Thermoluminescence

A small sample of ancient pottery will emit a faint blue light when heated to a sufficiently high temperature. This faint blue light is known as thermoluminescence, or TL and is over and above the background red glow that is emitted from all materials. The TL can be measured using a sensitive detector known as a photomultiplier tube. The intensity of the TL signal is proportional to the time which has elapsed since the clay was last heated, normally since the kiln firing, and can be used to date when the object was last fired.

TL from natural minerals

Sampling

Pottery
Only a fully qualified representative of Oxford Authentication Ltd is authorised to take a sample of powder. 100mg of powder is removed from an inconspicuous area of the object. Usually more than one sample is taken from each piece to verify that all the parts are of the same antiquity. Each piece is fully documented and photographed at the time of sampling, and the samples are sent to our laboratory in Oxfordshire for analysis.

Porcelain
Porcelain and stoneware are fired to a higher temperature than pottery, and the clay body is much harder. It must be sampled using a diamond core drill under running water. Two 3-mm diameter cylinders, 4mm long, are removed from the base or other unglazed part of the object. These are then cut into 200-micron (one fifth of a millimetre) slices using a fine diamond wheel. These slices are used for the TL measurement.

Drilling porcelain under water
with a diamond core drill.

The 3mm diameter core	Porcelain slices ready for TL measurement	The core is sliced on a water-cooled Buehler Isomet low-speed saw using a fine diamond impregnated blade

Bronze casting cores
Bronze antiquities are often cast around a clay mould and this casting core is trapped inside otherwise hollow sections, such as handles, legs, heads, or torsos. Once this core has been extracted through the bronze outer casing, the sample can be dated in the same way as pottery.

This Chinese bronze rhino dating from the Tang dynasty was authenticated using casting core from within the body. Courtesy: The Asian Art Museum, San Francisco.	Casting core embedded in a bronze section.

The Test

Powder samples are prepared by sedimentation in acetone: fine grains being deposited onto aluminium discs. After drying overnight, they are ready for TL analysis. Larger grains are not used for TL, but are analysed for their radioactive content. Bronze casting cores are prepared in a similar way.

Fine grains evenly deposited onto aluminium discs

Measurements are made on our RisØ TL readers, which we maintain at the state-of-the-art through regular involvement with the RisØ team. The TL readers have been programmed to run controlled sequences of heating and of laboratory irradiations. Radioactive analyses are carried out on our Elsec thick source alpha counters.

The RisØ TL reader.

TL analysis 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 (see CASE STUDIES, genuine and fake Tang ladies). We compare the natural or archaeological glow-curve with that obtained after a known laboratory irradiation. This enables us to calculate the total radiation dose absorbed by the object since it was last fired. Radioactive analysis from the thick source alpha counters gives the annual internal dose-rate. The environmental contribution to this total usually has to be estimated based on data from dated archaeological sites. The approximate age is given by:

Quoted age limits. All age ranges are quoted with ±20% limits.