The thermoluminescence phenomenon arises from the irradiation of mineral inclusions by the radioactive impurities present in every ceramic paste, from the ejection of electrons due to this external energy supply, from the confinement of these electrons freed from the attraction of the nucleus into traps or deformations of the crystalline lattice, and finally from the return of the trapped electrons into the atoms owing to the external energy contribution resulting from the increase of temperature.
All ceramics having been fired at a temperature of at least 500° C ; earthen cores found inside bronzes (or other alloys) cast following the lost wax technique.
The thermoluminescence is not very reliable in regards to the African and Chinese terracottas. Indeed:
- AFRICANS (principally in Mali and Nigeria) make modern forgeries by collecting old ceramic fragments on archaeological sites, by grinding these sherds and by recreating statues with the pulverized earth and an organic or mineral binder
- ASIANS (in China and Hong Kong) make recent fakes which they irradiate with gamma rays to induce the thermoluminescent phenomenon
To remedy the lack of reliability of the T.L. dating, the laboratory recommends to submit archaeological terracottas to a surface examination with the help of a binocular microscope, in order to reveal possible evidences of a long burying in the ground.
Thanks to a long-term collaboration with the ISIB institute, the Brussels Art Laboratory will be able to benefit from a new technique of ceramic dating (O.S.L). No forger will be able to get round this method since it is based on the estimate of the alpha dose received by the ceramics.