The evolution of strength, elasticity and rupture behaviour of birch tar made with ‘double‐pot’ techniques during tar cooking

Abstract

Birch tar was the first adhesive produced by humans. Its study has consequences for our understanding of human evolution and the development of specialised craftsmanship. One of the better-documented birch tar making methods is the 'double-pot' technique, where two containers are used, one containing bark, the other collecting the tar. Birch tar made with double-pots has low viscosity and bad adhesive properties. To obtain a usable adhesive, it must be reduced in volume by cooking. We investigate the evolution of tar's mechanical properties during cooking. We use lap-shear tests to investigate strength, stiffness and failure behaviour. We found that tar must be cooked for 90 min to acquire adhesive properties. When cooked for longer, strength and stiffness increase over a 30 min lasting time span. Cooking for even longer, beyond this 30 min window, produced a substance that could no longer be used as an adhesive. The implications are that tar cooking requires a high level of skill because specific signs indicating the desired properties must be recognised during the process. Tar cooking constitutes a supplementary investment in time and resources and appears to be associated with a certain degree of risk.

Ochre-based compound adhesives at the Mousterian type-site document complex cognition and high investment

“…When mixed with fresh bitumen, 55 wt % ochre increases its strength by a factor of 3. Nonetheless, our values measured on fresh bitumen are very low if compared to lap-shear data reported on other archaeological adhesives [which fall in the range of τ u = 0.1 to 1.5 MPa (9,(62)(63)(64)]. We therefore attempted to identify other possible performance gains of a high ochre load in bitumen.…”

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

“…The comparison of our values with theirs might be slightly offset, as they used wooden laps and we aluminium laps. However, this offset is only minimal, as shown by a recent comparison of wooden and aluminium laps for birch tar lap-shear testing (Schmidt et al 2022a). This study found aluminium laps to produce slightly higher overall strength values than wooden laps, suggesting that an offset of 30% (for τ (yield) ) or 15% (for τ u ) should be applied when comparing values produced with both types of lap materials.…”

“…It has previously been argued that tar made with double-pot distillation might require further cooking to thicken it and make it more usable (Schenk and Groom 2018). A recent study has shown that cooking time has an effect on the adhesive properties of tar (Schmidt et al 2022a). For our experiments, we decided to let the raised structure cool overnight to let the tar thicken naturally.…”

“…We also note that Schmidt et al (2021) performed their experiments using dry bark collected from dead trees and performed their lap-shear tests using wooden laps. If we add the offset of 30% (for τ (yield) ) or 15% (for τ u ) as suggested by Schmidt et al (2022a), in order to compare our aluminium to their wooden lap-shear tests, the following values are obtained. Schmidt et al's raised structure (left to cool down) could be expected to yield values τ u = 1.09 MPa and of τ (yield) = 0.91 MPa if they were conducted with aluminium laps.…”

Differences in birch tar composition are explained by adhesive function in the central European Iron Age

“…However, the authors note that degradation and potential re-heating of tar could have an unknown influence on the chemical signatures. A recent study showed that adhesive strength can be increased through post-production re-heating of birch tar [ 46 ], but chemical alteration during such processes, as well as a way of recognising them in archaeological contexts, has not yet been assessed. Consequently, it is not only the lack of direct archaeological data on the production method, but also any post-production processing, such as reheating or mixing, that we do not fully understand yet.…”