Abstract
Birch 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.
Highlights
Birch tar is the oldest known adhesive dating back to the European Middle Palaeolithic
On the other hand, we find significant differences between the strengths of birch tar made with different techniques, it becomes worthwhile to discuss the relationship between the investment imposed by specific techniques and the value of the tar that they allow to produce
Choices made during our experiments and the quality of our data Previous authors have performed impact tests along with lap-shear tests [25]. This combined approach may provide a more complete understanding of adhesives as it tests for bonding strength under static conditions like cutting with hafted stone tool and under impact conditions when a projectile is tipped with a hafted stone point
Summary
Birch tar is the oldest known adhesive dating back to the European Middle Palaeolithic. There are five pieces of birch tar known from the Palaeolithic record, all attributed to Neanderthals. The oldest two pieces were found in Campitello (Italy) and indirectly dated to ~ 200 ka [1]. Two pieces were found at Königsaue (Germany) and estimated to between 40 and 80 ka [2, 3]. The most recently found birch tar artefact comes from Zandmotor (The Netherlands) and is ~ 50 ka old [4]. At Inden-Altdorf (Germany), there are other artefacts with residues that were claimed to be birch tar [5] but detailed identification with Gas-Chromatography has yet to be undertaken. One of the questions surrounding these Palaeolithic birch
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