Heat treatment of Kalahari and Cape silcretes: impacts upon silcrete chemistry and implications for geochemical provenancing
Recent studies in southern Africa and eastern Australia have demonstrated the feasibility of using a geochemical fingerprinting approach to determine the source locations from which silcrete raw materials were procured prior to their use in stone tool manufacture. The impact of intentional heat treatment of silcrete upon its chemistry, however, is unknown, meaning that heat-treated silcrete artefacts have to date been excluded from provenancing studies. This investigation presents the first high-resolution experimental analysis of the impacts of heat treatment upon the chemical composition of silcrete. The study compares the composition of unheated control samples against samples heat-treated to target temperatures of up to 600 °C taken from four silcrete blocks from South Africa and Botswana. Chemical compositions of samples are determined using ICP-MS and ICP-AES. Experimental results indicate that heat treatment has a limited impact upon silcrete chemistry. Only 7 out of 65 minor, trace and rare earth elements analysed (Al2O3, Fe2O3, K2O, As, Cr, Cs and Cu) were depleted beyond expected error limits following controlled heating. There was no consistent pattern of elemental depletion across the four silcrete samples, although a greater number of elements were depleted from chalcedony-cemented Kalahari silcretes compared with microquartz-cemented Cape silcretes. We conclude that it is safe to use chemical data from heat-treated artefacts from the Cape as part of geochemical fingerprinting studies; however, we recommend that Cu and Cs concentrations be omitted from any statistical analyses until the effects of heat treatment upon these elements are fully understood. We echo the conclusions of previous studies by recommending that chalcedony-cemented silcrete artefacts that show signs of burning or intentional heat treatment be excluded from provenancing studies in the Kalahari and potentially elsewhere.
KeywordsHeat treatment Silcrete Kalahari Cape coastal zone Geochemistry
We thank the British Academy and Leverhulme Trust (Small Research Grant number SG142023) for funding the project “Heat treatment of silcrete raw materials: the implications of temperature-induced transformations for archaeological provenancing studies” and ALS Minerals (Sevilla) for geochemical analyses. PS was also supported by the Deutsche Forschungsgemeinschaft (DFG) (grant number SCHM 3275/2-1).
- Cochrane GWG, Webb JA, Doelman T, Habgood PJ (2017) Elemental differences: Geochemical identification of aboriginal silcrete sources in the Arcadia Valley, eastern Australia. J Archaeol Sci-Rep 15:570–577Google Scholar
- Delagnes A, Schmidt P, Douze K, Wurz S, Bellot-Gurlet L, Conard NJ, Nickel KG, van Niekerk KL, Henshilwood CS (2016) Early evidence for the extensive heat treatment of silcrete in the Howiesons Poort at Klipdrift Shelter (Layer PBD, 65 ka), South Africa. PLoS One 11. https://doi.org/10.1371/journal.pone.0163874 CrossRefGoogle Scholar
- Porraz G, Parkington JE, Rigaud J-P, Miller CE, Poggenpoel C, Tribolo C, Archer W, Cartwright CR, Charrié-Duhaut A, Dayet L, Igreja M, Mercier N, Schmidt P, Verna C, Texier P-J (2013b) The MSA sequence of Diepkloof and the history of southern African Late Pleistocene populations. J Archaeol Sci 40:3542–3552CrossRefGoogle Scholar
- Porraz G, Igreja M, Schmidt P, Parkington JE (2016) A shape to the microlithic Robberg from Elands Bay Cave (South Africa). South Afr Humanit 29:203–247Google Scholar
- Roberts DL (2003) Age, genesis and significance of South African coastal belt silcretes. In: Memoir 95. Council for Geoscience, PretoriaGoogle Scholar
- Schmidt P, Hogberg A (2018) Heat treatment in the Still Bay - a case study on Hollow Rock Shelter, South Africa. J Archaeol Sci Rep 21:712–720Google Scholar
- Schmidt P, Nash DJ, Coulson S, Göden MB, Awcock GJ (2017b) Heat treatment as a universal technical solution for silcrete use? A comparison between silcrete from the Western Cape (South Africa) and the Kalahari (Botswana). PLoS One 12:e0181586. https://doi.org/10.1371/journal.pone.0181586 CrossRefGoogle Scholar
- Singer R, Wymer J (1982) The Middle Stone Age of Klasies River Mouth in South Africa. University of Chicago Press, ChicagoGoogle Scholar
- Summerfield MA (1983a) Silcrete. In: Goudie AS, Pye K (eds) Chemical Sediments and Geomorphology. Academic, London, pp 59–91Google Scholar
- Summerfield MA (1983b) Petrography and diagenesis of silcrete from the Kalahari Basin and Cape coastal zone, southern Africa. J Sediment Petrol 53:895–909Google Scholar
- Thiry M, Milnes A (2017) Silcretes: insights into the occurrences and formation of materials sourced for stone tool making. J Archaeol Sci Rep. 15:500–513Google Scholar
- Wragg Sykes RM, Will M (2017) Guest editorial – silcrete as a lithic raw material in global context: Geology, sourcing and prehistoric techno-economics. J Archaeol Sci-Rep 15:492–499Google Scholar