Birch bark tar (manufactured by the heating of bark in airtight conditions) has long been prized for its sticky, water resistant, and biocidal properties. Throughout human history it has seen a wide range of uses, including as a sealant (for example, in waterprooing vessels), an adhesive (for hafting weapons, repairing ceramics, or assembling composite objects like jewellery), and in perfume and medicine.
Archaeological evidence demonstrates that our predecessors were making this substance as far back as the Palaeolithic (the earliest discovery dates back c.185,000-135,000 years), and its use has continued into the modern day in eastern and northern Europe. In western Europe and Britain, though, it has been generally believed that its use was limited to prehistory, with birch bark tar being displaced by pine tars during the Roman period. As we will explore during this month’s Science Notes, though, the identification of birch bark tar at two early medieval sites in the east of England indicates that this technology was in use much later here than was previously believed.
A new study by the University of Bristol and the British Museum, in collaboration with Oxford Archaeology East and Canterbury Archaeological Trust (published in the Journal of Archaeological Science: Reports – see https://doi.org/10.1016/j.jasrep.2019.102118), analysed two finds of birch bark tar: one from Hatherdene Close, Cherry Hinton, Cambridge, and the other from Ringlemere Farm, Woodnesborough, Kent. They represent the first identification of birch bark tar from early medieval contexts in the UK. But do such discoveries indicate a longer continuity of the technology’s use in Britain than was previously thought, or a reintroduction in the medieval period?
Hatherdene Close is the location of an early medieval cemetery, where 126 skeletons were excavated, many buried with grave goods including weapons and dress accessories, dated to the 5th-6th centuries AD. The birch bark tar at this site was found in the burial of a 7-9-year-old child with other grave goods, in the form of a dark brown ‘lump’ of material initially identified as a possible potsherd.
Meanwhile, Ringlemere Farm (see CA 208) yielded the remains of a 7th-century building and a substantial Anglo-Saxon cemetery spanning the mid-5th and early 6th centuries. A ceramic vessel, also dated to the 5th/6th century, found in a small pit set into the side of an earlier prehistoric barrow, was coated inside with a shiny black residue that turned out to contain birch bark tar.
Both examples of birch bark tar were analysed at two different institutions using the same technique of high-temperature gas chromatography–mass spectrometry, which was used to separate, identify, and quantify the molecular components of the samples.
This process revealed that samples from both objects contained a complex mixture of molecular components. In the lump from Hatherdene Close, the dominant compounds identified are known together to be characteristic of birch bark, and others were also identified that are produced when bark undergoes the heating process necessary to produce birch bark tar. The samples from the Ringlemere vessel also produced molecular components consistent with the presence of birch bark tar, albeit at a lower level, which may be due to a lower quantity of the tar within the samples.
The chemical composition of the two examples differs slightly due to the nature of the finds (‘lump’ vs surface residue) and the site contexts (grave good vs isolated find), however both do provide evidence for the presence of birch bark tar at the sites, suggesting that the two locations may have shared access to a network of some sort where resources and technologies were exchanged.
There is limited evidence for the continuing use of birch bark tar in the Roman period in Britain, but this could simply be the result of examples being overlooked in the archaeological record. Alternatively, these recent discoveries could be the result of a revival in the early medieval period coinciding with the arrival of settlers from northeastern Europe, where the continuity of birch bark tar technology was unbroken. Regardless of how they came to be, though, the identification of these examples of birch bark tar provides new information about the technology’s use in early medieval Britain.