Integrating SEM-EDS in a sequential residue analysis protocol: Benefits and challenges
Residue analysis can be a useful way to determine the past functions of archaeological tools, particularly when teamed with other functional investigations such as usewear and technological analyses. The most common approach for residue analysis is through the use of optical microscopes, which can be used to visually identify in situ residues directly from the tool surface (with reflected light microscopes, RLM), as well as in water extractions sampled from the utilised tool edges (with transmitted light microscopes, TLM). Recently, the scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS) has shown great potential for archaeological residue analysis as it can provide high-resolution images at very high magnifications as well as elemental analysis of adhering material. An advantage of this instrument is that it is capable of operating in low vacuum or "environmental" mode, allowing specimens to be examined uncoated and without additional preparation, so that residues can be documented/analysed in situ on the stone. In this paper, we propose a sequential protocol for the identification of tool residues using various optical light microscopes in combination with the SEM-EDS, on residues documented both in situ on stone tools and those removed from the stone substrate in solvent extractions. We also propose a new method for analysing extracted residues using the SEM-EDS that permits high resolution images of micro-residues, particularly starch and other fibres. We argue that, although both methods have limitations and instrumental challenges, when used in combination, provide a complementary means for documenting tool residues.