THE MYSTERIOUS BOX:
                        Nuclear Science and Art


                                   by
                             Bernard Keisch




                                Contents


  The Mysterious Box                                                   2
  How Old Is a Painting?                                              11
  Who Was the Artist?                                                 24
  Other New Tools for Art Authentication                              36
  One Mystery Solved                                                  42
  Reading List                                                        44

                 United States Atomic Energy Commission
                     Office of Information Services
           Library of Congress Catalog Card Number: 70-606040
                           1970; 1974 (rev.)


The Author

    [Illustration: Bernard Keisch]

Dr. Bernard Keisch received his B.S. degree from Rensselaer Polytechnic
Institute and his Ph.D. from Washington University. He is a Senior
Fellow with the Division of Sponsored Research of Carnegie-Mellon
University in Pittsburgh. He is presently engaged in a project that
deals with the applications of nuclear technology to art identification.
This is jointly sponsored by the U. S. Atomic Energy Commission and the
National Gallery of Art. Previously he was a nuclear research chemist
with the Phillips Petroleum Company and senior scientist at the Nuclear
Science and Engineering Corporation. He has contributed articles on art
authentication to a number of journals. For the AEC, in addition to this
booklet, he has written _The Atomic Fingerprint: Neutron Activation
Analysis_, _Secrets of the Past: Nuclear Energy Applications in Art and
Archaeology_, and _Lost Worlds: Nuclear Science and Archaeology_.




Nuclear energy is playing a vital role in the life of every man, woman,
and child in the United States today. In the years ahead it will affect
increasingly all the peoples of the earth. It is essential that all
Americans gain an understanding of this vital force if they are to
discharge thoughtfully their responsibilities as citizens and if they
are to realize fully the myriad benefits that nuclear energy offers
them.

The United States Atomic Energy Commission provides this booklet to help
you achieve such understanding.




The Cover

This painting, originally believed to be the work of the Dutch artist
Frans Hals (1580-1666), is a fake. Measurements of the naturally
radioactive isotopes, polonium-210 and radium-226, in lead white from
the paint proved that it was no more than 50 years old.

    [Illustration: _A Van Meegeren forgery of a Vermeer._]




                           The Mysterious Box


The New Jersey sun was high overhead and the day was hot. The three boys
walking along a deserted stretch of beach didn’t mind because they were
barefoot and in their swimsuits. Occasionally they would dash in and out
of the surf to cool off.

Suddenly Martin let out a yell as his toe hit something hard hidden in
the sand at the water’s edge. A moment later Bill and Harley were
helping Martin dig out a large wooden case. It was heavy, well built,
tightly sealed, and had foreign words written on it.

“Maybe it’s a pirate treasure chest,” said Martin, who was almost eight
and had just read _Treasure Island_ for the first time the week before.

“You’re crazy,” said Harley, who, nearly ten, was much older and wiser.

Bill, going on twelve, thought aloud, “It must be something worthwhile;
maybe we can sell it and buy those model rockets we wanted.”

The three boys soon found that they couldn’t open the box and that it
was too heavy to drag along the sand easily.

“Martin,” said Bill, “get Dad while Harley and I stand guard.”

Two hours later the box was at their house and everyone in the family
was trying to read what was written on it. About all that was readable
was a large “U” followed by what appeared to be two numbers. Some of the
other marks looked like old German script and there was a date, 1945.

“You know,” said Bill, “I bet that came from a World War II German
submarine that our Coast Guard or Navy sank.”

“Let’s open it up!” said Harley as Martin ran to get the screwdrivers.

Inside they found a thoroughly waxed carton that they had to cut open.
Everyone held their breath as their father lifted the top.

“Nothing but a bunch of pictures,” said Martin who was still hoping for
pirate treasure.

“Paintings can be worth a lot of money,” said Dad, “thousands or even
millions of dollars.”

“Well then we’re rich!” yelled Harley and Bill together.

“Not so fast,” said Dad. “First of all, we don’t know if the paintings
are really valuable. Also, it looks like these might be part of the art
treasures that the Nazis stole from the countries they conquered in
World War II. Maybe someone was trying to get them by submarine to a
neutral country, like Argentina, just before the end of the war, and the
sub was sunk. If they are real and stolen, they’ll have to go back to
their rightful owners. But cheer up, maybe there’s a reward.”

“How do we collect it?” asked Bill. “If the Nazis grabbed them, aren’t
they real for sure?”

“Not necessarily,” Dad continued. “The Nazis were fooled sometimes by
people who sold them fakes. There was one painting that Hitler’s
sidekick, Göring, bought that was supposed to be a 17th century painting
by Vermeer, a Dutch painter. Because Vermeer’s work is so valuable, it’s
usually impossible to buy one for any amount of money.

“Vermeer is regarded as a national hero by the Dutch. The matter was
investigated and the painting traced to Han Van Meegeren, a modern Dutch
painter who had only a fair talent. When Van Meegeren realized he might
be charged with treason by the Dutch for selling a Vermeer to the Nazis,
he confessed that he had painted it himself. He also confessed that he
had painted other forgeries that fooled some of the experts and were
sold for a lot of money.

“Many people, however, thought Van Meegeren was only lying to save
himself from the charge of treason, and the whole thing had to be
decided by a committee of scientific art experts appointed by a court of
law. Using the methods that were then available, the experts showed that
Van Meegeren had done a remarkable job of forgery and they were
convinced that he had been telling the truth about painting those
pictures.

“At the time, the important ways the experts used to examine a painting
included studying the work with X rays, which could show another
painting underneath, analyzing the pigments (or coloring materials) used
in the paint, and examining the painting for certain signs of old age.

    [Illustration: _Han Van Meegeren listens to the evidence at his
    trial in Amsterdam. In the background is “The Blessing of Jacob”,
    which was sold in 1942 as the work of Vermeer._]

    [Illustration: _An authentic Pieter de Hooch work, “The Card
    Players”, painted in the 17th century._]

    [Illustration: _A forgery of a Pieter de Hooch picture painted in
    the 20th century by Han Van Meegeren._]

    [Illustration: _“Head of Christ” by Van Meegeren._]

“Van Meegeren was well acquainted with these methods. He scraped the
paint from old paintings that weren’t worth much just to get the canvas
and tried to use pigments that Vermeer would have used. He knew that old
paint was very, very hard and impossible to dissolve; so he cleverly
mixed a chemical (phenolformaldehyde) into his paint, and this hardened
into Bakelite when he heated the finished painting in an oven.

“For some of the paintings, Van Meegeren became careless and the experts
did find traces of a modern pigment (cobalt blue) in the paint. They
also found the Bakelite. For one or more paintings, Van Meegeren did so
well that, in spite of all this evidence, a few people still weren’t
convinced that these paintings were painted by Van Meegeren and not by
Vermeer.”

Bill, who by this time was bursting with questions, interrupted, “You
mean they still aren’t sure about some of those paintings after 25
years? Aren’t there better ways of telling whether a painting is genuine
or not? You’re a scientist. Can’t scientists like you do something about
it now?”

“Yes, recently a method was developed to settle just such a question.
It’s based on measurements of natural radioactivity in one pigment that
all artists used hundreds of years ago. And the method was applied to
some of the Van Meegeren paintings including the best one of them all.”

“How did it come out?” asked Martin.

    [Illustration: _An X ray of part of the Van Meegeren forgery,
    “Christ and His Disciples at Emmaus”. In the white circle are traces
    of paint from the original painting that Van Meegeren scraped off to
    obtain the old canvas. When the painting was believed to be a
    genuine Vermeer, it was sold for about $300,000._]

    [Illustration: The complete painting.]

    [Illustration: _A Van Meegeren forgery of a Vermeer._]

“How does it work?” asked Harley.

“You mean paintings are radioactive?” exclaimed Bill.

“Can we do it to the paintings we found?” asked all three together.




                         How Old Is a Painting?


“One question at a time. I’ll tell you how the method works and what it
does if you’re really interested.”

“We’re interested! We’re interested!” chorused the boys.

“In the first place, this method works only in certain cases of
suspected forgery. Over the last 50 or 100 years, a number of paintings
have turned up that seemed, even to the best art experts, to be several
hundred years old. Some of these were genuine, and some were painted by
forgers who could not resist the high prices paid for works of art. The
National Gallery of Art, in Washington, D. C., thinking that there might
be a way of detecting these forgeries, gave its support to a group of
scientists who developed a method for this purpose.

“To understand how the method works, you need to know a little about how
radioactive atoms disintegrate to form atoms of other elements. In this
case we are interested in the natural radioactivity that occurs in
certain rocks. As a matter of fact, in almost all rocks in the earth’s
crust there is a certain small quantity of uranium.”

“I thought uranium was rare,” interrupted Bill.

“It is, but we’re talking about such small quantities that its difficult
for scientists using the most sensitive equipment to detect it. The
uranium in the rock decays to another radioactive element and that one
decays to another, and another, and another, and so forth, in a series
of elements that results in lead, which is not radioactive. In this
series are two radioactive elements, radium and a radioactive isotope of
lead, that help us to date paintings. To understand this, we must first
understand how radioactive elements decay.

“All radioactive elements have what is known as a ‘half-life’; that is,
in a certain period of time, half of the element disintegrates to
another form. In another equal period of time, half of what is left
disintegrates, and then half again, and so on. In the case of the
uranium, which starts the series I am describing, the half-life is over
4,000,000,000 years. Because of its long half-life there is plenty of
uranium around and will be for a long, long time. On the other hand,
radium, which I mentioned a moment ago, has a half-life of only 1600
years. In 1600 years, half of it would be gone, and in another 1600
years half of that would be gone, and so on.

“The radioactive lead that we’re interested in has a half-life of only
22 years. This means that if you start with a small quantity of this
radioactive isotope of lead, which is called lead-210,[1] then in only a
few hundred years it would have disappeared. However, in rock, where
there is uranium, the uranium keeps feeding the elements following it in
the series, so that as fast as they decay they are reproduced by the
element before them.”

    [Illustration: _The Uranium Series. In this simplified diagram, the
    double vertical arrows represent alpha radioactivity and the single
    slanted arrows represent beta radioactivity. The times shown on the
    arrows are the half-lives for each step._]

  Uranium-238
      ⇓^α   4½ billion years
  Thorium-234
      ↓^β   24 days
  Protoactimum-234
      ↓^β   1⅕ minutes
  Uranium-234
      ⇓^α   ¼ million years
  Thorium-230
      ⇓^α   80 thousand years
  Radium-226
      ⇓^α   1600 years
  Radon-222
      ⇓^α   3⅘ days
  Polonium-218
      ⇓^α   3 minutes
  Lead-214
      ↓^β   27 minutes
  Bismuth-214
      ↓^β   20 minutes
  Polonium-214
      ⇓^α   less than one second
  Lead-210
      ↓^β   22 years
  Bismuth-210
      ↓^β   5 days
  Polonium-210
      ⇓^α   138 days
  Lead-206
      (Not Radioactive)

“I don’t quite understand how that works,” said Harley. “What do you
mean ‘it keeps feeding it’?”

“Well, think of a series of lakes connected by waterfalls. At the top,
the highest lake has an enormous supply of water. Following the
waterfall coming out of the lake you find a smaller lake and then maybe
a medium-sized lake, and after another waterfall, a smaller lake, then a
tiny lake, and so on.

“As long as that big lake on top is full or nearly full, all the other
lakes, whether they are small or medium-sized, will still be getting
water as fast as it pours out. But if you cut off the supply of water
from the upper lake to the next lake, then the smaller lakes will in
time run dry. The same thing works with the radioactivity. In this
series headed by uranium, as long as uranium is present all the other
elements below it are kept supplied so that they don’t run out.”

“I understand that,” said Bill, “but how do we use that to date a
painting?”

“One of the pigments used by artists for over 2000 years is known as
lead white and it is made from lead metal. The lead metal in turn is
extracted from a rock called lead ore, in a process called smelting. The
radioactive lead, this lead-210 that I mentioned, behaves like ordinary
lead metal and goes along with it.

“The radium, which has a fairly long half-life, doesn’t follow the lead
metal, but is removed with other waste products in a material called
slag. Since the longer-lived ancestor of the lead-210 is removed, the
supply of lead-210 is cut off. (Or we can say that one of the waterfalls
is shut off.) The lead-210 will then decay with its 22-year half-life.”

    [Illustration: _The radioactive series that starts with uranium is
    like a series of lakes connected by waterfalls. As long as uranium,
    the big one on top, has water in it, the others will be full and the
    falls will keep flowing. But when the first waterfall is shut off,
    the small lakes below it will run dry._]

“I get it,” said Bill. “That means that when you take a sample of old
lead white paint, there shouldn’t be any radioactive lead-210 left.”

“That’s right. But that would only be true if you removed all the
radium. Actually, in the smelting process it’s more usual to remove only
90 or 95% of the radium. In that case, the lead-210 would decay only
until the amount left would be equal to the small amount of radium that
wasn’t removed. In effect, this would be like shutting off only part of
the waterfall.”

“So what do you find,” asked Harley, “if you measure the radioactivity
in a sample of lead white paint?”

“We find that if the paint is old, compared to the 22-year half-life of
the lead, let’s say 100 years old or more, then the amount of
radioactivity from the lead-210 in the sample of paint will be equal to
the amount of radioactivity from the radium in the sample. But if the
paint is modern, let’s say only 20 years old or so, then the amount of
radioactivity from the lead-210 will be greater than the amount of
radioactivity from the radium.”

Martin, who had been quiet through all this explanation, finally spoke
up. “Well, was it finally tried out? How did it work?”

“Hundreds of samples were analyzed. These samples were taken from
paintings of all ages, from some over 300 years old right up to others
only a couple of years old. The old samples always showed equal amounts
of radioactivity from lead-210 and radium while the modern ones always
showed larger amounts of radioactivity from lead-210 than from radium.
That meant that scientists had a way of definitely telling if a lead
white paint was modern or not.

“Eventually, the method was tried on a number of paintings believed to
be by Van Meegeren. Sure enough, every one of them showed that the paint
couldn’t possibly have been more than 30 or 40 years old and that Van
Meegeren probably was telling the truth when he said that he had painted
them. The paintings certainly were not genuine Vermeers from the 17th
century.”

“Okay, Dad,” said Martin, “can we use the method on any of the paintings
we found? Are any of these paintings supposed to be old enough so that
we can use this test?”

“Not so fast. To find that out we have to do a lot of checking first.”

“How do we go about it?” asked Bill.

“Let’s see now. There are nine paintings in the box you found. The first
thing we should do is take them down to a museum or gallery and let the
art experts look at them. Since we have a few weeks of vacation time
left, what do you say we take a trip down to Washington, D. C., and show
them to some experts at the National Gallery of Art?”

Over the next few weeks quite a few things happened to the boys and
their paintings. Three of them were discarded right away because they
were immediately recognized as being copies of no value. Two were
relatively modern paintings with the signature Alfred Sisley; if
genuine, they were less than 100 years old. The remaining four appeared
to be very old paintings. Two of them seemed to correspond to paintings
that disappeared during the Second World War. Photographs and X rays
were taken and sent to the museum in Holland, which had owned the
missing pictures, so that they could make a preliminary examination.

    [Illustration: uncaptioned]

  Radioactivity of Lead-210

    Lead-210 decaying with a half-life of 22 years. When no radium is
    present there is almost none left after 6 half-lives or 132 years.

  Radioactivity of Radium-226

    Over the same period of time, a small amount of radium decays very
    little because its half-life is about 1600 years.

  Radioactivity of Radium 226
  Radioactivity of Lead-210

    But when lead-210 decays in the presence of radium-226, the
    radioactivity of the lead-210 only decreases until it is equal to
    the radioactivity of the radium.

That left two that could have been old but whose origins were unknown. A
series of simple chemical tests were begun on these and the boys watched
experts take very small samples of paint for examination under the
microscope. After several months a list of the pigments present in the
paintings was prepared. All the pigments found were typical of old
paintings and the ordinary examinations and tests couldn’t prove whether
the works were old or not. Finally, it was decided that the only way to
tell if these paintings were truly old was to apply the test that Dad
had described to the boys.

The boys watched a painting restorer remove samples of nearly white
paint right at the edge of the paintings. He worked carefully, using a
very sharp scalpel and a stereo-binocular microscope, through which
objects appeared to be sixty times larger than they really were. The
sample of paint weighed approximately twenty-thousandths of a gram. The
boys and their father took the samples to a radiochemical laboratory
where they watched a radiochemist do the required analysis for lead-210
and radium in the samples.

First the chemist dissolved the paint in acetic acid. This removed the
lead white from the oil and from the small amounts of other pigments in
the paint. The solutions were then heated and stirred with a silver disc
hanging in the liquid. After several hours the disc still looked clean,
but the chemist said that a radioactive element, polonium-210, was now
plated onto the silver. Polonium-210 is a member of the uranium series
following the lead-210, and a measurement of its radioactivity would be
an accurate measurement of the radioactivity of lead-210.

The silver discs prepared from the two samples were each placed in an
instrument called an alpha-particle spectrometer. This instrument is
extremely sensitive and can measure the very small amounts of
polonium-210 prepared from the tiny sample of paint that they started
with.

While the instruments were making the measurements, which took a couple
of days, the chemist turned to the remaining solutions and began the
analyses for radium.

    [Illustration: _A painting being sampled under a stereo-binocular
    microscope._]

    [Illustration: _Lead white weighing twenty-thousandths of a gram (20
    milligrams). This is the amount needed to measure lead-210 and
    radium-226 to determine if the lead white is old._]

In a series of chemical steps, he purified the solutions, removing the
lead and other materials so that finally he had a small amount of
solution that contained little else but the original radium and a very
small amount of barium (an element that he deliberately added and one
which is very similar to radium in its chemical properties). By adding
dilute sulfuric acid, he prepared an insoluble material, barium sulfate,
which was barely visible suspended in the solution.

    [Illustration: _Polonium plating apparatus. A heated solution of
    lead white in acetic acid is stirred with silver discs for 4 to 8
    hours._]

    [Illustration: _The disc above appears clean after removal, but on
    its surface it retains a minute amount of polonium which can be
    measured._]

By forcing the solution through a special thin plastic filter having
tiny holes, the particles of barium sulfate together with the radium
that had been in the solution were caught on the surface of the filter.
This was mounted on a solid disc so that it too could be placed in the
alpha-particle spectrometer for the measurement of radioactivity from
the radium.

Two weeks later the results were ready. Dad, the boys, and one of the
experts from the museum met with the chemist to discuss them. For one of
the two paintings, the polonium-210 radioactivity was about ten times
that of the radium activity. The boys were disappointed because this
meant that the painting could not have been 300 or 400 years old as it
first appeared to be.

    [Illustration: _An alpha-particle spectrometer is used to measure
    the radioactivity of the radium and polonium prepared from the lead
    white._]

    [Illustration: _A plastic disc on which is cemented a filter
    containing a nearly invisible deposit of barium sulfate (BaSO₄) that
    “carried” the radium._]

But in the second painting the radioactivity from the polonium-210 and
from the radium-226 were just about equal. That meant that this painting
was at least 100 years old and, from its appearance, probably more. The
boys were excited.

“We have a really valuable painting!” said Martin.

“Not so fast, boys,” cautioned Dad. “We don’t know who painted it and we
don’t know exactly how old it is.”

The Gallery’s expert was happy too. He believed that the second picture
was a genuine Dutch painting from the 17th century. It was a landscape
and the artist might have been Aelbert Cuyp.

    [Illustration: _“The Maas at Dordrecht”, a genuine painting by
    Aelbert Cuyp._]

“What do we do now?” asked Harley. “How can we prove that the painting
was painted in Holland in the 17th century by Cuyp?”

“There is a method now being developed,” said Dad, “that could give us
that kind of information.”

“How does it work?” Martin asked.




                          Who Was the Artist?


“Do you know how criminals are caught by using fingerprints?” asked Dad.

“Sure we do,” said Martin. “Each person has a set of fingerprints that
is different from anyone else’s.”

Harley spoke up. “Did the artist leave his fingerprints on the
paintings?”

“Probably not,” said Dad. “Besides, they would have been wiped off long
ago. Also, who knows what each artist’s fingerprints were like?”

“Then what do you mean?” asked Bill.

“What I mean is, there is another kind of ‘fingerprint’ that scientists
are just now learning to use in all kinds of identification problems.
It’s not really a fingerprint, but it’s just as distinctive as a real
fingerprint.

“You see, in every material, no matter how pure you try to make it,
there are always other substances contained in it in very, very small
quantities, which are there only by chance. Usually the person making or
using that material doesn’t even know they are there, and the quantities
are so small they don’t do any harm. During the last several years,
scientists have developed extremely sensitive methods of analysis, which
have been applied to all kinds of problems.

“One such method is called neutron activation analysis. In this method
these small amounts of impurities can be detected in tiny samples of
material. This is quite important because only very small samples can be
taken from a precious painting without damaging it. Normally, a
scientist or an art restorer takes samples that are no bigger than the
head of a pin.”

“How can you do anything with a sample that small?” asked Bill.

    [Illustration: uncaptioned]

“With neutron activation analysis you can do a great deal. To give you
an example of how sensitive this method is, think of a bathtub
containing 500 quarts of milk. Add 1 drop of an acid containing a speck
of gold dissolved in it. After you mix the acid and milk thoroughly, you
won’t be able to tell by looking at it that anything was added. But if
you take a thimble full of liquid out of the bathtub, you can easily
tell with neutron activation analysis that gold was added to the milk.

“Scientists call low concentrations of accidental impurities ‘trace
elements’, and the amounts that are present are measured in parts per
million rather than percent. One part per million is one ten-thousandth
of a percent.”

Bill spoke up again. “So how does that make a fingerprint, Dad?”

“It works this way. Suppose an artist used lead white in several
paintings. Now if the lead white were absolutely pure it would contain
only lead, carbon, oxygen, and hydrogen. But the lead white the artist
used would also contain very small quantities of other elements, these
trace elements that I spoke of. In that particular batch of lead white,
certain trace elements will be present in a certain quantity. The kind
and amount of the trace elements will be present in that exact pattern
only in that batch of lead white.

“Now suppose you analyze the lead white from several paintings that you
know were painted by that particular artist, and you find that there is
silver, mercury, antimony, tin, and barium in every one of the samples.
Also, each of these elements is always present in a certain
concentration. Suppose also, that you have a painting which looks like
it was painted by that particular artist but you’re not quite sure.

“Well, if you take a sample of lead white from that unknown painting and
you find that the pattern of impurities is the same as in the paintings
you knew were genuine, then the ‘fingerprints’ match. The chances of
duplicating impurities of this kind by pure accident are extremely
small, just about as small as the chances of finding two people with the
same fingerprints. That’s why we call this a ‘fingerprint method’.”

“That sounds like a good idea,” said Harley. “Who thought it up?”

                      x = one part per million (ppm)
  A known Rembrandt.

                              x
                              x
      x           x           x
      x           x           x           x
      x           x           x           x
      x           x           x           x                       x
      x           x           x           x                       x
      x           x           x           x           x           x
    silver     chromium      zinc     manganese      iron       cobalt

  Unknown painting A

                                                                  x
                                                                  x
                                                      x           x
                                                      x           x
                                          x           x           x
                  x                       x           x           x
                  x                       x           x           x
      x           x                       x           x           x
      x           x           x           x           x           x
    silver     chromium      zinc     manganese      iron       cobalt

  Unknown painting B

                                          x
                                          x
      x           x                       x
      x           x                       x                       x
      x           x                       x                       x
      x           x           x           x                       x
      x           x           x           x                       x
      x           x           x           x           x           x
    silver     chromium      zinc     manganese      iron       cobalt

  Known forgery

                                                                  x
                                                                  x
                                                      x           x
                                                      x           x
                                          x           x           x
                  x                       x           x           x
                  x                       x           x           x
      x           x                       x           x           x
      x           x           x           x           x           x
    silver     chromium      zinc     manganese      iron       cobalt

    _Match the patterns of these lead white “fingerprints”. Unknown
    painting A is_ not _a Rembrandt; it_ is _by the same forger who
    painted the known forgery at the bottom. Unknown painting B is
    either by Rembrandt, one of his fellow citizens, or one of his
    students using the same paint._

“It was thought of many times by many people. But, it’s never been used
for identifying paintings. In 1964 in the Netherlands, two scientists,
named Houtman and Turkstra, analyzed about 40 different samples of lead
white, 20 of which came from Dutch and Flemish paintings. The rest were
samples of lead white not taken from paintings but obtained directly
from the manufacturers. They analyzed these samples for different
elements. These included silver, mercury, chromium, manganese, tin,
antimony, and a couple of others.

“They found that the concentrations of these elements in the lead white
from all the old Dutch and Flemish paintings were very similar. And the
trace element concentrations were quite different in the modern lead
white samples analyzed in the same way. At the time, they presumed that
it was because the lead white in the paintings was manufactured so long
ago. They may have been right to a certain extent.

“For example, they found that in all the old paintings there were from
10 to 30 parts per million of silver in the lead white, while in the
modern samples of this pigment there were generally less than 10 parts
per million of silver. All of them had been painted before the 19th
century, and all the samples of pure lead white were manufactured during
the latter part of the 19th century or during the 20th century. They
believed that the reason the silver concentration was lower in the more
modern material was because during the 19th century, lead refiners were
doing a better job of removing all the valuable silver from lead.

    [Illustration: _Silver concentrations in lead white. The
    concentrations generally decreased after the middle 1800s. Notice
    also how the concentrations were very similar for all the older
    paintings (before 1700) which were Dutch or Flemish._]

“However, in 1967 in Germany, two men, named Lux and Braunstein,
discovered that in some old paintings produced in Italy, lead white also
contained low quantities of silver just like modern material. They
believed that the higher concentrations of silver in lead white were
typical of Dutch and Flemish painters while the lower concentrations
were typical of Italian paintings of about the same age.

“The whole case is still unsettled because not enough measurements have
been made to show how reliable this method can be. That is, no one knows
if samples of paint from several paintings by one artist would all have
the same pattern of impurities in the same pigment. It may be that of
the many pigments present in an artist’s paintings only a few will be
suitable for use in this ‘fingerprinting’ method.”

    [Illustration: _Quartz vials (right) containing samples are sealed
    in the aluminum can on the left. They are then bombarded with
    neutrons in a reactor like the one in the picture below._]

“It sounds complicated,” said Bill.

“It is, and it’s going to take years of work before the method is
proven, if it is at all. It may turn out that you can’t tell one artist
from another, but only groups of artists like 17th century Dutch
painters or 19th century English painters.”

“Tell us something about neutron activation analysis,” said Martin. “How
do you measure such small amounts of impurities?”

“The best way to tell you how this works is to show you. How would you
boys like to visit a laboratory where neutron activation analysis is
being done?”

“Do you have to ask?” said Harley. “Of course we would!”

A few weeks later it was all arranged. At a laboratory close by a
nuclear reactor, the boys watched a radiochemist place a few specks of
material inside small quartz tubes that were then sealed. The tubes were
put in an aluminum can and placed in the nuclear reactor. The can was
fastened on the end of a long pole that was then submerged in a deep
pool of water. At the bottom of the pool the boys could see a bright
blue glow.

    [Illustration: _This type of nuclear reactor is used for neutron
    activation analysis._]

“So that’s what a nuclear reactor looks like!” said Bill.

“Yes,” said Dad. “Where you see the blue glow you can also see rows of
fuel elements. Each one contains slugs of uranium encased in aluminum.
This is one of a number of different types of reactors. But every
nuclear reactor is arranged so that the uranium atoms divide (or
fission) many, many times each second.

“When this happens, heat is produced that is carried away by the water,
and also many, many free neutrons are produced. Those samples, placed
down next to the reactor in the bottom of the pool are being bombarded
by the neutrons, and some of the elements in the samples absorb the
neutrons and become radioactive.”

After a while the samples were removed and carried back to the
laboratory in a lead box. A short while later, the radiochemist opened
the aluminum can, broke open the quartz capsules, and removed the
samples for analysis. The boys watched the chemist mount each sample on
a card and take it to a room where there was equipment for measuring
radioactivity.

    [Illustration: _Gamma-ray spectrometer. The sample to be measured is
    placed on a stand over a gamma-ray detector. The pulse-height
    analyzer is a device that sorts electrical impulses from the
    detector according to the energy of the gamma rays causing the
    impulses. The screen displays the gamma-ray spectrum and the
    electric typewriter automatically types out the data collected when
    the measurement is complete._]

One by one the samples were placed inside a shield consisting of a big
pile of lead bricks. When the heavy door was opened, the boys could see
a metal can inside the shield, which housed a detector (called a
lithium-drifted germanium detector) that measured the gamma rays emitted
by the sample. As each sample was placed near the detector the chemist
turned on a gamma-ray spectrometer to which the detector was connected.

    A tiny sample of lead white {sample} is sealed in a quartz vial
    {vial} which is bombarded with neutrons in a reactor.

    [Illustration: uncaptioned]

    Many of the atoms become radioactive, emitting gamma rays.

    [Illustration: uncaptioned]

    The sample is placed in a gamma-ray spectrometer and the gamma rays
    are separated according to their energy.

    [Illustration: uncaptioned]

  Gamma-ray spectrum
    Copper
    Zinc
    Antimony
    Lead
    Silver
    Height
    Antimony

    The location (energy) of each peak indicates what is present and the
    height indicates how much!

    [Illustration: _A gamma-ray spectrum as it appears on the screen of
    a pulse-height analyzer. The gamma-ray peaks are marked with the
    name of the element whose radioactive isotope emits the gamma ray;
    two for cobalt and zinc and one for cesium._]

There, on what looked like a small television screen, flashes of light
appeared that gradually formed a curve with many peaks and valleys.
After a few minutes the spectrometer was stopped and an electric
typewriter automatically typed out rows and columns of numbers.

The chemist explained, “This curve, which you see on the screen, is a
gamma-ray spectrum and tells us what elements are in the sample. The
typed-out data give us an accurate measure of the shape of the curve on
the screen. By measuring the gamma-rays’ energies we know what elements
in the sample were made radioactive. The height of each gamma-ray peak
tells us how much of that element is present in the sample.

“That gives us the information we need to calculate the concentrations
of the small quantities of materials in our samples. We can do this
because at the same time I irradiated a set of standards. Standards are
materials that are just like the samples except that they contain known
amounts of the impurities I am trying to measure.”

As the boys were leaving the laboratory, the chemist apologized for not
having enough time to explain the activation analysis procedure more
thoroughly, but he did give the boys a list of books to read on the
subject of radioactivity and radioisotopes.[2] They thanked him for his
help.

During the ride home, they discussed the paintings that were still
unproven.

“It’s too bad that the method of activation analysis fingerprinting
hasn’t been fully developed yet,” said Dad.

“Yes,” said Bill. “Then we could prove whether or not that last old
painting was really by Aelbert Cuyp as the expert from the gallery
believed. But what about those paintings that we found in the box that
were not so old?”

“Well,” said Dad, “if the activation analysis method were workable, we
might be able to prove if they were painted by Alfred Sisley. Meanwhile,
until the method is really developed we don’t know if we can do it that
way or not.”

“So what do we do now?” asked Martin.

“We’ll have to wait until scientists can thoroughly investigate this
method and several others that they’re working on.”

“Other methods!” exclaimed Bill. “What other methods?”

    [Illustration: _“The Banks of the Oise”, a genuine painting by
    Alfred Sisley._]




                 Other New Tools for Art Authentication


“There are several new tools that scientists are working on now,” said
Dad. “These involve methods that have been developed by scientists for
other purposes, but are now being explored for use in authenticating
works of art.

“For example, in Los Angeles, the county museum purchased an instrument
known as a Spark Source Mass Spectrometer. Like activation analysis,
this instrument will also measure small traces of impurities, but they
have just set that up and it will take them years to explore the use of
it for the type of problem we have been discussing.

“X-ray diffraction is another method that has been around for quite
awhile but hasn’t been used much for art identification until recently.
With X-ray diffraction, samples of pigments can be identified by the
pattern formed when X rays are bent by passing through the sample of
pigment.”

“How’s that?” asked Harley.

“There are 3 or 4 different compounds with about the same chemical
composition as lead white. Chemically, they are almost impossible to
distinguish. But with X-ray diffraction, a chemist can easily tell them
apart. The hope is that the type of lead white will indicate how it was
manufactured. Until the middle of the 19th century, lead white was
produced mainly by packing strips of lead in clay pots with a little
vinegar in the bottom. The clay pots were stacked in a large building
with layers of decaying organic matter on the floor. The building was
sealed for several weeks during which time the lead corroded in the
fumes and became covered with a white substance. The white substance,
lead white, was scraped off, ground, and washed to make the pigment.

“But, in the 19th century, when people began to learn more about
chemistry, they looked for faster ways of making lead white and some of
these methods produced a lead white of somewhat different composition.
By using X-ray diffraction, chemists now hope that they can tell how the
lead white was manufactured. This may provide another means of dating
the lead white in a painting.”

“Are there any other methods?” asked Harley.

    [Illustration: _The stack process for making lead white. Rows of
    clay pots containing lead and vinegar are packed to the ceiling of
    the building, and fermenting tanbark on the floor produces carbon
    dioxide and heat. The fumes of vinegar and the carbon dioxide
    corrode the lead in 2 to 4 months, and the corrosion is lead
    white._]

“Yes, isotope mass spectrometry is one. All lead consists of 4 different
isotopes or atoms of different weights. Three of these 4 are the end
products of a radioactive decay chain. Depending upon the history of the
rock formation in which the lead ore occurred, the relative amounts of
the lead isotopes vary in a special way. In other words, if we know the
different amounts of lead isotopes in the world’s lead ore deposits, and
we have a sample of lead white from a painting, we can tell from which
deposit the lead, which formed the lead white, came. If, for example, we
find that the isotope pattern in a sample from a painting is the same as
in lead ore from Australia, then the painting can’t be very old because
lead white wasn’t produced from lead mined in Australia until about 100
years ago.”

    [Illustration: _X-ray diffraction patterns from three different lead
    compounds that might occur in lead white. The middle one is the
    ideal lead white produced for over 2000 years. While some of the
    bottom compound may be found mixed with it, the compound shown at
    the top is only a 20th-century invention._]

  4PbCO₃ · 2PB(OH)₂ · PbO
  2PbCO₃ · PB(OH)₂
  PbCO₃

“How do you measure lead isotopes?” asked Harley.

“With an instrument called a mass spectrometer. This instrument is
capable of separating the lead isotopes. First, the atoms of lead in the
sample are electrically charged and ‘fired’ in a beam down the length of
a tube between the poles of a strong magnet. There, the charged atoms
(or ions) in the beam are deflected by different amounts according to
how heavy they are. Thus the different isotopes are separated. This
method is also still being studied and, although it shows great promise,
it will be some time before it can solve problems of art identification.
Also the study of the natural variation in isotopes of other elements,
such as sulfur, is useful for identification of other pigments as well.

    [Illustration: _Diagram of a simple mass spectrometer. The ionized
    atoms of lead travel in a beam at the same speed. The heavier atoms
    bend less than the lighter ones when the beam passes the magnet.
    Thus two beams emerge instead of one. Actually there are four
    isotopes of lead so there will be four beams._]

    [Illustration: _“Agostina”, a genuine painting by Jean Baptiste
    Camille Corot._]

“Another new method that shows great promise has been developed, but
this one is not applicable to the paintings that you boys found in the
box.”

“Why not?” asked Bill.

“Since the Second World War, the art forgery business has been growing
rapidly. For example, it has been said that of the 2000 pictures that
Corot, a 19th century Frenchman, is known to have painted, more than
5000 of them are in the United States. This may be only a humorous
exaggeration, but a large number of forgeries have been produced in the
last several years. These are usually supposed to be paintings that are
less than 100 years old. Present-day forgers like to forge paintings
that aren’t very old because it’s easier to get away with. Now this new
method, which will detect such recent forgeries, is based upon the
presence of carbon-14, a radioactive isotope of carbon, in our
atmosphere and in all things that grow on our planet.

“Ordinarily, carbon-14 is produced only by cosmic rays, and its
concentrations in the atmosphere and in growing things would remain at a
constant level. But since the middle of the 1950s the testing of nuclear
weapons has increased the amount of radioactive carbon in our atmosphere
by quite a bit. Many artist’s materials, such as linseed oil, canvas,
paper, and so on, come from plants or animals, and so will contain the
same concentrations of carbon-14 as the atmosphere up to the time that
the plant or animal dies.

“Therefore, linseed oil (from the flax plant), for example, produced
during the last few years will have a much greater concentration of
carbon-14 in it than linseed oil produced more than 20 years ago.
Scientists at Carnegie-Mellon University have shown that this method
will work. It is only a matter of making the measurements on the small
samples available from presumably valuable paintings.”

    [Illustration: _The changing concentrations of carbon-14 in our
    atmosphere. High levels of carbon-14 in linseed oil and other
    painting materials will indicate that a work of art is only a few
    years old._]

  Carbon-14 radioactivity
    Older materials contain less as the carbon-14 decays away.
    In this period, decrease is due to the burning of large quantities
          of coal and oil as industry grew. This diluted the newly
          formed carbon-14.
    Increases due to testing of atomic weapons in the atmosphere.
  Carbon-14 produced by cosmic rays only
    Neutron → Nitrogen → Carbon-14 + proton
    Carried down by rain in carbon dioxide

“There are also a number of other methods being studied including the
use of Messbauer Effect Spectroscopy to study pigments that contain
iron, thermoluminescent dating of pottery and terra-cotta statuary,
X-ray fluorescence analysis as a general tool, and neutron
autoradiography as a means of studying the technique of artists. You can
read all about them if you wish.”[3]

“It sounds like forgers are going to have a tough time in the future,”
said Harley.

“That’s right. It may even turn out that producing forgeries to pass all
these new tests will be so difficult and expensive that forgers will
stop trying.”




                           One Mystery Solved


A year later an important letter arrived at the boys’ house. Dad opened
it, read it quickly, and said, “Good news, boys! This letter is from the
Dutch government. Remember those two paintings that we thought might
have been stolen from a Dutch museum?”

“Yes,” said Bill.

“Well, it seems that after a year of studying them, the Dutch have
decided that they really are the paintings that were stolen.”

“That is good news,” said Harley. “At least we know that two of the
paintings we found are genuine.”

“What are they going to do with them?” asked Martin.

“Of course, they have to go back to their original owners. But this
letter says that the Dutch government wants us to come to Holland as
their guests as a reward for finding those paintings.”

    [Illustration: _These two paintings “The Lacemaker” and “The Smiling
    Girl” were thought to have been by Vermeer. A series of tests,
    including some of those described in this booklet, showed that the
    paintings are fairly old. However, some of the materials used are
    not typical of Vermeer, and the pictures are now thought to have
    been painted by a follower of the artist._]

“That’s great!” said Bill. “Looks like we’re getting something out of
finding that box after all.”

“Yes,” said Dad. “And don’t forget the other unidentified paintings may
also be genuine. We’ve proved that one is a fake, the experts believe
that three of the others are copies, and then there are the two that
might be Sisleys and are only waiting for a method to prove it. And we
have one more that science managed to prove was really old. I’m sure
that in a few years methods will be developed to tell us exactly who
painted it.

“And now let’s make arrangements for our trip to Holland.”




                              Reading List


_About Atomic Power for People_, Edward and Ruth S. Radlauer, Childrens
      Press, Chicago, Illinois 60607, 1960, 47 pp., $2.50. Grades 5-9.

_All About the Atom_, Ira M. Freeman, Random House, Inc., New York
      10022, 1955, 146 pp., $2.50. Grades 4-6.

_Atoms at Your Service_, Henry A. Dunlap and Hans N. Tuch, Harper and
      Row, Publishers, New York 10016, 1957, 167 pp., $4.00. Grades 7-9.

_Carbon-14 and Other Science Methods that Date the Past_, Lynn and Gray
      Poole, McGraw-Hill Book Company, New York 10036, 1961, 160 pp.,
      $3.95. Grades 9-12.

_Experiments with Atomics_ (revised edition), Nelson F. Beeler and
      Franklyn M. Branley, Crowell Collier and Macmillan, Inc., New York
      10022, 1965, 160 pp., $3.50. Grades 5-8.

_The Fabulous Isotopes: What They Are and What They Do_, Robin McKown,
      Holiday House, Inc., New York 10022, 1962, 189 pp., $4.50. Grades
      7-10.

_Inside the Atom_ (revised edition), Isaac Asimov, Abelard-Schuman,
      Ltd., New York 10019, 1966, 197 pp., $4.00. Grades 7-10.

_Introducing the Atom_, Roslyn Leeds, Harper and Row, Publishers, New
      York 10016, 1967, 224 pp., $3.95. Grades 7-9.

_Our Friend the Atom_, Heinz Haber, Golden Press, Inc., New York 10022,
      1957, 165 pp., $4.95 (out of print but available through
      libraries); $0.35 (paperback) from Dell Publishing Company, Inc.,
      New York 10017. Grades 7-9.

_Radioisotopes_, John H. Woodburn, J. B. Lippincott Company,
      Philadelphia, Pennsylvania 19105, 1962, 128 pp., $3.50. Grades
      7-10.

_The Story of Atomic Energy_, Laura Fermi, Random House, Inc., New York
      10022, 1961, 184 pp., $1.95. Grades 7-11.

_The Useful Atom_, William R. Anderson and Vernon Pizer, The World
      Publishing Company, New York 10022, 1966, 185 pp., $5.75. Grades
      7-12.

_Working with Atoms_, Otto R. Frisch, Basic Books, Inc., Publishers, New
      York 10016, 1965, 96 pp., $3.50. Grades 9-12.


Footnotes


[1]It is called this because 210 is the total number of protons and
    neutrons in its nucleus.

[2]See the reading list on page 44.

[3]See _Secrets of the Past: Nuclear Science and Archaeology_, which is
    listed on the inside back cover of this booklet.


PHOTO CREDITS

Cover courtesy Groninger Museum voor stad en Lande

    Page

      5      Yale Joel, _Life_ magazine, copyright © Time, Inc.
      6      Her Majesty the Queen, copyright © reserved
    7 & 8    Ullstein Bilderdienst
     10      Rijksmuseum, Amsterdam
     23      National Gallery of Art, Washington, D. C., Andrew Mellon
             Collection
   35 & 40   National Gallery of Art, Washington, D. C., Chester Dale
             Collection
     43      National Gallery of Art, Washington, D. C., Andrew Mellon
             Collection

                      ★ U.S. GOVERNMENT PRINTING OFFICE: 1974—747-556/15


The U. S. Atomic Energy Commission publishes this series of information
booklets for the general public. The booklets are listed below by
subject category.

If you would like to have copies of these booklets, please write to the
following address for a booklet price list:

  USAEC—Technical Information Center
  P. O. Box 62
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School and public libraries may obtain a complete set of the booklets
without charge. These requests must be made on school or library
stationery.

 Chemistry

 IB-303      The Atomic Fingerprint: Neutron Activation Analysis
 IB-301      The Chemistry of the Noble Gases
 IB-302      Cryogenics: The Uncommon Cold
 IB-304      Nuclear Clocks
 IB-306      Radioisotopes in Industry
 IB-307      Rare Earths: The Fraternal Fifteen
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 Biology

 IB-101      Animals in Atomic Research
 IB-102      Atoms in Agriculture
 IB-105      The Genetic Effects of Radiation
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 The Environment

 IB-201      The Atom and the Ocean
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 IB-414      Nature’s Invisible Rays

 General Interest

 IB-009      Atomic Energy and Your World
 IB-010      Atomic Pioneers—Book 1: From Ancient Greece to the 19th
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 IB-011      Atomic Pioneers—Book 2: From the Mid-19th to the Early
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 IB-012      Atomic Pioneers—Book 3: From the Late 19th to the Mid-20th
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 IB-002      A Bibliography of Basic Books on Atomic Energy
 IB-004      Computers
 IB-008      Electricity and Man
 IB-005      Index to AEC Information Booklets
 IB-310      Lost Worlds: Nuclear Science and Archeology
 IB-309      The Mysterious Box: Science and Art
 IB-006      Nuclear Terms: A Glossary
 IB-013      Secrets of the Past: Nuclear Energy Applications in Art
             and Archaeology
 IB-017      Teleoperators: Man’s Machine Partners
 IB-014,     Worlds Within Worlds: The Story of Nuclear Energy Volumes
 015, & 016  1, 2, and 3

 Physics

 IB-401      Accelerators
 IB-402      Atomic Particle Detection
 IB-403      Controlled Nuclear Fusion
 IB-404      Direct Conversion of Energy
 IB-410      The Electron
 IB-405      The Elusive Neutrino
 IB-416      Inner Space: The Structure of the Atom
 IB-406      Lasers
 IB-407      Microstructure of Matter
 IB-415      The Mystery of Matter
 IB-411      Power from Radioisotopes
 IB-413      Spectroscopy
 IB-412      Space Radiation

 Nuclear Reactors

 IB-501      Atomic Fuel
 IB-502      Atomic Power Safety
 IB-513      Breeder Reactors
 IB-503      The First Reactor
 IB-505      Nuclear Power Plants
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 IB-512      Thorium and the Third Fuel

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                     U. S. ATOMIC ENERGY COMMISSION
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                          Transcriber’s Notes


—Silently corrected a few typos.

—Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by
  _underscores_.