Kent Archaeological Review extract
 

Thermoluminescent Dating.
by Peter Couldrey.

In the earliest days of antiquarianism the question "How old is it?" was among the most important and most difficult to answer. Today, establishing a chronology remains an important part of the archaeologist's work; but the development of independent 'absolute' dating methods has greatly eased the problem. Among the numerous methods now available, thermoluminescence (TL) would appear to offer promising results because it can be applied to fired clay and flint. Thus pottery, the most common artefact on post-neolithic sites, can be dated directly without reference to typological sequences; and humble 'pot-boilers' (fire-crackled flints) may deserve more attention than just to be counted.

Already the method has been used with great success to detect forgeries and the spectacular, though somewhat controversial, dates of material from Glozel have been the subject of a BBC Chronicle programme. It is intended here to outline the theory behind the method, to mention some of the problems, and to consider its accuracy and its value to archaeologists. The following description is written with reference to pottery.

Most clays contain small amounts of radioactive impurities (thorium, uranium and potassium-40) which cause ionization to occur in mineral grains, thereby separating electrons from their parent atoms. Some of these electrons become trapped at defects in the crystal lattice of the mineral and remain there until the mineral is heated above a threshold level, in the range 300-500°C. Once this level is reached the electrons are freed and, in the process, light is emitted hence the name thermo-luminescence. The strength of the light provides a measure of the number of electrons freed. When clay is fired the number of trapped electrons is reduced to zero; and, on cooling, the process starts again. Thus the age of pottery can be found from the number of trapped electrons, the natural thermoluminescence (L), but is also dependent on the extent to which the mineral can trap electrons, its sensitivity (S), and the quantity of radioactive impurities present (R). The age is then calculated as L / S x R.

Unfortunately this simple theory is subject to several complications which limit the accuracy of the method. The assumption of a linear rate of accumulation of TL, implied above, is not strictly valid. Instead, the growth of the number of trapped electrons begins slowly and accelerates until a linear rate is reached. This is then maintained until saturation level is approached, when the rate declines. Without allowing for this pattern, dates for young samples would be too young, and those for old ones too old. It is possible to check for these effects, and methods of estimating the discrepancy due to the supralinearity of young samples have been suggested; but, because samples do not all behave in the same way, correction procedures are not firmly established.

Even when the rate of accumulation of TL has been estimated, problems may arise because, once trapped, electrons can leak out, resulting in a reduced age measurement. Though this is sometimes quite serious, it is rare in pottery minerals.

Problems also arise when the radiation dose received by the minerals is estimated. This is dependent not only on the radioactive impurities within the sherd, but also on those in the surrounding soil; so the radioactivity of the soil needs to be measured. Furthermore, the water content of the soil and the sherd affects the dose-rate, and should be estimated. Their degree of saturation when excavated is not enough; rather, an average value for the whole time of burial is required. The porosity of the sherd is also important, because of its ability to retain the radioactive gas radon, which is formed during the decay chain of uranium, and which contributes a varying amount to the dose-rate depending on how much of it escapes.

Although it is possible to estimate the effects these factors have on the amount of TL being measured, they each reduce the degree of accuracy that can be obtained. In an attempt to overcome these problems, several methods of dating have been developed. Each attempts to minimise the effects of one or more of these complicating factors by specialising in particular minerals, or restricting the size of the grains examined. For example, a method using zircon grains is unaffected by the environmental dose-rate because of the relatively large amount of radiation produced within the grains themselves.

The degree of accuracy that can be obtained does vary depending on the material to be dated and the method used. With so many variables providing an opportunity for error, the chance of obtaining any useful dates may appear remote. Yet the method has proved successful, particularly in detecting forgeries, and tests on artefacts of known age have confirmed the general theory. There is no simple routine which will guarantee a high degree of accuracy. Instead, each sample has to be considered individually and relevant methods applied. Some objects cannot be dated at a11, while others, excavated under ideal conditions, can achieve an accuracy of 5%, although a figure of ±10% is more realistic.

The critical factor affecting the range of dates that may be obtained is the time taken for the sample to approach the saturation level. For pottery this varies from 5,000 to 50,000 years, though for flints the range could be extended to 500,000 years. The problems of dating flint, though, are more complex than those associated with pottery. And, although analyses can show whether thermal pre-treatment may have been used in the manufacture of stone tools, more research is required before the dating of flint becomes common practice.

It should be emphasised, however, that the method is not restricted to the dating of artefacts. A wide range of materials can be used, subject to varying degrees of influence from the problems mentioned above. Thus the burnt mounds of Orkney have been dated to between 1000 BC and 400 BC using the stones from the structures. But with this flexibility it is the ability to date heated flint that is the greatest promise offered by the method. The range from 50,000 to 500,000 years is one that falls between radiocarbon and potassium-argon dating methods, and since the use of fire is known from the beginning of this period, thermoluminescent dating could become of considerable value to archaeologists.

Further Reading.

  1. M Aitken, 'Thermoluminescence and the archaeologist' in Antiquity (1977) LI pages 11-19. This article provides a more detailed account of thermoluminescence, includes an appendix on the collection of pottery samples for dating and a useful bibliography.
  2. The journal ARCHAEOMETRY frequently carries articles on research in thermoluminescent dating.
 
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