Drilling, the usual method of sampling, introduces some uncertainty.
It is also rare that any information about the radiation from the burial soil can be obtained, as art objects are usually thoroughly cleaned.
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.
Generally speaking, when a sample is drilled and there is no information available about the burial environment, one may expect up to 40 per cent uncertainty.
Warning about fakes using ancient materials What about airport x-rays and radiography? Thus, when one measures dose in pottery, it is the dose accumulated since it was fired, unless there was a subsequent reheating.
When pottery is fired, it loses all its previously acquired TL, and on cooling the TL begins again to build up.
By comparing this light output with that produced by known doses of radiation, the amount of radiation absorbed by the material may be found.
Most mineral materials, including the constituents of pottery, have the property of thermoluminescence (TL), where part of the energy from radioactive decay in and around the mineral is stored (in the form of trapped electrons) and later released as light upon strong heating (as the electrons are detrapped and combine with lattice ions).
Some regions known to present problems for TL include Indonesia and West Mexico; objects from these areas usually do not successfully yield TL dates.
Much stoneware is not so hard as porcelain and may be sampled by drilling.
The clay cores from lost wax metal castings may readily be tested.
While not so accurate as radiocarbon dating, which cannot date pottery (except from soot deposits on cooking pots), TL has found considerable usefulness in the authenticity of ceramic art objects where high precision is not necessary.
Since the university laboratories involved with TL are research facilities, they generally will not accept art objects for authentication on a routine basis.
A leaflet from Daybreak describing the TL technique in more detail and giving a bibliography will be provided to interested persons.