Thermo luminescence Dating
The thermo luminescence technique is the only physical means of determining the absolute age of pottery presently available. It is an absolute dating method, and does not depend on comparison with similar objects (as does obsidian hydration dating, for example).
Most mineral materials, including the constituents of pottery, have the property of thermo luminescence (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 de-trapped and combine with lattice ions). By comparing this light output with that produced by known doses of radiation, the amount of radiation absorbed by the material may be found.
When pottery is fired, it loses all its previously acquired TL, and on cooling the TL begins again to build up. Thus, when one measures dose in pottery, it is the dose accumulated since it was fired, unless there was a subsequent reheating. If the radioactivity of the pottery itself, and its surroundings, is measured, the dose rate, or annual increment of dose, may be computed. The age of the pottery, in principle, may then be determined by the relation: Age = Accumulated dose / Dose per year
Although conceptually straightforward, TL has proven to to be far from simple in practice. In all, close to two dozen physical quantities must be accurately measured to establish the relationship between doses of different kinds of radiation and light output, and to compute dose rate. A leaflet from Daybreak describing the TL technique in more detail and giving a bibliography will be provided to interested persons.
The phenomenon of thermo luminescence was first described by the English chemist Robert Boyle in 1663. It was employed in the 1950’s as a method for radiation dose measurement, and soon was proposed for archaeological dating. 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.S.. The Research Laboratory for Archaeology at Oxford, in particular, has played a major role in TL research.
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.
What is the accuracy of TL Dating?
Studies at Oxford back in the 70’s on Romano-British pottery indicated that when all quantities entering the age equation are measured, the TL date of a single potsherd will typically fall within 15 per cent of the known date. When dates of a number of shards associated together are averaged, the error is reduced typically to 7-10 per cent. This is for well-behaved samples only. The succeeding 30 years, and increased understanding of the dosimetry, have not brought much improvement.
Unfortunately, it is not possible to achieve this precision for the majority of art objects. 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. It is also rare that any information about the radiation from the burial soil can be obtained, as art objects are usually thoroughly cleaned.
This radiation may in some cases contribute over half the total dose. Finally, one has to make the measurements regardless of whether the TL of the clay is well-behaved or not. Some clays are hardly thermo luminescent 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. This is adequate for the purposes of authentication where the question is whether the piece was fired in antiquity or recently; it will not differentiate, say, between a classic Greek terra cotta and a Roman copy. In some categories of objects, from China, for example, the actual age is quite precisely known for short-lived styles, and it is possible to work “backwards” to get information about the environment in many parts of the world, and some other parameters not usually measurable for art objects. Using this information often reduces the uncertainty to 15-25 percent.
What Materials can be Dated by TL?
Nearly any mineral material which has been heated above 500C at a time one wishes to know isa candidate for TL dating. This includes all forms of pottery. Porcelains, being nearly vitrified, are a special case requiring a fairly large solid core sample, and TL dating of intact objects is not recommended because of the damage caused by sampling. Most porcelain dating is done for insurance purposes on broken objects. 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. Heated stone material, such as hearths, pot boilers, and burnt flints, has been dated as well. Some regions known to present problems for TL include Indonesia and West Mexico; objects from these areas usually do not successfully yield TL dates.
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