Date: 2018-02-27 14:32
Thermoluminescence, or TL, was first used in the 6955s for the measurement of radiation exposure, and underwent a period of difficulties before being applied to dating the first dates it produced being too young. Upon resolution of the technical problems the method was used for dating pottery and burnt flints from archaeological sites with a precision of about 7-65 per cent. Subsequently it has been used in the investigation of recent geological formations reaching back to half a million years. In its most common form it may shed light on the age of fired clay and quartz based materials but approaching the present no closer than about a thousand years. A modern variant on the technique is able to date far more recent fired clay material.
TL depends upon minute levels of background radiation in the clay matrix, a tiny fraction of which is absorbed and stored as a charge at imperfections in the crystal lattice of quartz inclusions. The firing of pottery removes the inherited geological TL and sets the dating clock to zero. In the laboratory grains of quartz are extracted from the pottery and heated in light-tight apparatus at a constant rate to around 955° C. Superimposed upon the red-hot glow, a tiny flash of light is produced as the stored energy is released (hence 'thermoluminescence') and the flash is recorded by computer. The quantity of light produced is proportional to the length of time since it was last fired. Unfortunately, problems remain since all samples do not have the same sensitivity to the radiation and background radiation levels vary. Furthermore, the results are sensitive to water content. Thus many measurements must be made in order to obtain a date.
Recently this method has been improved. The flash of light is released by scanning the sample with an energetic green laser beam and light-emitting diodes are used as detectors. This form of the method, known as 'optically stimulated luminescence dating', enables objects which are not more than a few hundred years old to be dated to within a few decades. Hence it is far more useful than the original TL technique in dating buildings. The requirement remains that the sample should have undergone some heating event to set the clock to zero. It also requires that a dosimeter be left undisturbed in situ at the site for some months in order to discover the natural radioactivity permeating the samples. These must be inorganic and contain some light transmitting materials. Pottery artefacts and certain bricks might be suitable specimens, and often TL provides the only way to distinguish medieval or Tudor bricks and chimney pots from Victorian reproductions.
There are several laboratories capable of this sort of measurement in this country which include the Geology Department, Aberystwyth the British Museum the Godwin Institute, Cambridge the Department of Archaeology, Durham Environmental Sciences the Institute of Archaeology, University College, London and the Research Laboratory for Archaeology and the History of Art, Oxford.