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Section 1. Introduction#

1.1 Scope of the Guide#

The present document serves as a guide to good practice for the collection and archiving of data produced by Thermoluminescence (TL) measurements (analyses) of archaeological materials, such as ceramics, in the context of the archaeological research. This guide does not elaborate on the methods involved in thermoluminescence analysis in general, but aims at informing researchers involved in archaeological studies about the key elements and important metadata that should be documented from thermoluminescence analyses during the determination of the age of archaeological materials.

It should be noted that specific metadata can be very important since they are descriptive of the procedure followed for the treatment of physical samples and the protocols or techniques used during the analysis which are solidly interconnected to the produced data. Special attention should be given to documenting such metadata, which allow not only the easy archiving but also the reuse of the datasets produced. This ensures the re-evaluation of samples and the comparison of results between laboratories.

1.2 What is Thermoluminescence?#

In summary, thermoluminescence is the emission of light during the heating of a solid sample, usually an insulating one, which has been previously excited. The source of the emitted light is the initial excitation, which is typically created by irradiation, while heating acts as a trigger which contributes to the releasing of this accumulated energy.

To elaborate on the above, a solid sample such as ceramic can be excited by ionizing radiation at a certain relatively low temperature. This irradiation can either take place in the laboratory or in a radiative environment. However, another version, more related to the archaeological use of thermoluminescence, is when a material is irradiated by the radiation field in its natural surrounding. At the end of this stage, the sample is placed in an appropriate instrument where its temperature is gradually raised at a constant heating rate. Then the emitted light (photons) is recorded as a function of the temperature using a light sensitive detector, such as a photomultiplier (Chen & McKeever, 1997) and a glow-curve is acquired (Fig. 1).

Age = paleodose / annual dose

Figure 1: Typical glow curve of a pottery sample recorded after artificial radiation administered in the laboratory


1.3 Applications of Thermoluminescence in Archaeology#

a) Dating

The most common and important application of thermoluminescence in Archaeology is dating of archaeological objects, mainly ceramics, such as pottery, bricks or terracotta. The rapid heating of such objects up to 500°C results in the weak, but measurable emission of light, which stems from some of the constituents minerals. The TL from the specimen is mostly due to TL sensitive mineral inclusions (mostly quartz) in the host clay matrix of the pottery. The basis of TL dating in archaeology is a definite event, the kiln firing. This event is regarded as the starting of the "TL clock" for archaeological dating, since any TL previously stored in the object is completely erased during the firing process. After the onset of the TL clock, the pottery starts to build up TL due to its exposure to the weak flux of nuclear radiation emitted by radio-active impurities in the pottery and the surrounding burial soil, the most active of which are the potassium-40 (K-40), thorium (Th) and uranium (U). The above radioisotopes have half-lives of more than 109 years, which results in a constant radiation flux.

Based on the above, the amount of the material's thermoluminescence can provide an estimation of the elapsed time since the pottery was fired. This is better described by Eq. 1 (Aitken, 1985), while the whole process is illustrated in Fig. 2.

Flow chart


(1)

The "paleodose" is calculated by the thermoluminescence of the pottery (archaeologically acquired thermoluminescence). Estimation of the "annual dose" is based on the appropriate calculation of the contribution of the alpha and beta particles of the radioisotopes mentioned above (K-40, Th and U) using other methods.

It is obvious that the thermoluminescence constitutes only a part of a complex process used to achieve an accurate and reliable dating of ceramics. Various measurements with several techniques should be conducted to get the correct age, rather than an estimation of it, for an archaeological object. This is better illustrated by Fig. 2.

Flow chart

Figure 2: Flow chart of the process for pottery dating


b) Authenticity testing of ceramic objects

Thermoluminescence can also be used in the battle against art forgery and has been widely employed in the field of authenticity testing. Porcelain or ceramic objects can be the subject of thermoluminescence testing in order to determine their archaeological value. A modern object which has only been fired recently will carry only a trivial level of stored TL energy, compared to its ancient prototype (Fleming, 1979). Consequently, it is relatively easy to distinguish between a fake object whose age is less than a hundred years and those aged more than five hundred.

c) Pottery provenance and transactions among ancient communities

Thermoluminescence can also provide information on the provenance of pottery and further shed light on local production and socio-economical relationships amongst ancient communities. Such information can complement archaeological observations of typology and the recognition of special fabric textures of excavated pottery and lead to a more concrete decisions regarding provenance. Several such studies have been undertaken (e.g. Vaz et al., 1997; Rasmussen, 2001) and are based on differences of TL sensitivity and/or characteristics of pottery of different provenance. These variances are the outcome of different tempering materials and concentrations of trace elements found in the clay used for the manufacture of pottery from different sources (geographical regions).

1.4 Current Issues or Concerns#

a) Efficiency and diffusion of the thermoluminescence data

In many cases, thermoluminescence data from various archaeological objects (mainly pottery) are produced to support the work of archaeologists, without being stored with additional information about the object itself (e.g. type), mainly because archaeologists most often neglect to provide such information. This limits the reusability of TL data for the benefit of the wider archaeological community, since they are only produced for a specific purpose (e.g. the age of an object). This situation is further worsened by the fact that such data are not published, since only the result of the process (e.g. the age) is of interest for the archaeologists. However, TL data could provide further information about the temporal or spatial distribution of pottery when compared with similar data of other objects.

b) The need for additional metadata

To ensure the use of TL data in other archaeological studies and to allow the comparison between data acquired in different laboratories or the repetition of the same measurement, several metadata elements should also be stored. This metadata includes both details of the under-study object (e.g. sampling procedure, excavation site, picture, etc) and the measuring conditions (method, number of sub-samples, doses, temperatures, etc).

1.5 Data and Metadata#

Definition of Data and Metadata is a controversial issue with multifold approaches, since it extensively depends on usage. In the case of thermoluminescence, raw data, i.e., the results of every measurement (directly produced by the scientific instrument without any intermediate calculations), are considered as data. In fact they are temperature-photon counts pairs, whose population depends on the heating rate and final temperature selected for the measurement. However, metadata also comprise all subsequent information resulting from the post-processing of raw data along with other sample-related and pre-measurement (sample treatment, operational conditions etc) information which allows the interpretation of results or comparison with previous measurements. Examples include the location of the archaeological material, the method/technique, the measurement parameters etc.


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