Ageing of fired-clay ceramics: Comparative study of rehydroxylation processes in a kaolinitic raw material and moon-shaped ceramic idol from the Bronze Age

• Unstable non-crystalline residua of clay minerals have the ability of rehydroxylation.
• The temperature and soaking time of the firing process affect the breakdown of kaolinite structure.
• French standard method is not efficient for the rehydroxylation studies of archaeological ceramics.
• Hydrothermal ageing at 230 °C for 10 h can prove rehydroxylation of kaolinite.
• The disorder of kaolinite structure shifts the temperature of peak at DSC curve to lower value.

ABSTRACTSources of raw materials and identification of the provenance of ancient ceramics as well as the estimation of the firing temperatures of ceramic findings are two principal problems archaeologists solve with material scientists. These two problems are closely interconnected. This paper is focused on the identification of clay mineral kaolinite in a ceramic body of archaeological ceramic idol in comparison with a reference kaolinitic raw material. Kaolinite identified in studied ceramic samples can be from a primary source or a product of rehydroxylation of metakaolinite of the ceramics. The rehydroxylation process is simulated here in laboratory conditions in an autoclave by modifications of standardized test methods. The rehydroxylation process was studied on a kaolinitic raw material and the acquired results were then applied to evaluate samples of a unique archaeological object from the Late Bronze Age — a moon-shaped ceramic idol, which was found at an archaeological site near Prague in the Czech Republic. XRF, XRD, TG-DTA, TG-DSC and IR analyses were used to study this process. XRD and TA proved the presence of the clay mineral kaolinite in its end parts. A small amount of kaolinite was also detected in the central part, but only by TA. It was proved that kaolinite in the end parts originated from raw material and that its presence in the central part was caused by the rehydroxylation of the metakaolinite. It was probably due to the use of the moon-shaped ceramic idol as an andiron in an open fireplace.

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