x
x
x x
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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.
[Illustration: _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.”
[Illustration: _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.
[Illustration: _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.
[Illustration: _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 {sample} is sealed in a quartz vial
{vial} which is bombarded with neutrons in a reactor.
[Illustration: uncaptioned]
Many of the atoms become radioactive, emitting gamma rays.
[Illustration: uncaptioned]
The sample is placed in a gamma-ray spectrometer and the gamma rays
are separated according to their energy.
[Illustration: uncaptioned]
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!
[Illustration: _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?”
[Illustration: _“The Banks of the Oise”, a genuine painting by
Alfred Sisley._]