When pottery gets covered in the ground, radiation from the earth starts to energize (excite) the electrons of these crystalline materials, putting them into “trap states.” This is a measure of the radiation dose.The longer the pottery is in the ground, the more radiation dose it will absorb, causing more electrons to be excited into trap states.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-).
By the mid-1960'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. The Research Laboratory for Archaeology at Oxford, in particular, has played a major role in TL research.
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.
It is an absolute dating method, and does not depend on comparison with similar objects (as does obsidian hydration dating, for example).
The thermoluminescence technique is the only physical means of determining the absolute age of pottery presently available.
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.