Release, in the form of a light pulse, of stored nuclear energy in a mineral substance when heated to perhaps 500°C/930° F. The energy originates from the radioactive decay of uranium and thorium, and is absorbed by crystalline inclusions within the mineral matrix, such as quartz and feldspar. The release of TL from these crystalline substances is used in archaeology to date pottery, and by geologists in studying terrestrial rocks and meteorites.
Thermoluminescent dating Crystalline substances found their way into the clay fabric of ancient pottery as additives designed to strengthen the material and allow it to breathe during kiln-firing at 600°C and above. Firing erased the huge level of TL energy accrued in geological times and sets a ‘time-zero’ for fresh energy accumulation over archaeological times, the TL intensity measured today being proportional to the pottery's age. TL 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.
Applications include dating of the world's earliest known pottery, Jōmon ware from Fukui Cave near Nagasaki, northwestern Kyushu Island, Japan (about 12,000 BC), and extensive authenticity studies among Tang dynasty (AD 618–907) funereal wares from China. Ancient bronzes may also be dated in this way, through their ceramiclike casting cores. The fabric of Australian Aboriginal cooking hearths offers further potential usage, extending the timescale covered by some 40,000 years. The dating has been used to date sediments and calcium carbonate deposits in caves, such as that at Caune de l'Arago, southern France, a lower Paleolithic cave site inhabited around 350,000 years ago. Because of TL's lower rate of accuracy, radiocarbon dating is preferred where suitable organic samples are available. Contamination of the sample is a problem, and the radioactivity of the surrounding soil has to be considered when evaluating the results.