The mechanical properties of heat-treated rocks: a comparison between chert and silcrete

In archaeology, heat treatment of stone is the process of making a new material for tool production. Its invention in the African Middle Stone Age was an important step in the evolution of transformative technologies and the cultural evolution of early humans in general. Although the chemical and crystallographic transformations in silica rocks, the only material class heat-treated in the Stone Age, begin to be well known, many of the mechanical transformations and their chemical origins remain a subject of controversy. The difference between different silica rock categories is also only poorly understood. In this paper, we investigate the thermally induced changes of three mechanical properties in the two silica rock types chert and silcrete: fracture strength, indentation fracture resistance (approximating fracture toughness) and elastic modulus. These tests are complemented by statistical analyses (Weibull modulus) and a quantitative fracture surface analysis. The results show that heat treatment transforms these silica rocks in terms of their fracture toughness and the uniformity of fracture. A comparison with published data on the structural transformations in the same samples identified the loss of chemically bound water and subsequent defect healing to be the chemical mechanism behind these mechanical transformations. These findings have important implications for the study of the interactions between chemical and structural processes and the mechanics of natural rocks or ceramics.