This experiment has shown that some residues were preserved at Star Carr in both slightly acidic clay and very acidic peat conditions, as well as in calcareous alkaline soils located off-site. The experiment has helped inform expectations about which residue types might survive archaeologically. Bird feathers, squirrel hair, softwood tissue, tree resin, and red ochre were preserved after both 1 and 11 month burial periods and across all three burial environments. If these results are taken at face value, one might hypothesise that all of the aforementioned residue types are identifiable on Star Carr lithics. Yet it would be unreasonable to make this assumption based on an experiment whose maximum duration was only 11 months. The experimental results are, however, sufficient for assessing the microscopic survivability of at least some archaeological residue types, particularly those that did not survive the duration of the experiment. For example, the bone and antler residues on flints in the wetland were not detected with reflected VLM after 11 months burial, suggesting that these residues will not preserve archaeologically in this acidic peat burial environment.
A more considered interpretation of the results questions the preservation of feathers and hair in the archaeological record at Star Carr, since no examples of keratinised organic matter have been found to date; for instance keratin overlying the bone portions of hooves or horns. Taking this into account, we hypothesise that the residue types that might be encountered on archaeological tools from Star Carr are tree resins, wood residues, plant cell walls, and red ochre, based on their good preservation on experimental tools (as indicated in Table 6). All of these residues were found to require secondary methods in addition to in situ viewing with reflected VLM to identify them securely. As such, we argue that other lines of evidence that move beyond suggestive visual characteristics are necessary for residue identification in many cases. This means that any amorphous lithic residues suspected to be anthropogenic should also be investigated through chemical analysis to understand their nature.
Additional techniques can be used to provide more secure identifications of residues observed in situ with reflected VLM. These might include viewing with transmitted light microscopy (extraction of residue required); scanning electron microscopy, SEM (with residues preferably examined in situ in a low-vacuum SEM chamber so no coatings are applied to the tool); or chemical characterisation with an SEM capable of energy dispersive X-ray spectroscopy microanalysis or SEM-EDS (preferably in situ); microscopic Raman spectroscopy, Micro-Raman (in situ or extraction); gas chromatography mass spectrometry or GC-MS (extraction required); microscopic Fourier transform infrared spectroscopy or FTIR-M (in situ or extraction); attenuated total reflectance Fourier transform infrared spectroscopy or ATR-FTIR (it is possible in some circumstances to use on small artefacts with in situ residues, but extraction is recommended where the residue is large enough to permit sample recovery in order to avoid the risk of physically damaging the artefact).
The reed tissues that were buried were only able to be securely identified if both the cell wall structure and green chlorophyll pigments were present. The chlorophyll was an obvious sign that the original reed residue had been relocated, but is very likely nonexistent in archaeological plant residues. At Star Carr, the finding of plant cell residues on ancient stone tools might be problematic if recovered from the organic peat contexts since the peat contains both reeds and wood. Worked wood planks (Taylor 1998) – the earliest example of carpentry in Northern Europe (Milner et al. 2013, 58) – are preserved at Star Carr, and it is very plausible that stone tools on site will contain traces of associated wood residues.
The preservation of tree resin residues is particularly noteworthy. Terpenoids, the main components of resins (Mills and White 1977, 13; Versteegh and Riboulleau 2010, 17), are chemically stable over long periods of time. Terpenoids are so durable they have been identified by GC-MS in Eocene and Miocene conifer seed cone fossils that are millions of years old (Otto et al. 2002). Resins are also protected from oxidation in anaerobic environments (Mills and White 1977, 24). Identifications of pine resin, used as a sealant for ships, as coatings for pottery, as fumigants or incense, and as a hafting material for stone tools, are also well known in the archaeological record (Pollard and Heron 2008).
Forming an assessment concerning the rate at which residue diagenesis occurs in the acidic archaeological environments at Star Carr is complicated by the fact that not all residues examined have diagnostic characteristics that allow for easy comparison between burial times of 1 month versus 11 months. However, some basic observations are possible. In the dry land unit, within the time between 1 and 11 months burial, bone, antler, muscle, fish and potato changed in designation from likely present to absent. Also in the dry land unit, hardwood changed from positively identified at 1 month to absent after 11 months. In the wetland unit, starch residues from potato were present after 1 month, but absent after 11 months. In the alkaline unit, hardwood was positively identified after 1 month, but became less identifiable after 11 months and was determined to be probably, but not convincingly, still present.
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