A modified Picro-Sirius Red (PSR) staining procedure with polarization microscopy for identifying collagen in archaeological residues

Use-wear and residue analyses of stone artefacts are widely used to better understand the behaviour and resource utilization of past peoples. There are numerous ethnographic reports describing the processing of animal parts, but identification of collagen, the principle component of animal protein, can be difficult because of the similarity in appearance to other non-collagenous residues (e.g. Lombard and Wadley, 2007). Additionally, damage to collagen structure caused by processing and taphonomic factors can alter collagen's microscopic morphological features and further prevent microscopic identification. This paper describes the trialling, blind testing and application of a modified Picro-Sirius Red (PSR) staining protocol not reliant on intact morphology which can be used to identify archaeological collagenous residues. The protocol allows for differentiation of Types I, II and III collagen and can detect the minuscule amounts in archaeological samples. The application of this staining protocol to ten grindstones from arid and semi-arid regions in south and central Australia supports the view that lack of collagen heretofore reported in residue studies is more likely the result of under-detection than its absence (see Monnier et al., 2012). The staining protocol represents an inexpensive, efficient and reliable process which can be added to contemporary use-wear and residue analyses. This addition allows more inclusive assessments of past function and promotes wider understandings of the behaviour of past peoples.