SYNCHROTRON-BASED X-RAY CHARACTERIZATION AND MAPPING OF HEAT-TREATED ARCHAEOLOGICAL TOOLSTONE

Experimental and ethnographic studies of flintknapping suggest that many archaeological cultures prepared the fine-grained, silica-rich rock typically used to make flaked stone tools by subjecting it to heat treatment, a process which exposes it to temperatures of several hundred degrees Celsius for several hours to days. This procedure improves the predictability with which the rock flakes, but the physical and chemical processes responsible for this change are not well understood, despite previous studies using techniques such as electron spin resonance. This is an interesting problem from a material science standpoint, but it also leaves archaeologists without a clear way to definitively determine when heat treatment was used in the production of ancient stone tools.

Using geologic samples of an orthoquartzite that was an important precontact toolstone in northern Alberta, we are employing the Canadian Light Source (CLS), Canada’s national synchrotron facility, to determine how synchrotron-based analyses can help address these problems. Our initial work involved X-ray absorption near edge structure (XANES), a form of spectroscopy which has indicated that heat treatment of these samples induces changes to the chemical coordination of some trace elements, and micron- to sub-micron scale migration of other trace elements, with virtually no alteration to the silica that comprises 99% of this rock. We are now studying the spatial dimensions of these changes by collecting high-resolution x-ray florescence (XRF) and x-ray diffraction (XRD) maps of our samples. This combination of data will allow a better understanding of the subtle physical and chemical changes induced by heat treatment. This, in turn, may reveal a means for securely identifying heat-treated flaked stone artifacts, an advance of great value in efforts to study the origins and development of a toolmaking method that likely represents one of humanity's earliest uses of pyrotechnology.