How Old Is a Painting?
“One question at a time. I’ll tell you how the method works and what it does if you’re really interested.”
“We’re interested! We’re interested!” chorused the boys.
“In the first place, this method works only in certain cases of suspected forgery. Over the last 50 or 100 years, a number of paintings have turned up that seemed, even to the best art experts, to be several hundred years old. Some of these were genuine, and some were painted by forgers who could not resist the high prices paid for works of art. The National Gallery of Art, in Washington, D. C., thinking that there might be a way of detecting these forgeries, gave its support to a group of scientists who developed a method for this purpose.
“To understand how the method works, you need to know a little about how radioactive atoms disintegrate to form atoms of other elements. In this case we are interested in the natural radioactivity that occurs in certain rocks. As a matter of fact, in almost all rocks in the earth’s crust there is a certain small quantity of uranium.”
“I thought uranium was rare,” interrupted Bill.
“It is, but we’re talking about such small quantities that its difficult for scientists using the most sensitive equipment to detect it. The uranium in the rock decays to another radioactive element and that one decays to another, and another, and another, and so forth, in a series of elements that results in lead, which is not radioactive. In this series are two radioactive elements, radium and a radioactive isotope of lead, that help us to date paintings. To understand this, we must first understand how radioactive elements decay.
“All radioactive elements have what is known as a ‘half-life’; that is, in a certain period of time, half of the element disintegrates to another form. In another equal period of time, half of what is left disintegrates, and then half again, and so on. In the case of the uranium, which starts the series I am describing, the half-life is over 4,000,000,000 years. Because of its long half-life there is plenty of uranium around and will be for a long, long time. On the other hand, radium, which I mentioned a moment ago, has a half-life of only 1600 years. In 1600 years, half of it would be gone, and in another 1600 years half of that would be gone, and so on.
“The radioactive lead that we’re interested in has a half-life of only 22 years. This means that if you start with a small quantity of this radioactive isotope of lead, which is called lead-210,[1] then in only a few hundred years it would have disappeared. However, in rock, where there is uranium, the uranium keeps feeding the elements following it in the series, so that as fast as they decay they are reproduced by the element before them.”
The Uranium Series. In this simplified diagram, the double vertical arrows represent alpha radioactivity and the single slanted arrows represent beta radioactivity. The times shown on the arrows are the half-lives for each step.
Uranium-238 ⇓α4½ billion years Thorium-234 ↓β24 days Protoactimum-234 ↓β1⅕ minutes Uranium-234 ⇓α¼ million years Thorium-230 ⇓α80 thousand years Radium-226 ⇓α1600 years Radon-222 ⇓α3⅘ days Polonium-218 ⇓α3 minutes Lead-214 ↓β27 minutes Bismuth-214 ↓β20 minutes Polonium-214 ⇓αless than one second Lead-210 ↓β22 years Bismuth-210 ↓β5 days Polonium-210 ⇓α138 days Lead-206 (Not Radioactive)
“I don’t quite understand how that works,” said Harley. “What do you mean ‘it keeps feeding it’?”
“Well, think of a series of lakes connected by waterfalls. At the top, the highest lake has an enormous supply of water. Following the waterfall coming out of the lake you find a smaller lake and then maybe a medium-sized lake, and after another waterfall, a smaller lake, then a tiny lake, and so on.
“As long as that big lake on top is full or nearly full, all the other lakes, whether they are small or medium-sized, will still be getting water as fast as it pours out. But if you cut off the supply of water from the upper lake to the next lake, then the smaller lakes will in time run dry. The same thing works with the radioactivity. In this series headed by uranium, as long as uranium is present all the other elements below it are kept supplied so that they don’t run out.”
“I understand that,” said Bill, “but how do we use that to date a painting?”
“One of the pigments used by artists for over 2000 years is known as lead white and it is made from lead metal. The lead metal in turn is extracted from a rock called lead ore, in a process called smelting. The radioactive lead, this lead-210 that I mentioned, behaves like ordinary lead metal and goes along with it.
“The radium, which has a fairly long half-life, doesn’t follow the lead metal, but is removed with other waste products in a material called slag. Since the longer-lived ancestor of the lead-210 is removed, the supply of lead-210 is cut off. (Or we can say that one of the waterfalls is shut off.) The lead-210 will then decay with its 22-year half-life.”
The radioactive series that starts with uranium is like a series of lakes connected by waterfalls. As long as uranium, the big one on top, has water in it, the others will be full and the falls will keep flowing. But when the first waterfall is shut off, the small lakes below it will run dry.
“I get it,” said Bill. “That means that when you take a sample of old lead white paint, there shouldn’t be any radioactive lead-210 left.”
“That’s right. But that would only be true if you removed all the radium. Actually, in the smelting process it’s more usual to remove only 90 or 95% of the radium. In that case, the lead-210 would decay only until the amount left would be equal to the small amount of radium that wasn’t removed. In effect, this would be like shutting off only part of the waterfall.”
“So what do you find,” asked Harley, “if you measure the radioactivity in a sample of lead white paint?”
“We find that if the paint is old, compared to the 22-year half-life of the lead, let’s say 100 years old or more, then the amount of radioactivity from the lead-210 in the sample of paint will be equal to the amount of radioactivity from the radium in the sample. But if the paint is modern, let’s say only 20 years old or so, then the amount of radioactivity from the lead-210 will be greater than the amount of radioactivity from the radium.”
Martin, who had been quiet through all this explanation, finally spoke up. “Well, was it finally tried out? How did it work?”
“Hundreds of samples were analyzed. These samples were taken from paintings of all ages, from some over 300 years old right up to others only a couple of years old. The old samples always showed equal amounts of radioactivity from lead-210 and radium while the modern ones always showed larger amounts of radioactivity from lead-210 than from radium. That meant that scientists had a way of definitely telling if a lead white paint was modern or not.
“Eventually, the method was tried on a number of paintings believed to be by Van Meegeren. Sure enough, every one of them showed that the paint couldn’t possibly have been more than 30 or 40 years old and that Van Meegeren probably was telling the truth when he said that he had painted them. The paintings certainly were not genuine Vermeers from the 17th century.”
“Okay, Dad,” said Martin, “can we use the method on any of the paintings we found? Are any of these paintings supposed to be old enough so that we can use this test?”
“Not so fast. To find that out we have to do a lot of checking first.”
“How do we go about it?” asked Bill.
“Let’s see now. There are nine paintings in the box you found. The first thing we should do is take them down to a museum or gallery and let the art experts look at them. Since we have a few weeks of vacation time left, what do you say we take a trip down to Washington, D. C., and show them to some experts at the National Gallery of Art?”
Over the next few weeks quite a few things happened to the boys and their paintings. Three of them were discarded right away because they were immediately recognized as being copies of no value. Two were relatively modern paintings with the signature Alfred Sisley; if genuine, they were less than 100 years old. The remaining four appeared to be very old paintings. Two of them seemed to correspond to paintings that disappeared during the Second World War. Photographs and X rays were taken and sent to the museum in Holland, which had owned the missing pictures, so that they could make a preliminary examination.
Radioactivity of Lead-210
Lead-210 decaying with a half-life of 22 years. When no radium is present there is almost none left after 6 half-lives or 132 years.
Radioactivity of Radium-226
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.
A painting being sampled under a stereo-binocular microscope.
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.
Polonium plating apparatus. A heated solution of lead white in acetic acid is stirred with silver discs for 4 to 8 hours.
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.
An alpha-particle spectrometer is used to measure the radioactivity of the radium and polonium prepared from the lead white.
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.
“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.