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Prehistoric periods have traditionally been defined by technologies that were innovated by people of the time. But these definitions do not capture the full scope of human ingenuity. Adhesives are easily overlooked, yet they are the earliest synthetic substances for which we have archaeological evidence. As far back as the Middle Palaeolithic (c.300,000-50,000 years ago), early humans were processing bark taken from birch trees and using the resulting tar to secure the handles of their stone tools and weapons.
‘We call birch tar a “synthetic” material because it has to be produced,’ explained Tabea Koch, whose PhD research at the Universities of York and Cote D’Azur included experimental archaeology initiatives exploring how the substance was created. ‘You can’t just walk through a forest and touch a tree and find something sticky, like you might with pine resin, it needs to be discovered and made from birch bark – it implies technology.’
This substance continued to be a popular choice throughout prehistory, with its use expanding to encompass new technologies and craft specialisations – such as repairing ceramics and fixing ornamentation to objects – as they emerged. Birch tar can even be found on Roman and early medieval artefacts, though its apparent predominance may be a quirk of preservation as much as reflecting the preferences of past populations. Natural adhesives like pine resin, bitumen, and beeswax are also archaeologically attested, while synthetic substances like animal-derived glues would have been easily available and may have been used much more than surviving examples suggest.
‘Birch tar is so prominent because it is so enduring; really, we are seeing only the substances that have survived,’ said Rebecca Stacey, Scientist at the British Museum. ‘We don’t get a full understanding of the range of adhesive technologies that would have been available – for example, you might expect hide glues to have been widespread during this period, but they don’t survive well in archaeological contexts.’
Also contributing to this partial picture is the fact that previous research into ancient adhesives has largely focused on their use in hafting, and on ceramics. Only a few isolated studies have looked at the role that adhesives played in the production and decoration of metal objects – and so a study, recently published in the Journal of Cultural Heritage (see ‘Further information’ below), has set out to shed more light on this particular phenomenon, carrying out chemical analysis of adhesive residues identified on 15 Iron Age objects from ten archaeological sites across England and northern France.
Diverse uses
Drawn from the collections of the British Museum, Hull and East Riding Museum, and the Strasbourg repository of the Institut national de recherches archéologiques préventives (Inrap), the artefacts were chosen due to the presence of visible black residue, which made them the most promising options to sample and test for surviving adhesives. They span the 5th century BC to the 1st century AD in date, and represent diverse objects, from jewellery (including brooches and a pendant) and one of the pair of Etruscan-style flagons from Basse-Yutz in France, to harness fittings and chariot terrets, as well as more martial items including a sword from Asby Scar in Cumbria, and the Battersea Shield, which was dredged from the Thames in London in the 19th century.
Having made their selection, the researchers had three main aims for their work: to establish if adhesives were preserved on different metal objects, to identify the chemical composition of any surviving substances, and to assess the potential impact of past conservation techniques on these residues. Gas chromatography-mass spectrometry was used to analyse 18 tiny samples of possible adhesives taken from the artefacts. This revealed that birch tar was again the main surviving substance that had been used, securing a wide range of ornamentation including coral and amber studs on brooches and harness gear, red glass inlays on the Battersea Shield, and coral inlays on the Etruscan-style vessel; it had also served as infill on the Truchtersheim pendant, from Bas-Rhin, France. This was not the only ancient adhesive detected, however; the team also identified traces of pine-derived products in four of the samples, while bitumen had been used as an infill on the Asby Scar sword pommel.
Application, conservation
As well as offering illuminating insights into ancient craftwork, the study also gave the team an invaluable opportunity to re-examine objects that have been in museum collections for a long time. One of these was the Battersea Shield, which was purchased by the British Museum in 1857. Some of its glass decorations did not survive intact, making it possible to examine what lay underneath, and how the ornaments had been attached.
Old reports suggest that the dark adhesive that can be seen is bitumen, but it was not analysed at the time and the current research indicates only the presence of birch tar and possibly waxes. Birch tar is not a substance used for conservation, and hence relates to the original adhesive.
‘Results like these show the value of museum collections for current and future researchers to learn more through new analytical techniques,’ said British Museum Curator Sophia Adams. ‘It is encouraging that objects found in the 19th century may still yield new information in current research. It also really highlights the importance of record-keeping and preserving paper archives: when we have detailed records of what has been done to an object in the past, it helps us to distinguish ancient and modern materials.’
The results of the study attest that ancient adhesives do survive on metal objects, and that they were used in a variety of ways, highlighting promising routes for future research. While the team acknowledge that the artefacts were mainly made of copper-alloy, and that further work is needed to fully appreciate the impact of iron corrosion on the preservation of adhesives, they also highlight that the sole iron artefact in the study (a brooch from Argam Lane, near Rudston in East Yorkshire) did have surviving traces of adhesive, too.
Just as the study has added important new information to our understanding of how adhesives were used in the past, it has also raised intriguing questions that future research may be able to address. For example, on some of the objects, both rivets and adhesives had been used to hold ornamentation in place, prompting queries about why fixings had apparently been doubled in this way. Further clues came from the Wetwang terrets, which were adorned with coral studs apparently secured by rivets. In places where a stud had fallen off, however, it was evident that the rivets did not penetrate fully into the underlying metal. What, then, was their purpose?
It may be that the rivets still had a practical role, holding ornaments in place until the adhesive beneath them dried – alternatively, they may have had a purely decorative function, becoming a form of ornamentation in their own right. Above all, this study highlights how technologies have evolved over time to meet the needs and creative aspirations of the people putting them to use.
Further information:
• Tabea J Koch, Sophia Adams, Melanie Giles, Aimée Little, Francesco Palmas, Frederik W Rademakers, Martine Regert, Yohann Thomas, Rebecca Stacey (2025) ‘Potential and limitations of adhesive identification on museum curated metal objects’, Journal of Cultural Heritage 71: 358-369; https://doi.org/10.1016/j.culher.2024.12.013 (open access).
• The Battersea Shield is soon to go on display in Secrets of the Thames, a new exhibition set to open at London Museum Docklands in April; for more details, see http://www.londonmuseum.org.uk/newly-announced/secrets-thames.
