A stab in the dark: testing the efficacy of watsonia exudate as glue for stone tool hafting

Composite tool manufacture is a complex technological behaviour that arose in the Middle Stone Age. The hafting of stone tools to handles or shafts seems to depend on the use of glue or adhesive. Chemical and microscopic analysis of glue or adhesive residues on lithic artefacts show the origins of these residues to be from various plant and animal sources, although attributing these residues to specific species has been problematic for various reasons. Experimental evaluation of the efficacy of glue for composite tool manufacture is a valuable method to identify possible sources of residues found on lithic artefacts. Various plant sources of glue have been identified in historical accounts, but some are yet to be identified in the archaeological record or need to be tested for their efficacy. The botanical remains of the genus Watsonia are particularly well represented in the archaeological record, where it is thought to have been foraged as a food source by Stone Age hunter-gatherers. During field observations, Watsonia underground storage organs (USOs) have been found to produce exudate. We test the efficacy of this exudate as a hafting glue for stone tools. Our results indicate that Watsonia exudate is nearly as effective as resin from Vachellia karroo (previously Acacia karroo) trees. These results lay the groundwork for identifying glue as part of composite tool technological components in the archaeological record that may not yet have been identified by other analytical methods, and highlight the role USOs might have played in human technology.