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.
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.”
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.
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.
“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.”
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.”