Viability of Raman microscopy to identify micro-residues related to tool-use and modern contaminants on prehistoric stone artefacts

Analyses of ancient micro-residues and usewear preserved on stone artefacts can potentially provide detailed information about how prehistoric humans used the artefacts to process materials such as food, pigments and/or adhesives. However, ancient micro-residues are likely degraded, and there are multiple potential sources of contamination, such as contact with sediments, groundwater, recent handling, storage materials or laboratory conditions, any of which can inhibit reliable identification of micro-residues and other traces of prehistoric use. In this pilot study, five stone tools from the archaeological site of Liang Bua (Flores, Indonesia) were used to evaluate the viability of Raman spectroscopy to identity ancient micro-residues preserved on stone artefact surfaces that are due specifically to prehistoric use as opposed to some form of ancient or modern source of contamination. Inorganic and organic deposits that occur commonly in the cave environment, including iron oxide, manganese oxide and biofilms, were identified in both the sediment and on the artefacts. Protein and saturated fatty acid micro-residues were identified on edges of all artefacts and may partially originate from modern handling. Proteins, plant fibres and other micro-residues associated with calcium nitrate are possibly archaeologically significant. Detection of plant fibres and starch grains may indicate either modern contamination or prehistoric contact with plant material that was transferred incidentally or during tool manufacture and/or tool use. These results demonstrate the viability of Raman microscopy to screen, at an early stage of archaeological residue analysis, for modern contaminants and micro-residues related to tool manufacture and/or tool use. This approach serves as a base for planning strategies and analytical protocols for future work that targets larger samples of artefacts, integrates Raman microscopy with GC-MS/LC-MS and includes more comprehensive studies of usewear.