Thermoluminescence dating of a ceramic requires two steps. The first consists of gauging the accumulated radiation (or 'archaeological dose') absorbed by crystals in the ceramic since its firing. Buried terra cottas are irradiated by radioelements in the objects themselves and by those in the soil in which they are buried. To measure accumulated radiation, one tracks the thermoluminescent properties of the crystals -- when heated, they release stored radioactive energy in the form of light. Technicians generally drill out small samples from a piece; only an ounce or two of material is necessary for the test. The second step consists of determining the amount of radiation absorbed yearly by the crystals. Dividing the 'archaeological dose' by the 'annual dose' gives the age of the terra cotta.
Thermoluminescence tests are an important factor when dealers and collectors judge authenticity. But TL has its limitations. First, in order to insure profitability, commercial labs often limit the number of samples they take from terra cottas for analysis, generally drawing them from only two parts of a piece. Scientists who run the labs say this number is insufficient in view of the con game now taking place with terra cottas.
Then there is the matter of who takes the samples. Since TL testing is a global business, it is impractical for directors of commercial firms to take all the samples themselves. Firms like Daybreak Nuclear in Guilford, Connecticut, and Oxford Authentication Ltd. in Wantage, England, employ a worldwide network of representatives who travel wherever the artifacts happen to be. While we have no reason to doubt the honesty of these representatives, the art world in which they operate is not always concerned with professional ethics. Dealers are capable of bribing the experts. Three times during my research I visited African dealers in Bamako who showed me terra cottas accompanied by certificates of authenticity from the largest commercial labs in Europe. They were fake terra cottas and, apparently, fake certificates.
Finally, there's reason to doubt the reliability of certain measurements of radiation attested in commercial certificates. Annual doses are crucial to determining an Object's age. "They are extremely difficult to obtain," says Max Schvoerer, a professor at the University of Bordeaux 3 and founder of the Center for Research in Applied Physics and Archaeology
|at the university. "In general, research scientists determine it by comparing measurements taken in the lab with those [from soil samples] at the place where the object was found. Because of the time and costs involved, commercial firms don't have the opportunity to do this. They thus either have to extrapolate from published reports about [the geology of] the region the object under analysis came from, or only measure the internal radiation of the object, which can sometimes result in considerable margins of error. In this case the test becomes merely an indication of antiquity, not a method of precise dating."
Directors of TL labs try to address these issues. They recognize, for example, that it would be preferable to take more samples. If there is serious doubt about an artifact, they will take further samples if the client requests. As for the wisdom of using representatives to take samples, Doreen Stoneham, director of Oxford Authentification Ltd., simply replies that "it's more practical." Lab directors refuse to discuss the possibility of corrupt practices, jealously guarding their employees'identities and qualifications. Victor Bortolot, director of Daybreak Nuclear, would say only that his team is composed of a dozen people, most of whom are art restorers.
As for the method used to measure annual doses of radiation, commercial labs recognize their weaknesses vis-a-vis university labs: the precision of their work cannot be compared with that of scientists who actually travel to the site where the artwork was found. Nonetheless, they say their calibrations still guarantee antiquity, which is usually all clients want. However, to avoid misunderstandings, most commercial labs have stricken the word "dating" from their brochures.
Commercial labs stress that TL tests are only one step in assessing an artwork's authenticity. Art market professionals, however, have every interest in promoting TL certificates as the final word. One couldn't dream of better support for a sales pitch. - Michel Brent