Direct dating archeology
TL dating is a matter of comparing the energy stored in a crystal to what "ought" to be there, thereby coming up with a date-of-last-heated.
In the same way, more or less, OSL (optically stimulated luminescence) dating measures the last time an object was exposed to sunlight.
These dates open the floodgates for researchers to ask and answer questions about the rock art that have baffled them for decades. In some sites, paintings continued to be made for more than a thousand years.
“This is astonishing,” says Pearce, “people returned to the same rock shelters over very long periods of time to make rock paintings very similar to those made centuries or millennia before.
The new dates were obtained using radiocarbon dating.
Over the decades rock art has proved extremely difficult to directly date.
During the 1960s and 70s, the Oxford University Research Laboratory for Archaeology and History of Art led in the development of TL as a method of dating archaeological materials.
The success of this project is based on very careful chemical characterisation of the composition of the paint and contaminants on the rock.
The energy released by stimulating the crystals is expressed in light (luminescence).
The intensity of blue, green or infrared light that is created when an object is stimulated is proportional to the number of electrons stored in the mineral's structure and, in turn, those light units are converted to dose units.
Where De is the laboratory beta dose that induces the same luminescence intensity in the sample emitted by the natural sample, and DT is the annual dose rate comprised of several components of radiation that arise in the decay of natural radioactive elements.
Artifacts which can be dated using these methods include ceramics, burned lithics, burned bricks and soil from hearths (TL), and unburned stone surfaces that were exposed to light and then buried (OSL).