Birch leaves as raw material for tar – production, identification and implications

Abstract Birch tar, an adhesive made from birch trees, was the first synthetic substance made by early humans. Its manufacture and use has provided important insights into the technological and cognitive skills of past humans. So far, research has focused on the use of bark as the raw material for birch tar. However, in a different context in southern Africa, adhesives were made from leaves. There, the foragers of the Middle and Later Stone Age produced biomass tar by burning the leaves of a variety of plants using a condensation process. This raises an obvious question: is it also possible to make birch tar from leaves? This study aims at answering this question and, if the answer is yes, investigate the production parameters and chemical composition of birch leaf tars. For this, we made tar from birch leaves and bark using the condensation method, maintaining experimental parameters as similar as possible. Experimental tars were analysed using gas chromatography-mass spectrometry and infrared spectroscopy to evaluate their chemical and molecular compositions. Tar could be produced from birch leaves, but yielded less than half as much tar per gram as bark. Leaf- and bark- derived tars share several chemical characteristics but can be distinguished analytically. Our findings expand the range of potential raw materials exploited for tar production and provide new criteria for the identification and interpretation of birch tar residues in archaeological contexts.