The molecular composition of birch tar and its infrared spectrum

Birch tar was the first substance made by humans that is otherwise not available in nature. The oldest artefacts date to ~ 200 thousand years ago in Europe but birch tar becomes more frequent around 45 thousand years ago and even more so from the Mesolithic onwards. Their study has important implication for our understanding of evolutionary processes such as cognitive capacity and cultural evolution. What remains unknown is the overall molecular composition of birch tar. Several approaches based on gas chromatography-mass spectrometry have shown the potential of studying solvent soluble molecules to understand the botanical origin of different tars, their manufacturing techniques and their post-depositional alteration mechanisms. However, birch tar’s soluble fraction cannot shed light on its mechanical properties. In this study, we conduct gas chromatography-mass spectrometry and spectroscopic analysis of seven bulk birch tar samples, including infrared band assignment and structural refinement of organic molecules in the insoluble fraction. We find that the overall proportion that cannot be analysed by gas chromatography accounts for 85–90% in birch tar. This phase consists of polymerised molecules with a structure analogous to asphaltenes but containing a significantly greater amount of oxygen-related functional groups. Our findings have implications for understanding the viscosity, adhesiveness and stiffness of birch tar and they call for caution in applying analytical techniques that only target soluble molecules in birch tar.