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]