Towards robust identification of Pleistocene adhesives: a critical review of current analytical approaches

Chemical identification of adhesive remains on prehistoric stone tools is of great interest for archaeologists, as the residues contain interesting information on tool use and the exploitation of natural resources by hominins. Adhesives were used to form a wrapping around the stone tool to protect the hand from the sharp edges and improve grip, or to secure a handle out of organic material to the stone tool. This invention, of adding a handle to a stone tool, marks a fundamental change in prehistoric technology. Adhesives can be manufactured from readily available exudates, like pine resin, but could also be man-made, in the case of birch tar that is obtained by dry distillation of birch bark. The glueing properties of the adhesives could be enhanced with the addition of an additive (e.g. charcoal, ochre, beeswax). Given that adhesive manufacture is considered to indicate planning abilities and complex thought, its identification in archaeological assemblages is important for understanding the evolution of human cognition. However, given long-term burial, organic residues on stone tools are generally significantly degraded, which raises numerous chemical challenges and interpretative difficulties that need to be tackled through close collaboration between archaeologists and chemists. Without this interaction between two vastly different research fields, studies can suffer from an overinterpretation of analytical data or a lack of understanding of the archaeological context. This review discusses the main pitfalls encountered in the chemical analysis of prehistoric adhesives and offers analytical recommendations to avoid them. Applying the analytical practices as proposed here will increase the reliability and credibility of the analytical results and allow a strong chemical foundation for the archaeological interpretations. The main focus is on the use of gas chromatography-mass spectrometry for the chemical identification of prehistoric adhesives; however, other commonly used analytical techniques are also briefly discussed.