“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.”