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.
“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) | |||||
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| A known Rembrandt. | |||||
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| silver | chromium | zinc | manganese | iron | cobalt |
| Unknown painting A | |||||
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| silver | chromium | zinc | manganese | iron | cobalt |
| Unknown painting B | |||||
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| silver | chromium | zinc | manganese | iron | cobalt |
| Known forgery | |||||
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| 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.
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.”
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.
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.
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
is sealed in a quartz vial
which is bombarded with neutrons in a reactor.
Many of the atoms become radioactive, emitting gamma rays.
The sample is placed in a gamma-ray spectrometer and the gamma rays are separated according to their energy.
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!
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?”
“The Banks of the Oise”, a genuine painting by Alfred Sisley.