Responses of South African Agate and Chalcedony When Heated Experimentally, and the Broader Implications for Heated Archaeological Minerals

ABSTRACT

Here we present a database of responses by South African agate and chalcedony to heat treatment. This will assist analyses of heated stone tools not only in South African archaeological sites, but wherever heated agate and chalcedony pieces were knapped. The minerals are abundant worldwide. To replicate potential heating methods during the Stone Age we placed some minerals in a wood fire, some under coals, and others were buried in sediments beneath fires. Thermal responses include lustrous flaked surfaces, pot lid fractures, semi-circular internal fractures, rough internal surfaces, and crazing. Aerobic heating is implied by pot lid fractures. To explain the thermal responses we analyzed the minerals using X-ray fluorescence, Raman spectroscopy, and carbon and sulfur analyses. Our chalcedony contains more water and impurities than agate, making it more vulnerable to thermal damage. Our method of combining field experiments with chemical analyses has global applications even though we expect that mineral components of agate and chalcedony will vary slightly in different parts of the world.

KEYWORDS:

• Experimental heat treatment
• minerals
• wood fires
• thermal responses
• XRF
• Raman spectroscopy
• carbon and sulfur analysis

Acknowledgements

We thank the Archaeology Department, University of the Witwatersrand, for access to the Rose Cottage Cave lithics. We are grateful to Dawn Green for supplying banded agate nodules and a large chalcedony block from the Drakensberg. We thank Professor Bruce Cairncross for commenting on the thin section of the Drakensberg chalcedony and Paul Goldberg for images of thin sections. Research funding was obtained by LW from the National Research Foundation, African Origins Platform Grant #91595 (www.nrf.ac.za). Opinions expressed here are not necessarily those of the National Research Foundation or the Centre of Excellence. Paloma de la Peña (PdlP) is funded by the Centre of Excellence, University of the Witwatersrand. We thank two anonymous reviewers for useful comments that improved this paper.

Notes on contributors

Lyn Wadley (Ph.D. 1987, University of the Witwatersrand, South Africa) is an honorary professor of archaeology in the Evolutionary Studies Institute, at the University of the Witwatersrand, Johannesburg. Her research interests include the Middle Stone Age, cognitive archaeology, and experimental archaeology.

Paloma de la Peña (Ph.D. 2011, Universidad Complutense de Madrid, Spain) is a post-doctoral fellow in the Evolutionary Studies Institute and a honorary research associate in the School of Geography, Archeology and Enviromental Studies, University of the Witwatersrand, Johannesburg. Her research interests include Middle Stone Age and Early Upper Palaeolithic.

Linda C. Prinsloo (Ph.D. 2009, University of Pretoria, South Africa) is a research fellow in the Centre of Archaeological Science, University of Wollongong, Australia. Her research includes the application of vibrational spectroscopy to identify organic residues on stone artifacts and rock art, thermal properties of rocks used to make stone tools, characterization of sediments, and characterization of glass trade beads to unravel trade routes around the Indian Ocean.

ORCID

Lyn Wadley http://orcid.org/0000-0003-0053-0813

Linda C. Prinsloo http://orcid.org/0000-0003-3929-1899