Adhesives have played a vital role throughout human history. Studying their composition and production methods offers insight into past technologies and helps reconstruct historical practices. This study focuses on the materials science analysis of Betula sp. (birch) bark tar, a widely used adhesive in prehistory times. By examining its molecular composition and production techniques, this research seeks to replicate ancient manufacturing methods using experimentally produced samples.
In this study, Gas Chromatography-Mass Spectrometry (GC/MS) was employed to analyze the chemical composition of the adhesives. To classify different production methods, machine learning techniques—including Principal Component Analysis (PCA), Linear Discriminant Analysis (LDA), and K-Nearest Neighbors (KNN)—were applied. The results indicate that LDA successfully differentiates between production techniques, suggesting its potential for identifying variations in tar preparation. However, since this study is based on experimentally produced samples, its application to archaeological specimens requires further validation.

Adhesives are essential components of everyday life and have been for thousands of years. The history of adhesives begins with a stone tool covered in birch tar found at Campitello Quarry, Italy, dating to around 200,000 years ago. This find demonstrated the use of adhesives by Neanderthals and their ability to manufacture materials through transformative processes. In Southern Africa, modern humans have been producing compound adhesives by mixing plant and mineral materials since at least 70,000 years ago, tailoring adhesives to different environments and uses. In recent years, the number of identified prehistoric adhesives has grown, and adhesive technology has become a proxy for discussing technological complexity across different hominin species. However, to fully evaluate and compare the adhesive technology of other human groups across space and time, more research is needed on the selection of adhesive materials, adhesive recipes, and the context in which adhesives and their tools were used.

With this dissertation, I contribute to enlarging the sample of identified prehistoric adhesives by analysing tools with adhesive residues from Steenbokfontein Cave (South Africa, Later Stone Age), Morín Cave (Spain, Middle-Upper Palaeolithic), and the Dutch North Sea (the Netherlands, Mesolithic). I employ a multi-analytical approach encompassing optical microscopy and experiments with the integration of data from chemical analysis of residues. The results of the analysis, combined with data from available literature, provide insights into several questions that enhance the debate on the technological complexity of Neanderthals and modern humans. What materials and additives were used by Neanderthals and modern humans to manufacture adhesives? Was there a difference in adhesive recipes depending on the context of use of the hafted tools? Was there a preference for hafting specific tools? Is there a difference between Neanderthal and modern human adhesive technologies in terms of raw materials exploited, use of additives, and context of use? Does adhesive technology reflect differences in technological complexity between Neanderthals and modern humans?

All the analysed adhesives were used by Neanderthals and modern humans to fasten their tools to organic handles. Adhesive residues have been identified on stone and organic projectile points, as well as on ‘common tools’ used in domestic tasks, strongly suggesting that adhesives were integrated into the domestic economy of Neanderthals and modern humans. No relevant differences in the use of adhesives were observed depending on the tools’ raw materials or functions.

Adhesives were mostly produced from natural resources available in the surrounding environment. At Steenbokfontein Cave, South Africa, adhesives were made using the resin or tar of conifer trees, specifically Podocarpus or Widdringtonia, both of which were available near the Cave and mixed with (mineral) additives. Similarly, at Morín Cave in Spain, the resin from a tree of the genus Juniperus, largely available in the environment, was likely used. However, there is evidence that some adhesives were selected over others equally available for their material properties. For instance, birch bark tar was preferred over pine resin for hafting bone points at the Dutch North Sea, a trend seen at many other Mesolithic sites. Furthermore, increasing evidence suggests that Neanderthals used additives, primarily iron oxides, to alter the material properties of their adhesives, similar to contemporaneous modern humans in Africa. This reflects Neanderthals' and modern humans' understanding of available natural resources, their distinct material properties, and the effects of their combinations.

Adhesive technology requires good knowledge of natural resources and their material properties, control of fire, enlarged cognitive functions, and forms of cultural transmission and social learning, qualifying it as a complex technology. The examination of adhesive remains in this thesis demonstrates that Neanderthals and modern humans share considerable technological parallels, highlighting Neanderthal technological sophistication. How Neanderthals selected, transformed, and employed adhesives suggests analogous procedures and reasoning to modern humans. Consequently, these insights likely reflect that Neanderthals had comparable cognitive and technological skills to anatomically modern humans.

The chemical analysis of archaeological residues can offer valuable insights into the technical complexity of ancient hominins, thus opening a window to their cognitive complexity and socio-economic exchange networks. The scarcity of residue findings in the archaeological record bears the need for non-destructive analytical tools. Various combinations of non-destructive techniques, for instance optical microscopy, infrared or Raman spectroscopy, have been implemented. These techniques are also often combined with destructive GC-MS analysis, which is the most common analytical method for these organic materials. However, currently, there is no systematic proposed way to analyse archaeological residues non-destructively. This project aims to verify the organic nature and test the reliability of identification of archaeological adhesive materials with a combination of non-destructive methods. For this purpose, SEM-EDS, FTIR microspectroscopy in reflectance mode, Raman microspectroscopy and XRD are implemented. A set of experimental adhesive replicas of pure materials and mixtures are examined in pristine form, but also after a three-year-long weathering experiment. The materials tested are pine tar, birch tar, pine resin, beeswax and mixtures of pine resin with beeswax, as well as pine resin with beeswax and ochre; these materials are representative for archaeological finds. Additionally, a set of archaeological samples were studied, consisting of Mesolithic bone/antler points with adhering hafting adhesive residues, form the Dutch North Sea. This research shows that the effectiveness of each method varies with the different materials and mixtures tested. In addition, degradation negatively influences the reliable verification and identification of the organic residue constituents significantly. A single method is usually unable to reliably identify weathered and degraded samples, creating the need for consideration of combined results from multiple methods. A protocol is eventually proposed for the systematic non-destructive analysis of unknown archaeological residues, based on the observations of this research. The protocol recommends SEM-EDS as a starting point of analysis for verifying the residue’s organic nature. The combination of micro-FTIR and micro-Raman follows, to reinforce the verification of organic matter and identify the residue constituents. Lastly, XRD can identify additives to the residue mixture and phases related to the artefact’s environment of burial.