Thermoluminescence (TL) is a faint violet-blue light that is emitted when a piece of fired pottery is heated at a sufficiently high temperature. The amount of TL is measured using a sensitive detector known as a photomultiplier tube. The intensity of the thermoluminescence is proportional to the time that has elapsed since the pottery was removed from the kiln; hence, whereas there is a relatively bright signal from an ancient pottery object, a modern piece of pottery will emit little or no light when it is reheated as only a short time has elapsed since it was fired and there has not been sufficient time for the thermoluminescence to build up to a measurable signal.

 

Raw clay will emit a strong thermoluminescent signal if heated without first being fired in a kiln. However, the act of firing drains away all the geological thermoluminescence acquired over millions of years, essentially setting the dating clock to zero. Once the pottery has cooled, the thermoluminescence begins to accumulate again at a constant rate. When a sample from a piece being tested is heated in the laboratory the intensity of the light seen is directly proportional to the time that has elapsed since the piece was last fired.

 

TL gives an absolute, objective measurement of the time since the clay at the sampling site was fired. In addition, it can detect unfired clay and restoration material. If samples are taken from several locations on the piece, TL can also indicate if sections were made from a similar clay.