The effects of heating ostrich eggshell on bead manufacturing: An experimental approach

Abstract

Archaeological ostrich eggshell (OES) bead assemblages often comprise a variety of colours. However, it is unclear if the range of colours seen in OES beads were caused deliberately by anthropogenic action, or accidentally by post-depositional taphonomic factors. In this study, OES fragments were heated to four different temperatures (200 °C, 350 °C, 550 °C, and 700 °C) to recreate colour variation observed in the OES bead assemblage from the Marine Isotope Stage (MIS) 1 occupations at Grassridge Rockshelter, South Africa. Beads were manufactured from the four samples of heated OES fragments, as well as from an unheated sample of control fragments. Time to perforation, successful bead manufacture and/or breakage, as well as loss of mass and fragility, were all documented to examine if heated ostrich eggshell performed differently from unheated ostrich eggshell during bead manufacture. Our results show that three of the four heated samples resulted in detrimental changes to ostrich eggshell properties, specifically the loss of mass and an increase in fragility, which negatively affected bead manufacture indicated by increased incidence of breakage and longer average times to perforation and completion. We compare these findings with the worked OES assemblage from Grassridge Rockshelter, South Africa, and our results indicate that unintentional heat treatment may have substantially impacted the colouration of the worked OES assemblage at Grassridge, especially with regard to heat treated preforms. Moreover, we argue that the impact of unintentional heat treatment on the colouration of worked OES assemblages, and especially preforms, should be more broadly considered across the archaeological record.