Authenticating Chinese porcelain
To authenticate a piece of porcelain, be it Chinese, the information sought is the age of manufacture of the support material. The most suitable tool for this investigation is thermoluminescence or the TL test. Like all dating methods, it can be used to date an event. In the case of TL, this is the last heat recorded by the material. As a general rule, this will be the firing of the material following its shaping.
The properties of thermoluminescence
The phenomenon of thermoluminescence and its applications have been known and widely described for several decades. Thermoluminescence properties are directly linked to the presence of crystals (mainly quartz) in terracotta. Quartz has the basic chemical formula SiO2. This means that quartz is composed of silicon and oxygen. On an atomic scale, a quartz crystal is a stack of small four-sided pyramids (tetrahedrons) in which there is an oxygen atom at each vertex, and a silicon atom in the middle of the pyramid. The oxygen atoms are shared with the adjacent tetrahedrons. This forms the basis of a perfect crystal. However, this crystal lattice contains defects: gaps, substitutions and the presence of interstitial ions. The existence of these defects is fundamental to TL dating, as they are capable of storing energy. These defects are filled by a steady supply of energy over time. This energy comes from natural radioactivity. As radioactivity is a regular phenomenon, it is correlated with the passage of time. As a result, the more time passes, the more energy is added to the crystals. In the laboratory, we heat the material and record and quantify luminescence (light emission). The amount of light emitted is proportional to the energy stored in the crystals, itself proportional to the time elapsed since the terracotta was manufactured.
Thermoluminescence method detects fake porcelain
Porcelain, like earthenware and stoneware, is fired at very high temperatures during manufacture. This high temperature results in partial or total amorphization (vitrification) of the material. Exposure to high temperatures alters the crystalline lattice, destroying the trapping centers or defects that store radioactive energy. This deprives us of all or part of the information we're looking for, making the material difficult to date by thermoluminescence. However, by adapting the TL measurement protocol, we can date virtually all porcelain. Whereas conventional thermoluminescence heats the material to 500°C, the alternative predose technique only heats to 200°C and focuses on a low-temperature signal at around 110°C.
The predose protocol is really dedicated to porcelain and earthenware, as it is the only way to obtain a date on this type of material heated to high temperature.
The development of counterfeiters' techniques has made thermoluminescence an indispensable tool for detecting fakes. However, as we saw in a previous article, TL tests must be combined with X-ray imaging (radiography or scanner) to detect new fakes.
