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TL and OSL Dating
Thermoluminesence (TL) and Optically Stimulated Luminesence (OSL) are related techniques which measure when objects
were last heated (TL) or when buried deposits were last exposed to light (OSL).
TL and OSL are known as 'electron trap' techniques. Some natural materials such as various stones and soils (and
also things made from them, such as pottery and stone tools) absorb or 'trap' naturally occurring electrons from
their surroundings. This happens at a known and regular rate until the material becomes saturated with electrons
after about 50,000 years. Since the world is much older than this, most objects are already saturated. However, if
these substances are heated (such as when pottery is fired in a kiln or stones are dropped in a fire) this releases
these trapped electrons and resets the 'clock' to zero. The object will then begin to trap electrons again. These
electrons can be released and counted in a laboratory to give a date since the object was fired (TL). Some soils
can have their electron 'clocks' reset simply by being exposed to sunlight. If they are then buried beneath later
deposits, they begin to trap electrons again and can be dated by similar methods (OSL). This produces a date for
the burial of the deposit.
The object to be dated is heated in a laboratory until it glows. Part of this is the ordinary glow of burning, the
remainder is due to escape of these trapped electrons and this is measured. These techniques can date objects up to
50,000 years old, although both are more accurate within the past 10,000 years. Even so, for the past 5000 years
they are less accurate than other dating methods like radiocarbon. They can be useful for dating early sites and
those that don't contain material suitable for radiocarbon or other dating methods.
Learn more:
- M. Aitken, 'Science-Based dating in Archaeology' (Thames and Hudson, London 1990)
- C. Renfrew and P. Bahn 'Archaeology: Theories, Methods and Practice' (3rd edition, Thames and Hudson, London 2000)
Return to Dating Methods.
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