Lap-shear and compression testing of ethnographic compound adhesives and implications for archaeology

Abstract The properties of ancient compound adhesives are gaining increasing research attention through substance identification and mechanical testing. Research on the latter relies on experimental replication as ethnographic cases of hafted stone artifacts are rare. Here we characterize the lap-shear and compression stress behaviors of a hitherto undescribed compound adhesive used for hafting stone scrapers by hideworkers in Ethiopia. We found that the Balanites aegyptiaca resin mixed with charcoal powder provides high stability and cures within a few hours. However, this adhesive suffered catastrophic failure across substrate when subjected to strong tensile or compressive force. The relative elasticity and curing contrast with gum-based compound adhesives commonly used in experimental replications, with charcoal or ochre as filler. In acacia-ochre adhesives, adding beeswax resulted in increased elasticity but reduced bonding strength. Our observations, tests, and comparisons suggest that adhesive selection is governed by the properties of the main ingredient and the intended task, in addition to raw material availability. In stone hidescrapers hafted to wooden handles, relative adhesive stiffness affords tool stability within the haft. Our results have implications for the interpretation of archaeological hafted technologies, including decision-making processes involved in the selection of raw materials for adhesive production and qualities optimized for specific tasks.