A method of dating that measures the amount of energy stored in crystals. Crystals like diamonds or more commonly quartz accumulate thermoluminescent energy over time by absorbing radiation. When exposed to sunlight this energy can be released and the ‘clock’ set back to zero. Therefore it is possible to measure the age of certain types of deposits (e.g. grains of sand), which may have been exposed to the sun in the past and subsequently covered over. Dating is complicated by the need to measure the amount of alpha, beta and gamma radioactivity being generated in the immediate area around any one sample because this may affect the accuracy of dating. Archaeologists therefore need to plan in advance what deposits might be dated by this method as once the area around a potential sample has been removed without appropriate recording the technique is unlikely to be successful.
When an object is heated to around 500°C/930° F it releases the stored thermoluminescent energy. In producing pottery crystalline substances are added into the clay fabric of pottery to strengthen the material and allow it to breathe during kiln-firing at 600°C and above. Firing erases the huge level of thermoluminescent energy built up by the crystals over geological time and sets a ‘time-zero’ for fresh energy accumulation. The thermoluminescence intensity measured today is therefore proportional to the pottery’s age. Thermoluminescence can date inorganic materials, including stone tools left as burnt flint, older than about 50,000–80,000 years, although it is regarded as less precise in its accuracy than radiocarbon dating.