Drilling Porcelain is a delicate process. Image by Doreen Stoneham
Sampling for Thermoluminescence Testing
In order to date a piece using this method, we take samples of the body using a diamond core drill under running cold water. The base is the normal sampling site. We do not drill in glazed areas for aesthetic reasons, and also because the glaze is vitrified, not crystalline, and cannot be dated. The cores are typically 5mm long by 3mm diameter.
Porcelain Dating: Preparation
In order to prepare the porcelain, we mount the cores in hollow fiberglass tubes and fix them in-situ with acrylic glue. When this is dry, we mount the tubes on a slow speed diamond cutter, and cut slices 200 microns thick using a thin diamond blade. Typically, we obtain six slices from each core with this process. It is possible to remove remnants of glue around the slices by washing in acetone before the sample is used for measurement.
TL Measurements
An automated TL reader is used to measure thermoluminescence in the porcelain samples. The TL reader is programmed to carry out a regime of irradiation with a calibrated beta source before subsequent heating, and TL output is recorded using a sensitive detector. The results are displayed as a graph of temperature vs TL intensity, called a glow-curve, and the lifetime dose is calculated from these measurements.
These results are put into the equation mentioned above, to calculate the approximate age of the sample.
Thermoluminescence results for ancient and modern porcelain. Image by Doreen Stoneham
TL Response of Ancient (Genuine) Porcelain
As you can see in the image on the left, Sn,(red) is a measure of the accumulated dose and is much higher than the background So (black). It is close to Sn+100 (green), the laboratory calibrating dose).
TL Response of Modern Porcelain
In the image on the right, showing the thermoluminescence of a piece of modern porcelain, Sn (red) is hardly raised above the background So (black)indicating a near zero accumulated dose and is far below Sn+100 (green), the laboratory calibrating dose).
Forging TL:
Forgery in China is very sophisticated, and forgers all around the world use a variety of methods to fake thermoluminescence, including the following:
- Irradiation: Since TL is a measure of accumulated radiation dose, many pieces are artificially irradiated so that measurements approximate the dose corresponding to a genuine piece. We have been alerted to this, and run a battery of tests to check if this has happened.
- Inserted Bases: Many modern pieces have inserted old bases, which are very skillfully disguised. If the object stands in acetone for some time, the glue will soften, and the base will part from the top. When we suspect an inserted base, we ask permission to sample from the upper part and reveal the forgery.
Thermoluminescence and Genuine Porcelain
TL is no problem if you are selling that piece of porcelain from Grandma’s attic, but if you are thinking of spending millions of dollars in the saleroom, get expert advice, get checks done on glaze composition and definitely have a TL test. And don’t forget to check out those bases.
Resources
The Telegraph. Ming ceramic used as a doorstop sells for $1.3million. (2012). Accessed October 8, 2012.
China Culture. Ming Dynasty Vase Breaks Auction Record at Sotheby’s. (2011). Accessed October 8, 2012.
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