Ethnographic toolstone heat treatment reveals distinctive motives and patterns of material transformation

The heat treatment of rocks to improve their fracture qualities for stone tool production is a key technological innovation of Pleistocene humans. Because the intentionality and technicality of this transformative process are often associated with cognitive prowess, the topic has attracted considerable archaeological research interest worldwide. Yet, with few ethnographic examples, assumptions about archaeological heat treatment are almost always based upon laboratory experiments. Here we report contemporary Konso craft specialists from southern Ethiopia, who represent the last known stone tool makers to regularly heat treat their toolstone. We found that, while material transformations commonly used as indicators of enhanced knappability (e.g., fracture toughness, elastic modulus, improved homogeneity, and newly formed lustre) are discernible in the studied heat-treated toolstones, other qualities of enhanced knappability (e.g., density and hardness) are also informative. Our results highlight the diversity and peculiarity of intentions and mechanisms through which toolstone workability is improved. The motives, techniques, and material transformations involved in Konso toolstone heat treatment afford important insights into various aspects of archaeological heat treatment that are unavailable with experiments alone.