What is thermoluminescence?

When a radiation is incident on a material, some of its energy may be absorbed and re-emitted as light of longer wavelength. The wavelength of the emitted light is characteristic of the luminescent substance and not of the incident radiation.

Thermoluminescence (TL) is the process in which a mineral emits light while it is being heated: it is a stimulated emission process occurring when the thermally excited emission of light follows the previous absorption of energy from radiation. Energy absorbed from ionising radiation (alpha, beta, gamma, cosmic rays) frees electrons to move through the crystal lattice and some are trapped at imperfections in the lattice. Subsequent heating of the crystal can release some of these trapped electrons with an associated emission of light.
If the heating rate is linear and if we suppose the probability of a second trapping to be negligible with respect to the probability of a recombination, the TL intensity is related to the activation energy of the trap level by a known expression. It is so possible to determine the trap depth.

Application on Archaeological findings

Material and objects of archaeological or historical interest that can be dated by thermoluminescence analysis are ceramics, brick, hearths, fire pits, kiln and smelter walls, heat treated flint or other heat-processed materials, the residues of industrial activity such as slag, incidentally fire-cracked rocks, and even originally unfired materials such adobe and daub if they had been heated in an accidental fire. 

Fundamental principles of dating technique

A non-negligible part of materials which ceramic is usually made of (like quartz and feldspars) is thermoluminescent: those materials have trap states that can capture electrons after interaction with alfa, beta and gamma rays existing in nature.
When these materials are heated to several hundreds of Centigrade degrees, electrons are evicted from trap states and energy is emitted in form of light: thermoluminescence (TL). Heating ceramic in a furnace resets TL accumulated by clay and other materials; from this time on, TL begins growing again as time passes; the more concentrated radioactivity where ceramic is, the quicker TL grows.
Thus by measuring TL we can date an object since the last time it was heated above 400°C. Since measured TL depends on time of exposition to natural radiations but also on the intensity of these radiations, to achieve a precise dating we need information about radioactivity of the area where the object was found.