Identification of heat-treated lithic artifacts via quantitative surface gloss characterization

Heat-treated siliceous artifacts from archaeological contexts provide valuable insights into the raw material economy and cognitive prowess of hominins responsible for such a sophisticated and transformative technology. Successful heat treatment induces molecular and structural changes that enhance the mechanical properties of toolstones, ultimately making them more knappable. Among the measurable transformations attained through heat treatment is a reduction in fracture toughness, which reduces the force required for fracture propagation. The internal changes during heat treatment also lead to significantly reduced surface roughness that is visually evident through a characteristically glossy appearance as a result of the improved microscopic smoothness. Reduced surface roughness (i.e., glossiness) is a reliable proxy for improved homogeneity and brittleness, hence fracture predictability. In this study, we present a simple, cost-effective, highly accurate, and field-ready technique for identifying heat-treated siliceous artifacts by measuring gloss as a function of the reduction in surface roughness. Our approach allows archaeologists to overcome the limitation of flat and opaque surfaces required for measurements by conventional gloss meters.