Documenting scarce and fragmented residues on stone tools: an experimental approach using optical microscopy and SEM-EDS

Residue analyses are widely applied to studies of stone tool function and can be a powerful method for determining the past tool use(s), especially when combined with other functional investigations such as usewear and technological analysis. Experimental work has shown that optical microscopes and the scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS) are reliable instruments for identifying intact tool residues. However, little experimental work has aimed to document residues that show various stages of degradation or when abundance is low. We combined traditional optical microscopy and the SEM-EDS to identify the advantages and challenges of each technique when looking at progressively smaller and more fragmented residues following more aggressive stages of cleaning, burial and soaking in a weak acid/base solution. We found that large quantities of intact residues on unwashed stone tools show distinctive morphological features under optical microscopes and the SEM-EDS can be used to document residues under extremely high magnifications and to determine their elemental compositions. After the various stages of washing, we found that residues became highly fragmented and were restricted to common stone features like the micro-cracks/scars along the working edge. These residues were often difficult to characterise using optical microscopes but the SEM-EDS proved highly useful. The weak acid/base solutions caused some residues to become physically altered or modified their elemental composition. Buried tools reduced the abundance of use-residues and introduced additional non-use-related contaminant particles that affected EDS measurements and lead to less reliable residue interpretations