Recent reports of discoveries of Chinese Ming porcelain which have lain for years in people’s houses selling for millions of dollars has rocked the art market. How does the purchaser confirm that the pieces are in fact genuine?
First there is the expert’s opinion – However it is not unknown for two experts to disagree.
There is also a scientific route which relies on comparing a sample of glaze or body with a database of excavated material.
Unfortunately, however sophisticated the analyses are, they rely on comparison, and there is the risk of a false positive result. In addition, these databases are published and the forger has access to them.
Finally there is thermoluminescence dating, the TL test, a so-called absolute method, which relies on measurement alone to calculate an approximate age. There are forgers who attempt to fake the age of porcelain, even via the TL tests, but today’s science is sophisticated enough to detect even the most careful forgery.
Thermoluminescence and the TL Test
Thermoluminescence is light emitted by some minerals, called TL minerals, when they are heated following irradiation. In porcelain the TL mineral we use for dating is quartz, which on heating emits TL in the blue-ultraviolet region of the spectrum.
Quartz crystals in the porcelain body are bathed in a constant flux of radiation emitted by radioactive impurities in the porcelain: Uranium (U), Thorium (Th) and potassium-40 (K-40) as well as environmental radiation. The radiation is constant, so that the thermoluminescence observed when a sample is heated is proportional to the total radiation dose absorbed over the lifetime of the piece.
Why Does TL Work?
The act of firing the porcelain during its manufacture drains the minerals of the thermoluminescence which has built up over geological time. In other words, at firing, the TL signal is zeroed, so a recently-fired object will emit little or no TL. Gradually, however, the thermoluminscence will build up again. The intensity of the TL on subsequent heating is proportional to the time which has elapsed since the last firing, usually the kiln firing. This measure of thermoluminescent intensity enables us to date the piece.
What Information do we Need to Date Porcelain?
- The radiation dose absorbed by the object since firing is calculated by irradiating slices of porcelain with known doses from a calibrated radiation source and measuring the thermoluminescence.
- The natural radioactivity in porcelain comes from uranium and thorium atoms in the clay as well as an isotope of potassium, K40. The contribution to the dose-rate from uranium and thorium is measured by counting alpha particles emitted from crushed sample using a technique called thick source alpha counting; the powder is spread on a scintillation screen which emits a flash of light every time it is hit with an alpha particle. The contribution from K-40 is calculated from chemical analysis of the clay.
- The environmental contribution has to be estimated as the object has been excavated. This is normally only a small fraction of the total dose-rate.
Age = (Lifetime Radiation Dose)/(Annual Dose from U/Th +K40 + environment)
Age Limits: We quote error limits of +/-20% and express the age as a 40% range.
This information together is sufficient to establish that the piece is either genuine or modern. Research of our results (unpublished) indicate that 92% of the pieces which fall within this limit are either genuine or modern. A small percentage will fall just outside these limits (which would be predicted) and the conclusions are clear. Of the remaining 8%, the pieces are either undatable (clay unsuitable) or misattributed.