Abrasive wear in heat-treated ostrich eggshell beads: implications for the archaeological record

Archaeological ostrich eggshell (OES) bead assemblages manifest in a variety of colours that result from exposure to high temperatures. It is unclear, however, whether this colouration is taphonomic, through post-depositional heat exposure, or the consequence of intentional heat treatment of OES to produce a desired colour (or for some related purpose). In this study, we investigate the latter question using controlled and actualistic heating techniques to determine (1) if OES beads remain usable following exposure to high temperatures and (2) whether abrasive wear that occurs prior to heat treatment can be distinguished from wear that occurs after heat exposure. Twelve sets of 20 OES beads were manufactured and subject to one of two investigative tracks. Track 1 simulated abrasive wear using a rock tumbler before heat treatment in either a muffle furnace or wood-burning fire. Track 2 beads were heat treated prior to abrasion. The results demonstrate that indirect exposure to a wood-burning fire generates sufficient heat to produce most colours known in archaeological OES assemblages, although controlling the fire to produce a desired colour is difficult. Beads exposed to high temperatures (≥ 550 °C) become extremely thin and fragile. In a furnace, abrasive wear that occurred prior to heat treatment was distinguishable from wear that occurred after heat treatment, but this distinction is less clear in beads heated in the fire. From these observations, we suggest that coloured OES and OES bead assemblages are more likely the result of unintentional post-depositional processes than deliberate anthropogenic heat treatment.