What causes failure (overheating) during lithic heat treatment?

Abstract

Heat treatment of lithic raw material, i.e. the intentional alteration of silica rocks for improving their knapping quality, is a process that may require great care and precisely controlled conditions in order to avoid failure due to overheating. The physical causes of overheating remain poorly understood leading to problems in the interpretation of heat-treated artefacts and/or fire-related taphonomic alteration of different types of silica rocks. This driving force of overheating is investigated by a set of experimental heat treatment sequences with different ramp rates and different volumes of flint with a well-defined mineralogical composition, porosity and water content. The results of this experiment show the main cause of heat-induced fracturing to be the vapour pressure in fluid inclusions within the rocks. Heterogeneous thermal expansion could be discarded. The interdependence between volume and heating rate is also shown. These results have implications for the study of archaeological heat-treated rocks, the understanding of taphonomic heat-induced fracturing of silica rocks and experimental flint knapping.