Part 2

“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) 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.

“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.”

“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.

“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.

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.

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!

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?”

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.

“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.”

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.

“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.”

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.”