Date Published: July 19, 2017
Publisher: Public Library of Science
Author(s): Patrick Schmidt, David J. Nash, Sheila Coulson, Matthias B. Göden, Graeme J. Awcock, Michael D. Petraglia.
Heat treatment was one of the first transformative technologies in the southern African Middle Stone Age (MSA), with many studies in the Cape coastal zone of South Africa identifying it as an essential step in the preparation of silcrete prior to its use in stone tool manufacture. To date, however, no studies have investigated whether heat treatment is necessary for all silcrete types, and how geographically widespread heat treatment was in the subcontinent. The aim of this study is to investigate experimentally whether heat treatment continued further north into the Kalahari Desert of Botswana and northernmost South Africa, the closest area with major silcrete outcrops to the Cape. For this we analyse the thermal transformations of silcrete from both regions, proposing a comprehensive model of the chemical, crystallographic and ‘water’-related processes taking place upon heat treatment. For the first time, we also explore the mobility of minor and trace elements during heat treatment and introduce a previously undescribed mechanism—steam leaching—causing depletion of a limited number of elements. The results of this comparative study reveal the Cape and Kalahari silcrete to respond fundamentally differently to heat treatment. While the former can be significantly improved by heat, the latter is deteriorated in terms of knapping quality. These findings have important implications for our understanding of the role of fire as a technical solution in MSA stone tool knapping, and for the extension of its use in southern Africa. Silcrete heat treatment—at least in the form we understand it today—may have been a strictly regional phenomenon, confined to a narrow zone along the west and south coast of the Cape. On the basis of our findings, silcrete heat treatment should not be added as a new trait on the list of behaviours that characterise the MSA of the southern African subcontinent.
Heat treatment was one of the first transformative technologies employed by our human ancestors in the southern African Middle Stone Age (MSA), with its use argued to shed light on socioeconomic processes, early resource management, and even cognitive and social capacities [1–6]. Its main application was to improve the flaking qualities of silcrete, a raw material used in tool manufacture throughout the Stone Age in the subcontinent. Controlled experiments on samples from the Cape coastal zone of South Africa indicate that silcrete in this area responds to heat treatment with significant improvement in workability, primarily due to the loss of chemically bound water during heating, and the formation of new Si-O-Si silica bonds [7, 8]. So far, six Middle and Late Stone Age (LSA) sites in South Africa have yielded published evidence of silcrete heat treatment—Pinnacle Point ; Diepkloof Rock Shelter ; Mertenhof Rock Shelter ; Klipdrift Shelter ; Blombos Cave ; Elands Bay Cave –and publications on several other sites are expected in the near future (Schmidt, personal data). Strikingly, in some of the assemblages of these sites, the vast majority of silcrete was heated prior to use. Because of this abundance of heat-treated silcrete in the archaeological record of the Cape, silcrete has, for some, acquired the status of a material that needs to be heat-treated to be knappable  (for pressure flaking ). To date, however, it has not been investigated as to whether heat treatment is necessary for all silcrete types, and how geographically widespread heat treatment was in the subcontinent.
The results of this comparative study suggest that samples of silcrete from the Cape coastal zone and Kalahari respond fundamentally differently to heat treatment. While the former can be improved significantly as a tool-making stone by heat, the latter is deteriorated in terms of its suitability, to the extent that heat treating silcrete from the Kalahari would have been a counterproductive technology. We attribute this to differences in the petrography, crystallography and mode of genesis of silcrete in the two regions.