Thermoluminescence (TL) dating is a radiometric dating technique used to determine the age of crystalline materials that have been heated in the past. It is particularly valuable in archaeology for dating artifacts such as pottery, burnt flint, and sediments, as well as geological materials like volcanic ash. The principle behind TL dating relies on the fact that crystalline materials, such as quartz and feldspar, absorb energy from natural background radiation (cosmic rays and radioactive elements like uranium, thorium, and potassium in the surrounding soil) over time. This absorbed energy causes electrons within the crystal lattice to become dislodged from their normal positions and trapped in structural defects within the crystal.

When the material is subsequently heated to a high temperature (e.g., during the firing of pottery in a kiln, a campfire, or a volcanic eruption), these trapped electrons are released from their defects. As they return to their stable positions, they emit light—a phenomenon known as thermoluminescence. This heating event effectively 'resets' the TL clock, as all previously accumulated trapped electrons are released. After this heating, the material begins to accumulate trapped electrons again from the ambient radiation.

To date an artifact using TL, a sample is taken from the object and reheated in a laboratory under controlled conditions. The intensity of the emitted light is measured, which is directly proportional to the amount of radiation absorbed since the last heating event. By also measuring the annual dose rate of background radiation at the archaeological site, scientists can calculate the time elapsed since the material was last heated. The older the sample, the more radiation it has absorbed, and thus, the more light it will emit upon reheating.

TL dating is particularly useful for periods beyond the effective range of radiocarbon dating (which typically extends to about 50,000 years ago) and for inorganic materials that cannot be carbon-dated. It can date materials from a few hundred years up to approximately 500,000 years old. However, its accuracy depends on several factors, including the consistency of the background radiation dose rate, the thermal stability of the trapped electrons, and careful sample collection to avoid exposure to light or heat, which could prematurely release trapped electrons and skew the results.