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TL/OSL dating, a method used in archaeology by CIRAM laboratories

Friday 9 February 2024

Luminescence dating by TL and OSL is, along with carbon-14 dating, one of the specialties of CIRAM laboratories.

We are aware of the time constraints to which archaeologists are subject, which is why we offer standard turnaround times of 3 months for TL (depending on the laboratory’s production load) and try to keep OSL turnaround times as short as possible. Luminescence dating methods are less well known than carbon 14, and their use remains limited in historical archaeology. They mainly refer to thermoluminescence (TL) and optically stimulated luminescence (OSL) dating.

TL dating, to date the last heating of materials

TL (thermoluminescence) allows the last heating of the material to be dated, whether clay or hearthstones (quartzite, flint, sandstone…). This method is frequently used in prehistory, where remains of other kinds are less abundant.

However, applications have been tested on materials more connected to historical issues, such as architectural terracottas for example. One of the most frequent uses lies in kiln dating, where TL is one of the most suitable methods along with archaeomagnetism.

Complementary study thanks to OSL dating

As a complement to thermoluminescence dating, OSL offers complementary perspectives. In fact, this method makes it possible to date a material’s last exposure to light. It’s not hard to imagine how interesting it can be to date a stratigraphic sequence on a site. Numerous applications have been developed in response to problems associated with the archaeology of historical and protohistoric periods. For example, OSL is used to date the installation layer of a megalith, the sealing of mortar in masonry…

The chronological range of these methods extends from a few hundred to a million years, which allows very wide use, on any type of archaeological site.

The fundamental principles of luminescence dating

Luminescence dating methods rest on common foundations. They rely on the ability of minerals (mainly quartz and feldspars) to record ambient radioactivity over time. The radioactivity absorbed comes from the earth’s surface, essentially from the decay of three radioelements:

  • Potassium (K);
  • Uranium series (U);
  • Thorium (Th).

The emission of particles (alpha and beta) and gamma radiation from these three elements occurs regularly over time and constitutes what is known as the dose rate. This annual dose (I) is supplemented by the effects of cosmic radiation, which vary according to burial depth, altitude and latitude. The quantity of radioactivity absorbed at the moment of measurement is called the archaeological dose (Qnat) or equivalent dose (De).

It is the ratio of these two quantities (Qnat and I) that gives the age between the object’s last heating and its study in the laboratory: