Powder samples and the clay casting cores of bronzes are prepared by sedimentation in acetone: fine grains between 2 and 10 microns are selected and deposited onto aluminium or rhodium-plated copper disks. After drying overnight, they are ready for TL analysis. Larger grains are not used for TL, but are analysed for their radioactive content on Elsec thick source alpha counters.

 

Porcelain and certain other types of clay cannot be tested using the fine-grain method. The pre-dose method is then used. The TL reader is programmed to measure changes in the 110°C peak of quartz (the pre-dose peak) in the clay. Each time the sample is irradiated and then heated, the pre-dose peak increases. The increase is related to radiation dose. The first increase is due to the natural dose which the piece has absorbed over its life-time. The sample is then given a laboratory irradiation and a second increase is measured. From these measurements an age of the piece can be calculated.

 

Measurements are taken on RisØ TL readers that run controlled sequences of the heating of the disks at a constant rate of 5°C per second to a temperature of 500°C and of the laboratory irradiations. Once a sequence has finished on the TL readers, the data files are analysed. The TL signal is displayed as a glow-curve, a graph of Intensity versus Temperature. The natural or archaeological glow-curve is compared to another that is obtained after known laboratory irradiation.

 

From this, the total radiation dose absorbed by the object since it was last fired can be calculated. The environmental contribution to this total has to be estimated based on data from dated archaeological sites. The approximate age is given by:

Total absorbed dose (from TL Analysis)

Approximate Age = -------------------------------------------------------------

Annual dose rate (from alpha counting)

 

Several measurements are made to ensure that the results are reproducible.

 

The datable TL signal comes from quartz and feldspar crystals in the clay, referred to as TL minerals. The TL signal for TL minerals grows linearly with age. However, if there are no TL minerals, or if there are other minerals which decompose on heating and produce TL, there will be a spurious signal that swamps the archaeological signal and the piece cannot be dated.