Most of the crystals emit light (luminescence) when stimulated. This stimulation can be, among others, heating (thermo-) or exposure to light of specific wavelength (opto-). The intensity of this emission depends on two parameters: the radioactivity of the environment in which the crystals were stored and the time during which they were submitted to this radiation.
The timer is reset each time the crystals are heated (typically at a temperature over 500°C) or exposed to bright light.
For a terracotta item collected on site, we can measure the radioactivity of the environment of the crystals extracted from the fragment to be dated (a few tens of grams). So, in addition to thermoluminescence measurements performed on the object, we provide a precise age of the last heating.
The same principle can be applied in assessing the ultimate exposure to sunlight of Aeolian sediments, for example, from OSL measurements.
For objects of art out of archaeological context, for which the burial environment is unknown, we proceed by assumption about the intensity of radioactivity. We provide an estimation of the antiquity of the last heating (two samples taken from the object by micro-drilling), which is considered as consistent or not with that presumed for the studied object. This is the test of antiquity.
Moreover, some problems can be encountered with the analysis of specific heated material (casting core, ceramics, stoneware …), because of poor reproducibility of signals or difficulties in exploiting the measurements. In such cases, the study can give no result (which does not mean that the object is recent or ancient, but only that we are not able to determine its antiquity).
In addition, TL technique can be used to characterize the state of heating of a ceramic or terracotta structure.
The test of antiquity using thermoluminescence being made on samples, to extrapolate the restult to the object as a whole, we propose to complete this analysis by the use of techniques such as Xray-imaging or a CT scan.
Download the document about thermoluminescence for objects of art
Download the document about thermoluminescence in archaeology (FRENCH)
Any question ? Any thought you would like to share ?