Discover CIRAM's thermoluminescence test protocols, measurements and results interpretation.

The thermoluminescence laboratory test

While dating materials is the most effective way of obtaining a chronological marker to authenticate an art object, it is important to distinguish between the event being dated and the event being dated. Thus, dating a material is not the same as dating its use by man.

Our scientists use carbon-14 dating for organic materials, but we also use other dating methods for inorganic materials. For heated objects, CIRAM uses thermoluminescence, which works on ceramics, porcelain and terracotta in general, with a chronological range of around 300,000 years.

Please note that materials such as stone, metal, glass and pigments cannot be dated. CIRAM laboratories will use analytical alternatives (electron microscope, chemical analysis...) to authenticate them.

The phenomenon of luminescence

In 1664, Boyle reported the phenomenon of heat-stimulated luminescence or thermoluminescence (TL). But its physical properties were not studied until the 1930s, thanks to the development of photomultiplier tubes. In the 1950s, the potential of thermoluminescence was explored for the dating of archaeological materials, particularly ceramics. But the application of TL to dating did not come to fruition until the mid-60s in England. It was not until 1967 that the first TL dating studies were carried out in France.

The principle of thermoluminescence

TL is based on the study of natural radioactivity and the ability of the crystals contained in a terracotta to accumulate the effects of this irradiation. This energy (also called irradiation dose) is proportional to the time the crystals are subjected to this irradiation. In other words, TL corresponds to the emission of light (luminescence) caused by a thermal (thermo) input of energy. Note that the intensity of this luminescence is proportional to the energy absorbed by the crystal, and therefore proportional to time.

TL tests are used to determine the natural radiation dose (QNat) received since time zero (i.e. the last time the material was heated). For true TL dating, two values need to be measured:

• QNat: actual irradiation dose ;
• I: annual radiation dose.

The TL study of objects outside their burial context does not allow for real dating. However, the QNat measurement will assess the age and provide objective information on the authenticity or otherwise of the object studied. For this purpose, we assume an annual radiation dose of between 3 and 6 mGy/year, which corresponds to the vast majority of cases encountered. This lack of annual dose measurement means that the relative accuracy of thermoluminescence age tests is of the order of 20-30% of the estimated age.

Distinction between forgeries and authentic objects

By examining thermoluminescence curves, it is possible to distinguish between modern fakes and authentic objects.

Unlike carbon-14, it is impossible to detect a significant gap between the dated event and the event being dated. Since the firing of terracotta is the last anthropogenic act recorded by the material. The relevance of the result to the problem posed leaves no room for doubt.

The thermoluminescence test

TL testing is relatively simple to perform, but the diversity of materials makes it difficult to adapt measurement protocols and interpret results. Porcelain, for example, is problematic for TL because its manufacturing process involves high-temperature firing, which results in total or partial amorphization of the material. As a result, the physical properties that condition TL operation are altered or even destroyed. In this case, the results of conventional TL will be unusable, and we will use "predose", an alternative technique.

The evolution of counterfeiters' methods, combined with their growing understanding of the analytical techniques we use, makes it increasingly necessary to use TL to detect fakes.

We'll see in a later article that thermoluminescence tests need to be combined with other investigative techniques to detect the so-called "new fakes", which are more successful.

The quest for authenticity in terracotta objects

Factual data derived from physico-chemical analysis, correlated with accumulated knowledge of the evolution of shapes and techniques, fuel the quest for authenticity.

To make a precise diagnosis, our scientists always propose a combination of clues, while remaining aware of the limits of each approach. If certainty is the absolute goal of authentication, it is rarely attainable. However, the detection of a fake will always be more obvious than the certainty of an authentic object. But thanks to our expertise and know-how, the result will always be reliable, objective and relevant.