Heat treatment technique and fuelwood density shape silcrete flake-edge geometry

Silcrete heat treatment was a transformative technological innovation in the Middle Stone Age, enabling toolmakers to improve the flaking properties of this raw material. Recent research has highlighted that silcrete heat treatment is not a uniform process, different silcrete sources and heating strategies can produce variable outcomes. Here we present experimental results testing how heat treatment affects the microscopic edge sharpness and geometry of silcrete flakes from the same source, comparing heated and unheated flakes and evaluating differences between two heating configurations: sand-bath heating (slow, insulated heating beneath sediment) and open-ember heating (direct exposure to embers). We heated silcrete nodules from a controlled source using these two methods (with various fuel woods), then flaked them and analyzed the resulting flakes via micro-CT. We used two complementary curvature methods: best-fit circle (BFC), which measures flake-edge curvature, and three-point stepping (TPS), which measures local curvature across the flake edge and adjoining surfaces. We compared heated flakes with unheated controls from the same silcrete source. Heat-treated flakes show sharper edges and smoother surrounding edges than unheated flakes. Within the heated sample, ember heating produced sharper edges than sand-bath heating. Fuelwood density moderated this effect mainly under sand-bath heating, where high-density wood fuels yielded sharper edges and smoother surrounding surfaces than low-density fuels, while ember-heated flakes were comparatively similar. These results show that heating technique and wood fuel properties impacted different components of silcrete flake edge geometry. Prehistoric toolmakers may therefore have balanced wood fuel availability, heat treatment preparation costs, nodule fracture risk, and desired flake edge form when selective among heat treatment strategies.