augite, 179

  Azurite, 33


  Ball Mills, 55–56

  Barium carbonate, 20

  ---- sulphate.
    _See_ Barytes.

  Barytes, 19, 20, 119–122

  ----, artificial, 133

  ----, correcting colour of, 121

  ----, detecting, in white lead, 120

  ----, low covering power of, 121

  Black chalk, 38

  ---- earth colours, 185–196

  ---- ----, trade names of, 202

  ---- earths, 6, 38–39

  ---- schist, 38

  Bitumen, 174, 175

  Blanc fixe, 19

  Blue earth colours, 183–184

  ---- ----, trade names of, 202

  ---- earths, 4, 33–36

  Bole, 31, 32, 152–154

  Bone breccia, 13

  Brown coal, pigments from, 175

  ---- earth colours, 168–175

  ---- ----, trade names of, 200

  ---- earths, 5, 36–38


  Calcareous marl, 110, 111

  ---- tuff, 12

  Calcining, 81

  ---- Ferric oxide, 161–164

  ---- furnaces.
    _See_ Furnaces.

  ---- lime, 88–90

  ---- ochre, 132–136

  Calcite, 11, 12, 14, 15

  Calcium carbonate, 12, 14, 15, 16

  ---- ----, action of acids on, 15

  ---- hydroxide, 16

  ---- sulphate.
    _See_ Gypsum.

  Calc sinter, 12

  ---- spar.
    _See_ Calcite.

  Caledonian brown, 36

  Cappagh brown, 36

  Caput mortuum. _See_ Colcothar.

  Carbon brown, 37

  ---- in limestone, 16

  Cassel brown, 37, 38, 174

  Celadon green. _See_ Green earth.

  Chalk, 13

  ----, black, 194–196

  ----, colour of, 103

  ----, correcting colour of, 104, 105, 106

  ----, covering power of, 106

  ----, grinding, 101

  ----, impurities in, 103, 104

  ----, precipitated, 107–109

  ----, preparation and properties of, 98–106

  Classification of earth colours, 4–8

  Clay, 21–23

  Clay, formation of, 113

  ----, impurities in, 114–119

  ---- in ochre, 128

  ----, levigating, 114–117

  Colcothar, 160, 161, 162

  Cologne earth, 173, 174

  Commercial nomenclature of earth colours, 197–202

  Crushers and Breakers, 43–45

  Crushing, 77–80

  ---- machinery, 43–60


  Disintegrators, 58–60

  Distemper, weatherproof, 94

  Dolomite, 18

  Draining and Drying, 66–77

  Drying appliances, 73–77

  Dyestuffs for improving earth colours, 85


  Edge runners, 48–55

  English red, 160


  Ferric hydroxide in ochre, 128–132

  ---- oxide, artificial ochre from, 143–144

  ---- ---- as by-product, 30

  ---- ----, burnt, 158–164

  ---- ----, calcining, 161–164

  ---- ---- in lime, detection of, 91

  ---- ----, native, as pigment, 154

  ---- ---- pigments from alum sludge, 164–167

  ---- ----, range of colours, 29

  ---- ---- shading, 28

  ---- ----, violet shades from, 164

  Ferrous sulphate, artificial ochre from, 139–143, 146–148

  Filter-cloths, cleaning, 72

  Filter-presses, 70–73

  Furnaces, calcining, 158, 162, 163, 166


  Granulator, 43

  Graphite, 38, 39, 185–194

  ---- as a lubricant, 194

  ---- as anticorrosive, 194

  Graphite in the manufacture of lead pencils, 191–193

  ---- for crucibles, 193

  ----, refining, 189–192

  Green earth, 176–180

  ---- ----, artificial, 180

  ---- ----, improving, 178

  ---- ---- colours, 176–184

  ---- ----, trade names of, 201

  ---- earths, 5

  Grey earth colours, trade names of, 202

  Grey earths, 38

  Gypsum, 18, 19, 111, 112


  Heavy spar. _See_ Barytes.

  Hematite, 155

  ----, brown, 23, 30, 31

  ----, red, 28, 30

  Hydro-extractor, 66–70


  Improving earth colours, 84, 85

  Indian red, 29, 160

  Iron cream, 29

  ---- glance, 154

  ---- in limestone, 17

  ---- ore, bog, 25, 31

  ---- ----, micaceous, 28

  Ironstone, brown, 23, 24, 25

  ----, clay, 24

  ----, red, 28–30


  Kaolin, 21, 22, 112–119


  Lazulite, 183

  Lemnos earth. _See_ Bole.

  Levigation, 60–65

  Lime, absorption of carbon dioxide by, 93

  ----, action of, on casein, 94

  ----, ----, on colours, 93, 98

  ----, calcining, 88–90

  ----, caustic, preparation of, 87–94

  ----, double compound of oxide and carbonate, 93

  ---- from mussel shells, 98

  ----, impurities in, 91, 92

  ---- in clay, 22

  ---- ----, eliminating, 117–119

  ---- in ochre, 129

  Lime in the preparation of artificial ochre, 140–144

  ----, moulding, 96–98

  ----, quick, 16

  ----, slaked, 16

  Limestone, 11–18

  ----, suitability of, for colour-making, 92

  Limonite, 25


  Magnesia, carbonate of, 123, 124

  ---- in lime, 91

  ---- in limestone, 17

  Magnesium silicate, 21

  Malachite, 35

  ---- blue, 183

  Marble, 11, 14, 15

  Minerals, testing for suitability as pigments, 172

  Mine sludge, 32

  Mixing earth colours, 81–84

  Moulding, 85, 86

  Mountain chalk, 12

  ---- milk, 12

  Muffle, burning ochre in the, 158–160

  Muriacite, 19

  Muschelkalk, 13


  Ochre, 24, 25, 26

  ----, blue.
    _See_ Vivianite.

  ----, calcining, 132–136

  ----, English, 138

  ----, green, 180

  ----, pit, 148–150

  ----, Roman, 137, 138

  ----, Siena, 137, 138

  ----, testing, 130–132

  ----, toning with chalk, 144

  ----, toning with clay, 144

  ----, vitriol, 146–148

  Ochres, 128–150

  ----, artificial, 138–146

  ---- as by-products, 146–150

  ----, burnt, 158–164

  ---- from various deposits, 136–138

  ----, Italian, 137, 138

  Oolithic limestone, 13

  Organic matter in lime, 91


  Pastel crayons, 126

  Pearl white, 94

  Permanent white, 19, 122

  Pipeclay. _See_ Kaolin.

  Preparation of colour earths, 40–86

  Pulverisers, 56–58


  Raddle, 29, 155–158

  ----, impurities in, 156

  ----, testing, 157

  Raw materials for earth colours, 8–39

  Red earth colours, 151–167

  ---- ----, trade names of, 200

  Red earths, 4, 27–33


  Sampling raw earths, 9

  Selenite, 18

  Shading pigments with permanent white, 19

  Siena, Terra di, 25, 26, 27, 168–170

  ----, ----. _See also_ Italian ochre.

  Siderosilicate, 171

  Sifting, 77–80

  Soapstone, 20, 21.
    _See_ also Steatite.

  Spanish brown, 174

  Sprudelstein, 15

  Steatite, 20, 21, 125, 126

  Stamps, 45–48


  Talc, 20, 21, 124, 125

  Terra sigillata. _See_ Bole.

  Testing purity of raw earths, 10

  Trade names of earth colours, 197–202


  Ultramarine, 33

  Umber, 36, 170–174

  ----, Cologne, 173, 174

  ----, true, 170–173


  Vandyke Brown, 38, 174

  Vermilion, 151

  Verona earth. _See_ Green earth.

  Vienna white, 95–98

  Vivianite, 33, 34, 184


  White earth colours, 87–126

  White earth, trade names of, 111, 198, 199

  White earths, 4

  White raw materials and pigmentary earths, 11–23

  Witherite, 20

  Working earth colour deposits, 9


  Yellow earth colours, 127–150

  ---- ----, trade names of, 100

  Yellow earth, 150

  Yellow earths, 4, 23–27


       PRINTED IN GREAT BRITAIN BY RICHARD CLAY & SONS, LIMITED,
         PARIS GARDEN, STAMFORD ST., S.E., AND BUNGAY, SUFFOLK.




                        The Manufacture of Paint

                    THE PRACTICAL HANDBOOK FOR PAINT
                 MANUFACTURERS, MERCHANTS AND PAINTERS

                                   BY

                      J. CRUICKSHANK SMITH, B.Sc.

                  Second Edition--Revised and Enlarged

                 DEMY 8VO. 73 ILLUSTRATIONS. 300 PAGES


CONTENTS

  CHAP.       PART I

     I.--SCOPE OF SUBJECT AND DEFINITION OF TERMS.
    II.--STORING AND HANDLING RAW MATERIAL.
   III.--TESTING AND VALUATION OF RAW MATERIAL.


  PART II

    IV.--PLANT AND MACHINERY.


  PART III

     V.--THE GRINDING OF WHITE PIGMENTS.
    VI.--THE GRINDING OF EARTH PIGMENTS.
   VII.--THE OXIDE OF IRON PIGMENTS.
  VIII.--THE GRINDING OF BLACK PAINTS.
    IX.--THE GRINDING OF CHEMICAL COLOURS.
     X.--THE GRINDING OF PIGMENTS IN WATER.
    XI.--THE GRINDING OF COLOURS IN TURPENTINE, GOLD
           SIZE, AND SPECIAL MEDIUMS.


  PART IV

   XII.--MIXED OR PREPARED PAINTS.
  XIII.--ENAMELS AND ENAMEL PAINTS.


  PART V

   XIV.--MODERN CONDITIONS WHICH AFFECT THE SELECTION
           AND APPLICATION OF PAINT.

    XV.--THE DESIGNING, TESTING AND MATCHING OF PAINTS.

   XVI.--COST CHARGES--COST OF HANDLING--CARRIAGE AND
           DELIVERY OF GOODS--COST OF MATERIALS--MACHINERY
           AS AFFECTING MANUFACTURING COST--ELECTRICITY AS
           MOTIVE POWER--MANUFACTURING ONCOST--PRICES--THE
           FUTURE OF THE INDUSTRY.

       Price 12s. 6d. net (Post Free, 13s. 3d. Home and Abroad).


                              PUBLISHED BY
                        SCOTT, GREENWOOD & SON,
                  8 Broadway, Ludgate, London, E.C. 4




                       The Chemistry of Pigments

           BY ERNEST J. PARRY, B.Sc., (LOND.), F.I.C., F.C.S.
                                  AND
                     JOHN H. COSTE, F.I.C., F.C.S.

          _Demy 8vo._      _5 Illustrations._      _280 Pages_


CONTENTS


Chapter I.--Introductory

Light--White Light--The Spectrum--The Invisible Spectrum--Normal
Spectrum--Simple Nature of Pure Spectral Colour--The Recomposition
of White Light--Primary and Complementary Colours--Coloured
Bodies--Absorption Spectra.


Chapter II.--The Application of Pigments

Uses of Pigments: Artistic, Decorative, Protective--Methods of
Application of Pigments: Pastels and Crayons, Water Colour, Tempera
Painting, Fresco, Encaustic Painting, Oil-Colour Painting, Ceramic Art,
Enamel, Stained and Painted Glass, Mosaic.


Chapter III.--Inorganic Pigments

White Lead--Zinc White--Enamel White--Whitening--Red
Lead--Litharge--Vermilion--Royal Scarlet--The Chromium
Greens--Chromates of Lead, Zinc, Silver and Mercury--Brunswick
Green--The Ochres--Indian Red--Venetian Red--Siennas and
Umbers--Light Red--Cappagh Brown--Red Oxides--Mars Colours--Terre
Verte--Prussian Brown--Cobalt Colours--Cœruleum--Smalt--Copper
Pigments--Malachite--Bremen Green--Scheele’s Green--Emerald
Green--Verdigris--Brunswick Green--Non-arsenical Greens--Copper
Blues--Ultramarine--Carbon Pigments--Ivory Black--Lamp
Black--Bistre--Naples Yellow--Arsenic Sulphides: Orpiment,
Realgar--Cadmium Yellow--Vandyck Brown.


Chapter IV.--Organic Pigments

Prussian Blue--Natural Lakes--Cochineal--Carmine--Crimson--Lac
Dye--Scarlet--Madder--Alizarin--Campeachy--Quercitron--Rhamnus--Brazil
Wood--Alkanet--Santal Wood--Archil--Coal-tar Lakes--Red
Lakes--Alizarin Compounds--Orange and Yellow Lakes--Green and Blue
Lakes--Indigo--Dragon’s Blood--Gamboge--Sepia--Indian Yellow,
Puree--Bitumen, Asphaltum, Mummy--Index.

       Price 12s. 6d. net (Post Free, 13s. 3d. Home and Abroad).


                              PUBLISHED BY
                        SCOTT, GREENWOOD & SON,
                  8 Broadway, Ludgate, London, E.C. 4




Transcriber’s Notes


Minor use of boldface in the advertisements is not shown in the Plain
Text version of this book.

Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in the original book; otherwise they
were not changed.

Simple typographical errors were corrected; unbalanced quotation
marks were remedied when the change was obvious, and otherwise left
unbalanced.

Illustrations in this eBook have been positioned between paragraphs
and outside quotations.

The index was not checked for proper alphabetization or correct page
references.