TL and OSL
Thermoluminesence (TL) and Optically Stimulated Luminesence (OSL) are techniques which measure the energy stored in heated or fired objects (TL) or can show when buried deposits were last exposed to light (OSL).
TL and OSL are known as ‘electron trap’ techniques. Quartz crystals in clay, glass, burnt stone and sedimentary soils are made of lattice crystal structures which absorb radioactive electrons (which become trapped in the lattice) at a standard rate over time. When an object is heated to high temperatures (TL) or exposed to sunlight (OSL), particular types of electron trapping mechanism are set to zero. From then on, the rate of electron accumulation starts again and can be measured against the surrounding radioactivity of the soil (which must also be sampled as a control – without this the techniques are much less accurate).
The energy represented by the trapped electrons can be released (and measured) by heating in a laboratory, which produces a range of luminescent glows showing as light. When detached from the ordinary glow of burning, these can be recorded and measured against a curve showing the levels of radioactivity present over the past 50,000 years, although both techniques are more accurate for the past 10,000 years. Even so, for the past 5000 years they are less accurate than radiocarbon dating. TL is very good for dating ceramics which exceed in age the current range of radiocarbon, and OSL can measure the point when layers were covered by silting or deposition, allowing the date of structures to be measured by measuring the last exposure of the underlying soil.