A Review of Heat Treatment Research

The recognition of intentional heat treatment of stone artifacts in the archaeological record has been a major research topic since the initial study by Crabtree and Butler (1964). Ethnographic and archaeological records reveal two methods of intentional thermal alteration: controlled heat treatment most commonly used to enhance the flaking quality of stone, and thermal shock fracturing often applied to produce angular facets on rounded nodules of tough lithologies. During controlled heat treatment, the stone was heated slowly and evenly to the temperature needed to bring about the desired flaking properties, which become more like those of high quality lithologies like obsidian. The response of a lithology to heat treatment is variable, and the same improvements in flaking properties can be accomplished by a variety of heating and cooling rates, provided the heating/cooling is not too rapid and a maximum temperature (350–400°C for cherts) is not exceeded. The improved workability makes soft hammer and pressure flaking more efficient, allowing greater conservation of raw material and manufacture of extensively retouched took. Controlled thermal alteration was normally applied to microcrystalline silicious materials; it began in the Middle Palaeolithic (around 110,000–90,000 B. P.) and was widespread around the world, but in most areas it became obsolete in the Early Bronze Age.Heat-treated artifacts can be recognised by a marked luster increase, particularly a high contrast between lustrous and non-lustrous fracture surfaces. Color changes during heating, generally towards more reddish hues, are less consistent. Heat treatment results in reduced fracture toughness of microcrystalline siliceous materials; this can be readily identified by mechanical testing. Quartz grains recrystallize and become more equigranular during heat treatment, allowing fractures to propagate more readily and accounting for the enhanced knappability. In addition, healing of microcracks may occur in very fine-grained siliceous materials.