Test to determine age in ceramics is not foolproof or scamproof
Seattle Times art critic
Thermoluminescence, or TL, testing is a way of measuring radiation damage in solid material. Its most common use is in industry and medicine, for monitoring exposures of radiation in people and workplaces. In the art world, TL is a way to establish the authenticity of fired-clay objects.
TL can tell whether a piece was fired in the distant past or only recently. Typically, the test is able to measure the time that has passed since the object was fired to within plus or minus 30 percent of the actual time span, depending on the type of clay and the testing method used.
An object created 1,000 years ago will yield a TL result in the range of 700 to 1,300 years — clearly indicating an ancient, rather than a modern, date of firing, according to conservation scientist John Twilley of New York.
After firing, natural radiation deposits energy in the ceramic year by year. Heating a sample to about 400 degrees Celsius releases this stored energy in the form of measurable light. Subsequent exposure of the sample to a known amount of radiation, followed by a second light measurement, allows the ceramic's historical radiation exposure to be gauged and its age calculated.
"You are basically calculating a ratio between two amounts of light," Twilley says.
However, even reputable labs can be fooled by fakes.
"I have tested pieces previously tested in China," Twilley says. "In one case, the piece had been drilled repeatedly — the bottom looked like a sieve it had so many holes — but despite all that sampling, they failed to determine that the whole surface had been covered with modern material. It was entirely reconstructed."
That's why Julian Thompson, Chinese art expert for Sotheby's in London, insists that TL testing be seen "as part of the jigsaw of evidence and only viewed in the context of a thorough and critical expert examination." Following is his list of ways TL tests can be falsified:
• Technical errors. Inaccurate results may come from problems with equipment or inexperienced operators.
• Fake certificates. A lab certificate can be photocopied and altered, substituting a different photograph or changing the results.
• Composite objects. Some new ceramic pieces may incorporate clay from old objects. The base of a jar could be old, but the top might be a reproduction. A TL test shows the age only of the part that is sampled.
• Recarved objects. An old piece of pottery, such as a Han dynasty tomb brick, can be carved into a new shape. The test result will indicate that the piece is as old as the material it's carved from.
In addition, real antiques can be artificially enhanced to add value in a marketplace that prizes uniqueness. A little animal can be placed in a subject's folded hands, for instance, or the legs of a horse can be broken and reattached at more dramatic angles.
• Switched samples. If a laboratory accepts a sample drilled by an outside source along with a photograph, it risks authenticating a work based on a sample taken from some other piece. The certificate then may appear to authenticate a fake.
• Applying radiation. Modern porcelain objects can be irradiated with a controlled dose that will cause a TL test to indicate it is much older. For this reason, TL on porcelain must be used only to confirm a known provenance and an expert judgment of the piece.