The creation of residue suites from butchery of whole animals and cutting of plants probably created complications for understanding each residue type individually because residue types tended to overlap. For instance, the original residues on flakes used to process a bird took several forms, including feathers, muscle tissue, fat, and blood, and there are several kinds of plant tissue represented within 'reeds', 'softwood' and 'hardwood'. Future residue burial experiments could refine the design used here to better isolate individual residues for diagenesis testing and avoid using residue types that are more accurately suites of residues.
The use of SEM can improve the visual (and where available, chemical) characterisation of residues. SEM was used to collect 140 micrographs of the reference collection residues that had been previously examined with VLM. The expectation was that the SEM technique would improve characterisation of all residues by allowing additional features of the residues to be seen. However, review of the SEM reference image library and comparison with VLM images of the same residues showed that although SEM produced images with clear microtopography, often no new visual characteristics were revealed beyond the structures already visible with VLM. Nevertheless, SEM was found to improve the ability to see different types of pit formations and other structures in the hardwood and softwood residues examined. SEM may thus be an ideal tool for investigating in situ wood residues since the structure and arrangement of pits on vascular cells can be more easily visualised with this form of microscopy.
The major advantage of using SEM is that the depth of field, and hence the area of residue that is in focus at any one time, is far greater than what is achievable with reflected VLM, even when image stacking software is used to produce a composite image. SEM is thus very well suited to recording micro-topographical information. Because SEM is a non-light based technique, it avoids the production of shadows and other optical artefacts that can hinder interpretation of visual characteristics. Using an SEM in backscattered electron (BSE) mode also offers the ability to easily gain an understanding of residue and substrate composition, and microanalysis shows the elements present in the residue itself, which can provide helpful clues as to the residue origin and inform on the course of further chemical analyses. One of the benefits of reflected VLM is that colour information is maintained that can provide key clues as to residue origin. Reflected VLM also allows one to partially view the internal structure of transparent or semi-transparent residues, as was the case with the animal hair and starch granules. Conversely, SEM only captures the surfaces of samples, but does so in great detail. Hence, we agree with Borel et al. (2014) that SEM and VLM should be considered complementary rather than alternative techniques.
Techniques in addition to reflected VLM, such as SEM, SEM-EDS, Micro-Raman, GC-MS, FTIR-M, and ATR-FTIR can be used to better document and characterise lithic residues lacking diagnostic structures, thereby offering more robust means of identification. Adoption of additional techniques offers avenues for hypothesis-testing the microscopic observations, and can bring a greater degree of objectivity to the interpretation of suspected anthropogenic residues. Such a multi-analytical approach will improve not only the analyst's confidence in their results, but also improve acceptance of the results by the wider archaeological community.
In addition to the present experiment, other major aspects of the residue research programme currently underway include: 1) testing the benefit (level of specificity attainable, interpretability, time required and ease of use) of various chemical characterisation techniques for the identification of unknown lithic residues, and 2) hypothesis-testing initial microscopic identifications of residue types with chemical characterisation techniques. Several chemical characterisation techniques have already been tested for the identification of unknown in situ residues thus far identified on Star Carr lithics. The results so far promise to shed light on cases where microscopic identification of the residue can be incorrect, and also articulate what expectations of archaeological residues from wetland deposits are reasonable.
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