Another super quick post but a link worth including here: http://www.oxfordauthentication.com/
One thing to be aware of is that thermoluminescence testing works only with fired items, in other words items like porcelain, earthenware or bronze. If you don’t know what thermoluminescence testing is have a quick read of the snippet below from their website:
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.
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 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.
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.
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.
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:
Approximate AGE = Total absorbed dose (from TL analysis) Annual dose-rate (from alpha counting)
Quoted age limits. All age ranges are quoted with ±20% limits.
Reprinted from http://www.oxfordauthentication.com/
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