An evaluation of the infrared 630cm-1 O-H libration band in bone mineral as evidence of fire in the early archaeological record

FTIR spectroscopy has played an important role in recent attempts to understand the use of fire in prehistory. It has been used in the identification of heat altered sediment and bone. For the latter, the presence of the OH libration band at ca. 630cm-1 in the FTIR spectrum of an archaeological bone has been assumed to be indicative of bone that has been altered by fire. However, no ad-hoc research has explored whether this FTIR band could result from other ambient temperature diagenetic processes, or what the effects of heating variables may be on the appearance of this band. Here, I report a study designed to address this lacuna, and apply the results to the collection of fauna at Wonderwerk Cave from the Oldowan context. Using samples of cortical bone from micro- and macrofauna, I carried out a series of heating experiments to explore the change in FTIR spectra depending on temperature and duration of exposure to heat. Results demonstrate that the 630cm-1 peak is indeed diagnostic of burning, and indicate that microfauna are particularly useful indicators of burning activity when subjected to FTIR analysis. I hypothesize that the 630cm-1 peak is the result of the formation of pure hydroxyapatite, which appears to form above temperatures of 537°C. The results of this research were applied to the micro- and macrofauna collection of Wonderwerk Cave, South Africa, from the Earlier Stone Age context of 1.78Ma and older. Our results demonstrate that the majority of the bones from this context were burnt above 537°C.