Adhesive Strength and Rupture Behaviour of Birch Tars Made with Different Stone Age Methods

Birch tar is the earliest adhesive produced by Neanderthals in the European Palaeolithic. Experimental studies have proposed a number of aceramic (i.e. without the use of ceramics) birch tar–making techniques, but to date, no archaeological evidence for any of these techniques has been brought forward. The likelihood of one of those methods being actually used in the Stone Age may be investigated by evaluating the method’s complexity, efficiency and the performance of the tar obtained. In this study, we experimentally produce birch tar with four common experimental aceramic techniques. These tars are tested for their mechanical properties using lap-shear tests. We found that tar made with the aboveground condensation method is strongest in terms of its adhesive strength. The tar also shows the most consistent behaviour throughout our test series. The weakest adhesive was observed in tar made with an underground (i.e. oxygen-depleted) technique resembling double-pot distillation (raised structure). Two supplementary methods yielded intermediate values: the recently proposed cobble-groove method produces tar with plastic behaviour. Tar made with another underground technique (pit roll) is almost as strong as condensation method tar. This study adds to our understanding of adhesive performance of birch tar made with different aceramic techniques. It also allows understanding the potential use of one method instead of another.