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5. Results: visual characterisation of residues in situ

One of the main findings of this experiment is that the visual appearance of many residue types is insufficient to make a confident identification during in situ analysis with reflected VLM. This finding was used to generate a tripartite division of residue types as 'diagnostic', 'distinctive', or 'non-distinctive' (see Table 7) using VLM. The purpose of this scheme is to emphasise that the identifiability of residues varies according to type, and unless diagnostic feature(s) are present and can be discerned with the type of microscopy employed, only a suggestion, not a reliable identification, can be made regarding the origin of the residue. Decomposition further exacerbates the difficulty of identifying residues. Examination of the reference collection in conjunction with experimentally degraded residues revealed few residues with morphologically diagnostic structures (Table 7). For instance, the residue types in the diagnostic category have potential for archaeological identification by reflected VLM only if found in an adequate state of preservation that allows diagnostic features to be located. However, some residues may be so poorly preserved that no diagnostic features remain. Residue types in the distinctive and non-distinctive categories require additional techniques beyond reflected VLM for accurate identification. Further techniques to identify the residue in question might include: viewing on glass slides with transmitted light microscopy, SEM, chemical characterisation with energy dispersive X-ray spectroscopy (EDS), microscopic confocal Raman spectroscopy (Micro-Raman), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), Fourier transform infrared microscopy (FTIR-M) or gas chromatography mass spectrometry (GC-MS).

Table 7: Distinctiveness of in situ residues using reflected VLM
Diagnostic Distinctive Non-distinctive
BoneX
AntlerX
MuscleX*
Blood filmX
Animal fatX
Fish scalesX
Bird feathersX
Mammal hairX
Potato starchX
Reed cellsX
Softwood tracheidsX
Hardwood vessel elementsX
ResinX
Red ochreX
*Muscle residue presented a special case for identification dependent on observation condition. Fresh muscle in the reference collection was diagnostically identified by the presence of myofibres with clear light and dark banding cross striations, but after burial the remaining suspected muscle residue showed no banding.

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