Thermoluminescence (TL) is a faint blue light that is emitted when fired pottery is heated at a sufficiently high temperature. When clay is fired in a kiln, the geological thermoluminescence is released. During its lifetime the pottery absorbs radiation from its environment and it is this which creates thermoluminescence. The older the pottery, the more radiation it has absorbed and the brighter the pottery sample glows. By measuring the TL, it is possible to calculate how much radiation has been absorbed and thereby determine an approximate age of the object.
At the forefront of this expertise is Oxford Authentication Ltd., founded in 1997 by Doreen Stoneham. A physicist with nearly three decades of experience at Oxford University’s Laboratory for Archaeology and the History of Art, which pioneered the process, Mrs. Stoneham perfected the technique for testing porcelain and is regarded as the foremost authority in the field of thermoluminescence testing.
Oxford Authentication laboratory is the only facility in the world dedicated solely to thermoluminescence testing with an application for the authentication of antique ceramics, and is the only TL testing laboratory in the world working to full International Quality assurance standards ISO 9001. It is also monitored by the National Radiation Protection Board every year to ensure radiation sources.
In Asia, Antique Authentication is the sole agent for Oxford Authentication.
TL testing can be done on fired clay such as earthenware, stoneware, porcelain and the casting cores of bronzes.
Regardless of cultural origin, any piece 18th century or older that has been fired above 500°C and containing sufficient quantities of quartz and feldspar, and radioactive impurities in the clay, ie. uranium, thorium and potassium, can be tested.
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.
For earthenware objects, a sample of approximately 100 mg is obtained in powder form from inconspicuous areas on the object by using a hand-held drill fitted with a tungsten carbide dental bit. Several samples are often required to verify that all parts are the same material and antiquity. All powder samples are taken in a dark room lit by a red or very dim yellow light in order to prevent the risk of draining away some of the TL with exposure to any form of strong artificial light or daylight.
For high-fired objects such as porcelain or stoneware, sampling is done using a 4-mm diameter diamond core drill under running water to prevent heating. Two 3-mm diameter cylinders, 4-mm long, are removed from the base or other unglazed areas on the object. These are then cut into 200-micron slices using a fine diamond wheel and used for TL measurements. These slices are further crushed for radioactive and chemical analysis in order to calculate the date of last firing.
Bronze antiquities are often cast around a clay mold and this casting core is trapped inside otherwise hollow sections, such as handles, legs, heads and torsos. Once this core has been extracted through the bronze outer casing, the sample can be dated in the same way as pottery.
Under ideal conditions, archaeological sites that have been excavated under controlled conditions and have been monitored in situ can be dated with a precision of ±6%. However, when authenticating an object that has been removed from the archaeological site much of the information has been lost, therefore the quoted age limits cannot realistically be better than ±20%. In most cases this is sufficient to distinguish an authentic piece from a modern copy. The practical limit for obtaining reliable TL results for objects most recently made is late 18th century, i.e. Qianlong Period (1735-1796). This means we are only able to obtain results for pieces 18th century or older.
A single page certificate with a foiled company seal, embossment and signatures is issued for each piece outlining presumed period of manufacture, sample areas and laboratory findings. Additional details of laboratory methods and techniques will also be included.
In some cases, the report will also contain information or comments on false irradiation, restoration material, differing clay or TL properties, and too insensitive or unsuitable clay for TL testing.
WARNING:
It has come to our attention there are fraudulent Oxford Authentication reports in circulation in the market. We advise any questionable reports be checked with Antique Authentication or Oxford Authentication by filling in the form below.
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