| Abstract: | Stone tool manufacture by many prehistoric and recent societies was characterized by deliberate heating of fine grained siliceous rocks to improve their flaking properties. Extensive mechanical testing of heated and unheated cryptocrystalline and macrocrystalline quartz lithologies has shown that thermal treatment causes a consistent marked reduction in fracture toughness. This mechanical property can be used as an objective measurement of the flaking qualities of stone materials, and a reliable criterion for the recognition of intentionally heated artefacts in the archaeological record. X-ray diffraction studies and scanning electron microscopy have demonstrated that the change in fracture toughness with heating is the result of recrystallization. The poorly ordered, strongly interlocking cryptocrystalline fabric of the unheated samples becomes more equigranular and better crystallized with thermal treatment. As a consequence, fractures propagate more readily in heated samples, accounting for their better flaking properties. |
