Earthenwares and ceramics

Thermoluminescence is the most appropriate test to guarantee the authenticity of earthenwares and ceramics.

Thermoluminescence age test

Thermoluminescence is based on an ingenious principle: the crystals contained in earthenwares (quartz, felspars) have the ability to accumulate energy over the time. When heated, these crystals release this energy in the form of light, a phenomenon known as thermoluminescence. This light emission is directly linked to the time elapsed since the object was last fired.

Thermoluminescence curves for a terracotta from the Ban Chiang culture. The average age of the last firing is between 1800 and 2400 years.

Storage of Luminous Energy: the quartz and feldspars crystals found in earthenwares and ceramics have the ability to store energy. This energy comes from surrounding natural ionising radiation, such as cosmic rays and radioactive elements in the soil.

Light Emission on Heating: when the object is subjected to a source of heat, for example during its initial firing, the electrons trapped in these crystals are released from their « electron traps ». This release of electrons is accompanied by the emission of light, a phenomenon known as thermoluminescence.

Resetting the "Electrons Traps": after this initial firing, the « electron traps » in the crystals are reset and begin to trap electrons again as a result of continued exposure to the surrounding natural radiation.

Luminescence Measurement: to date the object, a sample of these crystals is heated in a controlled manner in the laboratory. During this process, the trapped electrons are released, emitting light. The amount of light emitted is directly proportional to the time elapsed since the object was last fired. By measuring the intensity of this luminescence, it is possible to date the last time the object was fired.

Study of engobes and polychrome decorations

It is often important to check the nature of the polychrome decoration on earthenwares. Ancient ceramics may have lost their original slip and have modern polychromy.

Pigment Analysis is then used to verify the authenticity of the polychromes coatings and validate the integrity of the object. Stratigraphic analysis based on slips micro-samples is carried out using optical microscopy and SEM-EDX to identify and characterise the original polychrome layers beneath the modern overpainting.

Tang dynasty polychrome terracotta with cement and vermillion based red restoration overpainting on remains of an earlier orange litharge-based polychrome (arrows).