CHEMICAL CHARACTERISATION OF SURFACE ORGANIC RESIDUES ON ARCHAEOLOGICAL POTTERY: INVESTIGATING VESSEL USE, PALAEODIET, AND THE ACCURACY OF 14C-DATES

Two major classes of organic residue are recognised preserved in archaeological pottery vessels, namely: visible deposits on the interior surface, and absorbed residues within the vessel wall. Both classes represent the remains of organic natural materials processed in vessels and occur most commonly on vessels broadly classified as 'cooking' pots, although vessels would have been used for a wide range of activities involving the heating of organic materials. A major area of interest in such residues comes from the potential to use their biomolecular and stable isotope compositions to investigate: the use of vessels, the nature of commodities processed in vessels, dietary components and agricultural activities. Absorbed residues have been much more widely chemically analysed and applied than surface residues, with an increasing range of plant and animal products being recognised and significant archaeological questions addressed. The attraction of absorbed residues is their high frequency of occurrence, e.g. on average ca. 50% of 'British 'cooking' pots contain significant concentrations of organic residues relating to the lifetime of use of vessels. In contrast, surface residues have been much less studied, mainly because of their relative rarity. Where chemical investigations have been undertaken different classes of compound, e.g. carbohydrates and proteins, are seen to dominate surface residues, compared to absorbed residues, which are dominated by lipids. Up to now studies of surface residues have been limited to small numbers of vessels, whereas several investigations of absorbed residues have demonstrated the value of chemically analysing much larger numbers, several hundred vessels, to reveal changes in human activity through time and across geographical regions. One very significant application of surface residues has been their use in radiocarbon dating by accelerator mass spectrometry (AMS). Surface organic residues are particularly suitable for this purpose as they probably derive from a single 'cooking' event and thus have 'young' ages. Recognising this English Heritage (EH) has funded 14C-dating on >250 residues, which have been included in statistical (Bayesian) models, together with dates from other sources. This modelling suggests that such residues are more likely to produce anomalous ages than other sample types. Around 15% of samples ages are not reproducible between radiocarbon laboratories and have very poor agreement with the other information included in the chronological models. It seems there is contamination in some of the dated residues that cause these anomalies. As the chemical compositions of the dated residues is unknown, these anomalies are currently unexplained. Moreover, the opportunity for wider interpretations of vessel use and past diet are missed. The EH dating programme provides a unique and rapidly evolving sample set for undertaking systematic investigations of the organic chemistry of carbonaceous surface residues from archaeological ceramics. In this project we aim to bring together chemical and radiocarbon analyses of surface residues to: enhance interpretations of vessels use and diet from 14C-dated residues from Britain, and provide a robust explanation(s) for the anomalous 14C-dates obtained from a proportion of such residues. The specific aims of the project will be to: determine the chemical composition of the residues and assign their origins by chemical and stable isotopic analysis, in order to pinpoint chemical criteria for identifying the most suitable residues for 14C-dating, including the identification of marker compounds for residues likely to produce anomalous ages. The project will provide the student with training in a range of novel micro-analytical methods aimed at deriving molecular level chemical and stable isotope information for automated multivariate data treatment for organic residue classification in this key aspect of archaeology.