The thermoluminescence 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 thermoluminescence TLwhere 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.
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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. A leaflet from Daybreak describing the TL technique in more detail and giving a bibliography will be provided to interested persons.
The phenomenon of thermoluminescence was first described by the English chemist Robert Boyle in It was employed in the 's as a method for radiation dose measurement, and soon was proposed for archaeological dating. By the mid'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. While not so accurate as radiocarbon dating, which cannot date pottery except from soot deposits on cooking potsTL 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. The TL laboratory at Daybreak was established in to make TL available to the art community in general. When dates of a number of sherds associated together are averaged, the error is reduced typically to per cent. This is for well-behaved samples only.
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.
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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 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. 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 per cent. Nearly any mineral material which has been heated above C at a time one wishes to know is a 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.
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Some regions known to present problems for TL include Indonesia and West Mexico; objects from these areas usually do not successfully yield TL dates. These use pottery of the appropriate period to construct objects. Some of these are quite easy to detect; some quite difficult. For example figures, normally modeled, may be carved out of brick or assembled out of fragments.
It must be realized that TL dating is but one of the criteria for judging authenticity. The expertise of the conservator may be of equal or greater importance in many cases. Some problem areas include Northern Nigerian ceramics, especially Nok, which are becoming quite scarce. Ife ceramics are virtually all fake or stolen, if genuine!
New Nigerian and Asian bronzes may have introduced old cores, so it is imperative that the interface between metal and core be examined very carefully before the assumption can be made that the age of the bronze is the age of the core. Chinese unglazed ceramics constructed from fragments or carved from brick are a particular concern.
Glazed objects generally cannot be pieced together in this way without re-firing which would defeat the purposebut be sure the glaze is glass and not a synthetic resin! Often we recommend radiography of objects to ascertain the state of restoration before proceeding with sampling. We reserve the right not to sample and date an object based on concerns about tampering. Since the TL age is proportional to radiation dose, it is logical to be concerned about the effect of airport security x-rays and radiography done to examine the object.
In general it is not a problem. Airport security x-rays devices use very high sensitivity detectors so that the x-ray dose is in fact quite small, perhaps adding a week or month to the age, well below the uncertainty of dating.
Radiography, if many films are taken, may be more of a problem, so we recommend that samples be taken prior to exposure. It may also be possible to compute an approximate correction, but in almost every case the effect is small. Due to concerns about bioterrorism in the wake of the events of this past autumn, the US Postal Service has begun limited sterilization of mail by electron beam.
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This will destroy the dose information carried in the pottery and rendered it unsuitable for TL dating. There have been rumors circulating lately about recently fired Chinese pottery being artificially irradiated to circumvent TL dating. While this is certainly something we watch for, there is little real cause for concern. There are several reasons why this dose tampering is difficult to impossible to achieve successfully. First, it is difficult to get the dose right without considerable research into the properties of the clay and access to expertise in TL measurements.
Second, it is very difficult to get that dose sufficiently uniform over the extent of the entire object. It also and obviously requires a sophisticated means of irradiation, not easily available here, let alone in China. There are many considerations that we will not detail so as not to offer 'aid and comfort to the enemy'. The 'impossible' part is that different size grains in the clay actually have different doses in a naturally irradiated ceramic, but will have the same dose in the artificially irradiated example.
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