Heat treatment significantly increases the sharpness of silcrete stone tools

Abstract

Humans were regularly heat-treating stone tool raw materials as early as 130,000 years ago. The late Middle Stone Age (MSA) and Late Stone Age (LSA) of South Africa's Western Cape region provides some of the earliest and most pervasive archaeological evidence for this behaviour. While archaeologists are beginning to understand the flaking implications of raw material heat treatment, its potential functional benefits remain unanswered. Using silcrete from the Western Cape region, we investigate the impact of heat treatment on stone tool cutting performance. We quantify the sharpness of silcrete in its natural, unheated form, before comparing it with silcrete heated in three different conditions. Results show that heat-treated silcrete can be significantly sharper than unheated alternatives, with cutting forces halving and energy requirements reducing by approximately two-thirds. The data suggest that silcrete may have been heat treated during the South African MSA and LSA to increase the sharpness and performance of stone cutting edges. This early example of material engineering has implications for understanding Stone Age populations' technological capabilities, inventiveness and raw material choices. We predict that heat-treatment behaviours in other prehistoric and ethnographic contexts may also be linked to increases in edge sharpness and concerns about functional performance.

Testing inter-observer error under a collaborative research framework for studying lithic shape variability

“…This is especially pertinent in our case as the shapes obtained from these materials are likely to be more representative of the actual African stone tools that have been recorded in the main project. However, we note that heat-treated silcrete may achieve a grain as ne as int (Key et al 2021), and that obsidian can be even ner-grained than int; since both silcrete and obsidian are raw materials commonly found in African Middle Stone Age assemblages, we suggest that the int used here acts as a suitable middle ground in terms of granularity and can therefore be considered as broadly comparable to those raw materials studied in the main project.…”

“…The reference tools varied in both size and shape, encapsulating a range of morphologies characteristic of the empirical sample to be studied in the main project (African Middle Stone Age assemblages). While int is not a feature of African lithic assemblages, it could be considered representative of the nergrained materials, such as obsidian, chert, and heat-treated silcrete, exploited during the Middle Stone Age (Key et al 2021;Sahle et al 2013). The tools were produced on akes and retouched using: (1) direct freehand hard hammer percussion (quartzite hammerstones); (2) direct soft free hand hammer percussion using an antler hammer; (3) handheld pressure aking using an antler tine supported in a tanned leather pad.…”

Testing inter-observer error under a collaborative research framework for studying lithic shape variability

“…The reference tools varied in both size and shape, encapsulating a range of morphologies characteristic of the empirical sample to be studied in the main project (African Middle Stone Age assemblages). While flint is not a feature of African lithic assemblages, it could be considered representative of the finer-grained materials, such as obsidian, chert and heat-treated silcrete, exploited during the Middle Stone Age (Key et al 2021 ; Sahle et al 2013 ). The tools were produced on flakes and retouched using: (1) direct freehand hard hammer percussion (quartzite hammerstones), (2) direct soft free-hand hammer percussion using an antler hammer and (3) handheld pressure flaking using an antler tine supported in a tanned leather pad.…”

Recognizing heat‐treated chert: A comparison between infrared spectroscopy and fracture surface analysis

“…Heat treatment is a process that causes mechanical changes in rocks. It leads to an improvement of silica rocks' knapping quality and of the sharpness of cutting edges (see among others, Crabtree and Butler 1964;Purdy and Brooks 1971;Collins and Fenwick 1974;Flenniken and Garrison 1975;Anderson 1978;Domanski and Webb 1992;Torchy 2013;Key et al 2020.…”

“…Heat treatment is a process that causes mechanical changes in rocks. It leads to an improvement of silica rocks’ knapping quality and of the sharpness of cutting edges (e.g., Anderson, 1978; Collins & Fenwick, 1974; Crabtree & Butler, 1964; Domanski & Webb, 1992; Flenniken & Garrison, 1975; Key et al, 2020; Purdy & Brooks, 1971; Schmidt et al, 2019; Torchy, 2013).…”