Heat treatment of silcrete raw materials

Establishing the source of raw material used in stone tool manufacture can provide insights into the development of a range of important human behavioural traits, including mobility and the ability to plan in-depth. Working in Botswana, the project Principal Investigator (Professor David Nash) and colleagues Dr Stewart Ullyott, Dr Martin Smith and Dr Laurence Hopkinson, developed a novel geochemical approach (see ProSAIK project)  to provenance (source) silcrete, a type of silica-cemented duricrust with flint-like properties widely used for stone tool manufacture across southern Africa. The results of this research indicate that silcrete raw materials were transported up to 295km during the Middle Stone Age (MSA) to sites where they were then used for tool-making.

The approach has the potential to be applied at South African MSA sites, as well as Later and Early Stone Age sites in Botswana. An issue, however, is whether it needs modification for stone that was heat-treated prior to knapping. Heat-treatment – the deliberate heating of stone by fire – was a method widely used in the South African MSA to improve tool-stone quality. Heating strengthens silcrete by causing a loss of chemically bound ‘water’ from the rock structure, which, in turn, leads to the formation of new chemical bonds. An unwanted side effect, however, is the chemical synthesis of molecular water, which is evacuated as high pressure steam. The impacts of such steam leaching upon trace element chemistry – key to geochemical fingerprinting – are unknown.

The project began in May 2015 and ran to October 2016. Supported by a £9,500 grant from the British Academy, it aimed to use an experimental approach to establish the impact of heat-treatment upon silcrete chemistry. We will use a range of techniques on heated and unheated samples in laboratories at the University of Brighton and at project partner the University of Tübingen. These will include near- and mid-infrared (NIR, mid-IR) and x-ray diffraction (XRD) to determine mineralogical and crystallographic transformations associated with heating, plus inductively-coupled plasma mass spectrometry (ICP-MS) and atomic emission spectroscopy (ICP-AES) to assess chemical changes. Our results will demonstrate how, and at what temperature, chemical transformations occur, with implications for future provenancing work.