The mechanical properties of heat-treated rocks: a comparison between chert and silcrete
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ORIGINAL PAPER
The mechanical properties of heat-treated rocks: a comparison between chert and silcrete Patrick Schmidt 1,2
&
Gerald Buck 2 & Christoph Berthold 2,3 & Christoph Lauer 2 & Klaus G. Nickel 2,3
Received: 27 June 2018 / Accepted: 12 September 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract In archaeology, heat treatment of stone is the process of Bmaking^ a new material for tool production. Its invention in the African Middle Stone Age was an important step in the evolution of transformative technologies and the cultural evolution of early humans in general. Although the chemical and crystallographic transformations in silica rocks, the only material class heattreated in the Stone Age, begin to be well known, many of the mechanical transformations and their chemical origins remain a subject of controversy. The difference between different silica rock categories is also only poorly understood. In this paper, we investigate the thermally induced changes of three mechanical properties in the two silica rock types chert and silcrete: fracture strength, indentation fracture resistance (approximating fracture toughness) and elastic modulus. These tests are complemented by statistical analyses (Weibull modulus) and a quantitative fracture surface analysis. The results show that heat treatment transforms these silica rocks in terms of their fracture toughness and the uniformity of fracture. A comparison with published data on the structural transformations in the same samples identified the loss of chemically bound water and subsequent defect healing to be the chemical mechanism behind these mechanical transformations. These findings have important implications for the study of the interactions between chemical and structural processes and the mechanics of natural rocks or ceramics. Keywords Bending tests . Indentation fracture resistance (IFR) . Crystallography of tool stones . Knapping quality . Archaeology
Introduction In prehistoric archaeology, heat treatment of stone to produce tools by controlled fracturing (stone knapping) is commonly understood as one of the earliest efforts of humankind to deliberately alter the properties of naturally available materials. The earliest examples date back to the African Middle Stone Age (Brown et al. 2009; Porraz et al. 2013; Mourre et al. 2010; Schmidt and Mackay 2016; Delagnes et al. 2016; Schmidt et al. 2015) and were interpreted to be a proxy for Bmodern
* Patrick Schmidt [email protected] 1
Department of Prehistory and Quaternary Ecology, Eberhard Karls University of Tübingen, Schloss Hohentübingen, 72070 Tübingen, Germany
2
Department of Geosciences, Applied Mineralogy, Eberhard Karls University of Tübingen, Wilhelmstraße 56, 72074 Tübingen, Germany
3
Competence Center Archaeometry–Baden Wuerttemberg, Eberhard Karls University of Tübingen, Wilhelmstraße 56, 72074 Tübingen, Germany
behaviour^ (McBrearty and Brooks 2000; Sealy 2009) or Bcomplex cognition^ (Wadley 2013). In later periods, heat treatment is often
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