Thermoluminescence Dating of Ceramics

Thermoluminescence dating is based on the principle that natural radiation is stored by means of electrons in the crystal lattice of quartz or other minerals present in stone and other deposits. These in turn are present in the materials used for making ceramics.

When during the TL test the porcelain is heated above a certain temperature (600°C) or higher, its heat energy is released by emitting light waves. The intensity of the light is proportional to the time during which natural radiation energy accumulated in the substance. With the emission of that energy, the level of radiation energy stored in the material is reset to zero.

As the firing temperature of ceramics is usually exceeding 600°, this means the stored radiation energy is reset to zero at the time when a ceramic item is fired in the kiln, and the radiation energy starts accumulating anew from this point.

Thermoluminescence (or TL) dating uses this principle, measuring the emitted light glow to determine the period of time that passed since an item was fired.


Drawbacks:

  • Exposure to an accidental fire may not necessarily damage ceramics, but their energy level can be reset.
  • A thermoluminescence test also resets the stored radiation energy, which afterwards accordingly has a thermoluminescence age equalling zero.
  • Any other exposure to the minimum amount of heat will reset the accumulated radiation energy
  • Other forms of energy may also cause a release of the stored radiation energy. Additional causes for resetting the energy level are: grinding, crystallization, exposure to sunlight or heat.
  • When an item passes through X-ray safety inspection at an airport, or customs inspection at a sea port, the stored energy can be reset or reduced, resulting in an unreliable age test result.


Defeating this dating test:

Exposing ceramics artificially to radiation will enable fakers to artificially produce a high level of stored radiation energy.
Currently, it seems that some use X-rays for this, but the "loading" with this sort of high energy can result in a TL age that is much higher than an item could possibly have, if genuine.
For example, a "Tang" dynasty item may show an age of 2000 years. This would expose the fact that it was artificially irradiated.
However, as soon as a way is found to accurately "add" the proper radiation dose, the thermoluminescence dating method is not reliable anymore.

However, it appears that thermoluminescence dating is feasible only for porcelain of an age that is in the hundreds, or thousands of years. With Chinese ceramics this would mean that only ceramics older than the Qing dynasty can be measured properly. Interpolation is used to compensate for this. However, this again means that short-term measuring is simply not completely reliable.


Remark:

Think that exposing porcelain to X-rays is difficult for the average person, as he does not have access to the equipment?
Think again... in the more recent past in parts of the Chinese world there were lots of shops specialized in doing various examinations, including X-raying for general practitioners or physicians, who did not have the necessary expertise or equipment. Although now many seem to have their own X-ray machines, I am sure that there are still many of those old shops to be found. For a fee they will use it!


And while you are here, it is recommended to have a look at the "Case Studies" on this authentication site. They are good examples of how fakers try to cheat the tests.



Scientific Authentication


The problem is...

Personally I feel uncomfortable with this method of dating. Like Radiocarbon Dating that has been used for decades in archaeology for dating organic materials, it is based on an axiom. The assumption is that the level of natural radiation has always been the same as it is now. If, however, any major radiation event occurred in the past couple of hundred years, that is, if radiation was not always constant, then the theory is flawed.
Do we know whether it has been constant? As mentioned above, I feel uncomfortable with this. Particle physics is a science that is still too young to have reliable data going back such long time.



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