Using 3D digital microscopy and SEM-EDX for in-situ residue analysis: A multi-analytical contextual approach on experimental stone tools

Abstract

Residue analysis is a sub-discipline in archaeology that uses a variety of techniques which can be useful for determining the function of prehistoric stone tools. Past experimental studies have revealed the non-diagnostic nature of residues under reflected light microscopy and the challenges of identifying them as plant or animal and assigning them to a specific origin (i.e. use-related vs incidental). Recently, new contextual approaches and analytical techniques have addressed these limitations. Prior studies have suggested that use-related and incidental residues can be identified on stone tool surfaces based on differences in distribution patterns. Location, residue frequency, and associations with other residues are often the main criteria used in these studies to distinguish use-related from incidental residues, while other contextual information such as directionality, aspects of crushing and smearing, or dispersion from the working edge are often ignored. Moreover, the integration of chemical information provided by other analytical techniques into these contextual approaches that address residue mapping has received little attention. In this study, we address these issues and combine contextual information recorded through 3D digital microscopy with elemental compositions obtained through SEM-EDX to better identify different animal and plant residues on experimental stone tools. Based on an experimental sample tested under outdoor field conditions, we show how identification issues in fresh undamaged residues are not only restricted to the ambiguity between different use-related remains, but also the ambiguity between the latter and incidental residues. We demonstrate the versatility of 3D DM in recording in-situ contextual information of fresh residues (distribution patterns, invasiveness, disposition of specific tissues indicating tool-use direction, and aspects of crushing and smearing) and how supporting this information with the elemental composition obtained through SEM-EDX and element maps can help to improve identification of ambiguous residues, both use-related and incidental. Although more experiments are needed, the multi-analytical contextual approach tested here provides an example of in-situ characterization of fresh undamaged residues in experimental tools that can be conducted before performing post-depositional experiments or using other analytical techniques and extraction protocols.