On the performance of birch tar made with different techniques

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.

High-resolution sediment analysis reveals Middle Bronze Age byre-houses at the site of Oppeano (Verona province, NE Italy)

“…The production of wood tar resulted in a round patch of blackish material resembling the shape of the several hearths that were excavated at Oppeano. This simple arrangement is in line with recent experimental findings on tar production [ 101 , 102 ], and was corroborated by organic petrology in SMT 3. Wood tar could be related to several activities carried out in the domestic space, some of which linked to animal husbandry.…”

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

“…Like Schmidt et al (2021), we found that there are complex stress distributions present in the bonded areas of the analysed birch tar. This led to nonlinear behaviour even before failure.…”

“…Our findings on the usefulness of birch tar before cooking might be applicable to birch tar made with aceramic versions of the double-pot technique. Recently, Schmidt et al (2021) found that birch tar made with a two-chamber-based method proposed by Kozowyk, Soressi, et al (2017)-the so-called raised structure-allowed the production of slightly more viscous tar (they bonded laps and performed lap-shear tests with it without reduction). However, they found that when the structure was opened after 5 h the collected tar was three times less strong than when the structure was left to cool for 20 h. Their strength values obtained from tar made with the raised structure in 5 h are in agreement with our tar boiled for 85 min (taking into account that wooden laps were used in Schmidt et al, 2021), with τ u = 0.42 + 0.45 À 0.21 MPa compared to our own value of τ u = 0.63 + 0.38 À 0.18 MPa.…”

“…Lap‐shear tests resulted in stress strain diagrams plotting apparent shear stress τ in MPa (as obtained from the force applied by the testing machine/bonded area) over elongation strain (%). As proposed by Schmidt et al (2021), we extracted two values from these stress/strain diagrams: maximum shear stress τ u (MPa; the maximum stress value as read on the diagrams) and shear strength τ (yield) (MPa; the stress at which the curve deviates from the tangent slope of the linear increase of τ ). Thus τ (yield) corresponds to the stress at which the sample stops behaving elastically (this is more‐or‐less equivalent to the yield strength σ in tensile material testing; τ (yield) is marked in Figure 2c).…”

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

“…The performance of tar making methods has also been investigated in the past. Schmidt et al (2021) compared the adhesive strength of tar made with the aboveground condensation method (i.e. birch bark is burned beside tilted flat river cobbles) with tar made with the underground raised structure method (Osipowicz, 2005;Groom et al, 2013;Kozowyk et al, 2017b;Schenck and Groom, 2018).…”