Over the same period of time, a small amount of radium decays very
little because its half-life is about 1600 years.
Radioactivity of Radium 226
Radioactivity of Lead-210
But when lead-210 decays in the presence of radium-226, the
radioactivity of the lead-210 only decreases until it is equal to
the radioactivity of the radium.
That left two that could have been old but whose origins were unknown. A
series of simple chemical tests were begun on these and the boys watched
experts take very small samples of paint for examination under the
microscope. After several months a list of the pigments present in the
paintings was prepared. All the pigments found were typical of old
paintings and the ordinary examinations and tests couldn’t prove whether
the works were old or not. Finally, it was decided that the only way to
tell if these paintings were truly old was to apply the test that Dad
had described to the boys.
The boys watched a painting restorer remove samples of nearly white
paint right at the edge of the paintings. He worked carefully, using a
very sharp scalpel and a stereo-binocular microscope, through which
objects appeared to be sixty times larger than they really were. The
sample of paint weighed approximately twenty-thousandths of a gram. The
boys and their father took the samples to a radiochemical laboratory
where they watched a radiochemist do the required analysis for lead-210
and radium in the samples.
First the chemist dissolved the paint in acetic acid. This removed the
lead white from the oil and from the small amounts of other pigments in
the paint. The solutions were then heated and stirred with a silver disc
hanging in the liquid. After several hours the disc still looked clean,
but the chemist said that a radioactive element, polonium-210, was now
plated onto the silver. Polonium-210 is a member of the uranium series
following the lead-210, and a measurement of its radioactivity would be
an accurate measurement of the radioactivity of lead-210.
The silver discs prepared from the two samples were each placed in an
instrument called an alpha-particle spectrometer. This instrument is
extremely sensitive and can measure the very small amounts of
polonium-210 prepared from the tiny sample of paint that they started
with.
While the instruments were making the measurements, which took a couple
of days, the chemist turned to the remaining solutions and began the
analyses for radium.
[Illustration: _A painting being sampled under a stereo-binocular
microscope._]
[Illustration: _Lead white weighing twenty-thousandths of a gram (20
milligrams). This is the amount needed to measure lead-210 and
radium-226 to determine if the lead white is old._]
In a series of chemical steps, he purified the solutions, removing the
lead and other materials so that finally he had a small amount of
solution that contained little else but the original radium and a very
small amount of barium (an element that he deliberately added and one
which is very similar to radium in its chemical properties). By adding
dilute sulfuric acid, he prepared an insoluble material, barium sulfate,
which was barely visible suspended in the solution.
[Illustration: _Polonium plating apparatus. A heated solution of
lead white in acetic acid is stirred with silver discs for 4 to 8
hours._]
[Illustration: _The disc above appears clean after removal, but on
its surface it retains a minute amount of polonium which can be
measured._]
By forcing the solution through a special thin plastic filter having
tiny holes, the particles of barium sulfate together with the radium
that had been in the solution were caught on the surface of the filter.
This was mounted on a solid disc so that it too could be placed in the
alpha-particle spectrometer for the measurement of radioactivity from
the radium.
Two weeks later the results were ready. Dad, the boys, and one of the
experts from the museum met with the chemist to discuss them. For one of
the two paintings, the polonium-210 radioactivity was about ten times
that of the radium activity. The boys were disappointed because this
meant that the painting could not have been 300 or 400 years old as it
first appeared to be.
[Illustration: _An alpha-particle spectrometer is used to measure
the radioactivity of the radium and polonium prepared from the lead
white._]
[Illustration: _A plastic disc on which is cemented a filter
containing a nearly invisible deposit of barium sulfate (BaSO₄) that
“carried” the radium._]
But in the second painting the radioactivity from the polonium-210 and
from the radium-226 were just about equal. That meant that this painting
was at least 100 years old and, from its appearance, probably more. The
boys were excited.
“We have a really valuable painting!” said Martin.
“Not so fast, boys,” cautioned Dad. “We don’t know who painted it and we
don’t know exactly how old it is.”
The Gallery’s expert was happy too. He believed that the second picture
was a genuine Dutch painting from the 17th century. It was a landscape
and the artist might have been Aelbert Cuyp.
[Illustration: _“The Maas at Dordrecht”, a genuine painting by
Aelbert Cuyp._]
“What do we do now?” asked Harley. “How can we prove that the painting
was painted in Holland in the 17th century by Cuyp?”
“There is a method now being developed,” said Dad, “that could give us
that kind of information.”
“How does it work?” Martin asked.
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.
[Illustration: uncaptioned]
“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