By the mid-1960's, its validity as an absolute dating technique was established by workers at Oxford and Birmingham in England, Riso in Denmark, and at the University of Pennsylvania in the U. The Research Laboratory for Archaeology at Oxford, in particular, has played a major role in TL research.
Among the reasons for this is the small amount of material that may be taken for testing.
Drilling, the usual method of sampling, introduces some uncertainty.
Finally, one has to make the measurements regardless of whether the TL of the clay is well-behaved or not.
Some clays are hardly thermoluminescent at all; some may not have a straight-line relationship between dose and TL; spurious luminescence due to chemical or pressure effects may mask the radiation-induced TL; occasionally, a condition called "anomalous fading", where part of the TL is unstable, may lessen the accuracy of the dose measurement.
When pottery is fired, it loses all its previously acquired TL, and on cooling the TL begins again to build up.