On the performance of birch tar made with different techniques

Abstract

AbstractBirch tar is one of the oldest adhesives known in human history. Its production has been discussed in the framework of early complex behaviours and sophisticated cognitive capacities. The precise production method used in the Palaeolithic remains unknown today. Arguments for or against specific production pathways have been based on efficiency or process complexity. No studies have addressed the question whether birch tar made with different techniques is more or less performant in terms of its properties. We therefore investigate the adhesive performance of birch tar made with three distinct methods: the open-air condensation method and two variations of underground structures that approximate the double-pot method in aceramic conditions. We use lap-shear testing, a standard mechanical test used for testing the strength of industrial adhesives. Tar made in 1 h with the condensation method has a shear strength similar to, although slightly higher than, tar made underground if the underground process lasts for 20 h. However, tars from shorter underground procedures (5 h) are significantly less strong (by a factor of about 3). These findings have important implications for our understanding of the relationship between the investment required for Palaeolithic birch tar production and the benefits that birch tar represented for early technology. In this regard, the simple and low-investment open-air condensation method provides the best ratio.