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Detailed near-infrared study of the ‘water’-related transformations in silcrete upon heat treatment

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Abstract

In archaeology, lithic heat treatment is the process of modifying a rock for stone tool production using fire. Although the earliest known cases of heat treatment come from South Africa and involved silcrete, a microcrystalline pedogenic silica rock, its thermal transformations remain poorly understood. We investigate the ‘water’-related transformations in silcrete using direct transmission near-infrared spectroscopy. We found that SiOH is noticeably lost between 250 and 450 °C and hydroxyl reacts with H2O, part of which is trapped in the structure of the rocks. This water can only be evaporated through heat-induced fracturing at high temperatures, imposing maximum temperatures for silcrete heat treatment of approximately 500 °C. Between 250 and 450 °C new siloxane bonds are formed according to the reaction 2SiOH → Si–O–Si + H2O, which can be expected to transform the rock’s mechanical properties. The tolerance of silcrete for relatively fast ramp rates can be explained by its pore volume and low SiOH content, ensuring good water evaporation. These results shed light on the processes taking place in silcrete during heat treatment and allow for a better understanding of the parameters needed for it.

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Acknowledgments

We thank the Deutsche Forschungsgemeinschaft (DFG) for funding of the research project Heat Treatment in the South African MSA that made the present study possible (Grant No: CO 226/25-1, MI 1748/2-1, NI 299/25-1) and for funding the Agilent Cary 660 spectrometer used for parts of this study (MI 1748/1-1). We also thank Junichi Fukuda and an anonymous reviewer for their corrections and suggestions during the review process for this publication.

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Authors and Affiliations

  1. Department of Prehistory and Quaternary Ecology, Eberhard Karls University of Tübingen, Schloss Hohentübingen, 72070, Tübingen, Germany

    Patrick Schmidt

  2. Department of Geosciences, Applied Mineralogy, Eberhard Karls University of Tübingen, Wilhelmstraße 56, 72074, Tübingen, Germany

    Patrick Schmidt, Christoph Lauer, Gerald Buck & Klaus G. Nickel

  3. Institute for Archaeological Sciences, Eberhard Karls University of Tübingen, Rümelinstrasse 23, 72070, Tübingen, Germany

    Christopher E. Miller

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  1. Patrick Schmidt
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Correspondence to Patrick Schmidt.

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Schmidt, P., Lauer, C., Buck, G. et al. Detailed near-infrared study of the ‘water’-related transformations in silcrete upon heat treatment. Phys Chem Minerals 44, 21–31 (2017). https://doi.org/10.1007/s00269-016-0833-6

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