F. E. Rogers1 University of Dayton Dayton, Ohio 45409
Chemistry and Art Thermoluminescence a n d forgery
In 1953 Farrington Daniels, the chemist, suggested a technique based on thermoluminescence for the datine of pottery and other objects of ceramic art. In 1971, ~e;ket Cetimkaya, a Turkish farmer, was arrested for allegedly making and selling "ancient" pottery. Before we explore the interesting relationship between these two points in time it is necessary to understand why pottery dating is important. Much of what we know about ancient societies comes not from any written records hut rather from the artifacts left behind for the archeolopists' analysis. From the tools, pottery, and figurines of antiquity, archeologists can con: struct a picture of the social and religious character of a n ancient race where no written records exist. Therefore, every ceramic piece unearthed carries a small message of how the people a t that particular time thought and felt about themselves, their environment and their gods. It therefore becomes incumbent on science to play its vitally important role of assisting the archeologist in establishing the authenticity of the artifacts on which such far reachine conclusions are founded. (In this connection. students of-antiquities were immeasure'ahly aided by W. F.Libby's work in Carhon-14 datine.) For e x a m ~ l e an . "ancient" fieurine depicting a man kneeling in abrayer-like pose sa;s a great deal about the people, if it truly is ancient. Questions of age can be based on the style of execution hut such stylistic criteria are not as unequivical as scientific ones. Such considerations probably did not enter the mind of Cevket Cetimkaya who no doubt wanted to improve his station in life. What better way could he do it than to fill a need for Hacilar pottery a t about $7,000 each? Cevket in fact assisted the British archeologist James Mellaart in his excavation a t Hacilar, a 7000-year-old settlement in southwestern Turkey. he wealth-of material found at Hacilar suggested to some historians that Anatolia (modern Turkey), rather than Mesopotamia was the cradle of civilization. After excavation ceased in 1960, all manner of Hacilar pottery appeared in the antiquities market. Stylistic irregularities were frequent enough to merit further attention so the scientist of the Oxford Archeological Lahoratories stepped in and adapted the thermoluminescence technique to the problem. Thermoluminescence is the ability of certain materials to give off visible or near visible light when strongly heated. The light is emitted at temperatures lower than that a t which the red-hot glow appears. In antiquity, the pottery was made from clay containing small amounts of long-lived, radioactive uranium, thorium, and potassium which decay with the emission of alpha and beta particles. The energy from these isotopes and the radiospecies in the soil surrounding the buried pottery is absorbed by
For the first part in this series, see Rogers, F. E., J. CHEM. EDUC., 49,418 (1972). 1 Present address: 23 Van Buren St., Dayton, Ohio 45402. 2 A series of articles an the uses of thermoluminescence for the dating of antiquities appear in Archaeornetry, Vol. 13, parts 1 and 2, August (1971).Cutken, M. J., in "Thermoluminescence of Gealogical Materials" (Editor: McDougall), Academic Press, New York, 1968, pp. 369-378. 388
/ Journal of Chemical Education
Figure 1. Thermoluminescence curves for ceramic vessels. Curve A , Natural thermoluminescence for vessel fired in antiquity. Note absence of TL Delow 200°C. where electron tram are not rufficientlv stable. Curve B, shows the low level glow for a recently fired vesel. Curve C, red hot glow.
the fabric of the ceramic piece. This radiation causes electrons to he hoosted to higher energy levels in which they are trapped. The electrons in these metastable traps are released a t elevated temperature with the emission of light. The deeper the trap, the higher the temperature required. Electrons released a t 350°C, for example, are stable at amhient conditions for several hundred thousand years. Since measurements are made on milligram size samples the light levels are very low. However these are measurable when a photomultiplier tube with a high efficiency in the hlue-ultra violet region is used. The analysis itself may he approached qualitatively or quantitatively; i.e., (a) is the piece very old or recent and ( b ) exactly (+lo%) how old is the piece? The first question is answered by a comparison (Fig. 1) of the glow curves of the questionable piece and an authentic antiquity of the same era (Fig. 2). If they have similar thermoluminescence spectra, they are related in time. Quantitative work consists basically of these parts; first measure the glow curve of pottery samples, then determine the dose of artifical radiation necessary to reproduce the same glow curve in the sample. Finally, by chemical analysis of a pottery sample and adhering soil find the annual dose rate to which the fahric of the pot has been subjected by the radioelements. These values are combined to give an estimate of the time necessary to build up a radiation dosage equivalent to the thermoluminescence spectra.2
Figure 2. Double-headed vessels. Thermoluminescence has shown that the vessel on the left was fired In antiquity while the other, fired recently. is a forgery.
To reconstruct then, at the time of fabrication the ceramic is fired in a kiln which reduces the thermoluminescence to zero. In the intervening millenniums, the local ionizing radiation gradually populates the various electron traps. Therefore if the pottery or figurine is an authentic Hacilar ceramic, heating to 400°C will produce significant light. The ceramics that Cevket allegedly made did not lu-
minesce as Figure 1 shows, and hence were bogus. In the past, Cevket Cetimkaya has been arrested for smuggling antiquities hut never convicted. Now he is up against the kind of unequivical evidence that science can supply and should he survive this onslaught, he will have to hone up on thermoluminescence.
Volume 50, Number 6, June 1973 / 389
