Choices made during our experiments and the quality of our data
Previous authors have performed impact tests along with lap-shear tests . 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. We decided not to test for impact strength as there are no indications that any of the known Palaeolithic birch tar artefacts were used for hafting stone tools to projectiles or handles (compare: [1, 2, 4]).
We chose to use aluminium receptacles for the two main raised structure experiments. In this way, sediment contamination could be limited and a relatively large quantity of uncontaminated tar could be collected after the experiments (because the earthen walls of the lower container were covered by aluminium, Fig. 1). One of the possible effects of this protocol is that the use of aluminium containers, obviously not available in the Palaeolithic, could influence the quality of the tar produced with the raised structures. Although the nature of the container is not expected to have an influence on the volatile components of the raised structure tar (which is lost through evaporation), it appears possible that an aluminium container allows to collect more of the low viscosity fraction of the tar because it is more impermeable than other natural materials. These low viscosity components might be absorbed by a container made from a more porous natural material. We had therefore conducted another raised structure test, using a more porous stone receptacle. The comparison between the tar from this stone container and tar made with the aluminium container, both produced during 5 h, showed that tar collected in a more porous stone container had a ~ 40% lower maximum shear stress value and a comparable, although slightly lower, shear strength value. Thus, using an aluminum container has the opposite effect to what we expected. This result suggests that the use of aluminum containers might cause the shear strength of tar made with the raised structure to be overestimated. As it stands, our quantitative comparison between condensation method tar and raised structure tar must be regarded with caution, although the overall trend is most likely correct based on our results from this comparison.
We also found that there are complex stress distributions present in the bonded areas of birch tar analysed with lap-shear tests, leading to non-linear behaviour even before failure. This can be expected to depend (at least in part) on the thickness of the bond and the quality of the bonding surface (see for example: ). Measuring bond thickness was not possible with the wooden laps used for this study, as their thickness is not uniform across the bonded surface (i.e., they were not sufficiently plan parallel). Measurements were therefore repeated at least ten times for the samples (except for tar made with a stone receptacle that was repeated 5 times, as this was only done to verify the validity of our experimental protocol). As highlighted above, we consider only the apparent shear stresses τ, attributing all measured forces to be shear only. This is not entirely true, as can be seen from the curves in Fig. 2, which are not linear even for tars that fail catastrophically. One of the reasons for this is that organic materials, such as some of our birch tar samples, do not respond to stress with elastic deformation up to their failure. They show plastic deformation by viscous processes or other creep phenomena. Furthermore, horizontal elongation in lap-shear tests cannot be expected to be linear because the tar samples’ thickness at the bond varies with elongation (depending on Poisson’s ratio of the adhesives). Thus, τu recorded by lap-shear tests is not a good indicator of the resilience of natural tars against shear. We still use this value here because previous authors have provided lap-shear data [19, 25, 26] with which our data may be comparable. Although our tests cannot yield absolute values of G, our approach to measuring τu provides comparability with previous works [25, 26] that reported similar values.
The performance of birch tar made with different production techniques
What does performance of adhesives made in the Stone Age actually mean? Two cases may be distinguished. Adhesives that have to work only one time (as may be true for a projectile hafting) can most likely be qualified by either the maximum shear stress they endure (including a plastic deformation modifying the shape of the joint permanently) or by the total energy they absorb during the shearing process. For such a rupture energy evaluation, other experiments with better strain control are needed. The other case consists of adhesives used for repeated actions (cutting, scraping, etc.). These may be better qualified by shear strength τ(yield). A nearly brittle nature of the failure (the sample behaving almost elastically until the breaking point) might in this case be an advantage because tools either hold or break loose. A plastically deformed haft will perform less well in successive use cycles.
Our results highlight that tar made with the condensation method is similarly, although slightly more, performant when used as adhesive than tar made with the raised structure can be. A larger difference exists when raised structure tar is collected from the structure directly after the surrounding fire burned out. Our τu measured on 5 h-raised structure birch tar (0.417 + 0.45 − 0.21 MPa) is in accordance with, although lying slightly above, previously published lap-shear values of birch tar made with the double-pot method (using a metal container) that was subsequently boiled to thicken it (0.32 + 0.19 − 0.18 MPa, see: ). The raised structure is a good approximation of the double-pot architecture in aceramic conditions (for a detailed description of the double-pot, see: ) and it also appears to produce birch tar with similar properties. It is noteworthy however, that the aceramic raised structure allows the production of birch tar with similar strength as the metal-based double-pot without requiring the supplementary step of tar reduction by boiling. The reason for this might be a better availability of oxygen in the raised structure due to incomplete sealing because of the wet sediment. The strength of the raised structure tar can be improved by a factor of two, if the tar is allowed to cool slowly overnight. The reason for this might be oxidative reactions in the slowly cooling tar or tar reduction by degassing. Only further studies may shed light on these processes. The condensation method produced birch tar with the highest τu of the tested production methods. Our maximal strength (1.14 + 0.46–0.52 MPa) is well in accordance with previously published strength values of condensation tar (1.145 + 0.403 − 0.438 MPa, see: ).
The cost and return of birch tar made with different techniques
The differences in adhesive performance of our samples are best discussed with regards to the investment in time and effort required by different production methods. Recently, Blessing and Schmidt  found that the raised structure is the most efficient of the aceramic production techniques in terms of material requirement (supporting previous arguments made by: ). However, it can be inferred from the data in Badino et al.  that birch bark was readily available in northern Europe during the Late Middle Palaeolithic. It is therefore unclear whether efficiency in terms of required bark over tar yield may possibly have imposed constraints for making the birch tar artefacts from Zandmotor and Königsaue. Another raw material-related factor is that the raised structure requires the collection of firewood, imposing supplementary constraints on the environment in which tar is made and which are absent for the condensation method.
If time investment is compared for both techniques, the difference between the raised structure and the condensation method seems to be negligible. Both produced similar amounts of tar per hour in most experiments . It is also noteworthy that the raised structure imposes a minimum requirement of time, which is roughly 4–5 h , while the condensation method allows to produce usable amounts of tar in approximately one hour  (e.g., the 0.87 g weighing smaller birch tar piece from Königsaue can be produced in ~ 1 h 20; these times cannot be compared in terms of attention required, see for example , but they do still represent time requirements). Thus, in terms of time requirement, the condensation method may be far more advantageous if tar is needed rapidly. In the light of our finding that condensation tar is similar to tar made with the raised structure in 20 h and superior to raised structure tar made in 5 h (in terms of shear strength at least), we note that this simple open-air technique provides the best value for the time investment it requires. Whether raised structure tar produced without the cooling phase may be improved by a supplementary step of tar reduction, in which the low viscosity tar is boiled over an open flame to produce a more viscous product, cannot be answered at this point. However, additional cooking of the tar would require larger investment than investigated here.