Stable carbon isotopic characterisation of free and bound lipid constituents of archaeological ceramic vessels released by solvent extraction, alkaline hydrolysis and catalytic hydropyrolysis

Abstract

A sequential degradation scheme, involving solvent extraction, alkaline saponification and catalytic hydropyrolysis (open-system pyrolysis assisted by high hydrogen gas pressure), was used for recovering both free and covalently-bound lipid organic compounds from two archaeological ceramic sherds sampled from the interior of cooking vessels and from a ceramic control sample used for cooking modern milk. Various organic molecular products released by this regime were identified and quantified using gas chromatography–mass spectrometry (GC–MS), and the stable carbon isotopic ( δ 13 C ) values of the major products were measured using gas chromatography–combustion–isotope ratio mass spectrometry (GC–C–IRMS). Significant amounts of fatty acids and fatty acid-derived hydrocarbons could be released by alkaline hydrolysis and hydropyrolysis, respectively, following initial solvent extraction of the ceramic. No significant lipid signal bias, in terms of both carbon number distributions of compounds or stable carbon isotopic signatures, could be discerned though between free, hydrolysable and tightly-bound molecular components in this study. So, conventional analysis of free fatty acid components appears, from this limited data set, to provide an accurately representative insight into the total fatty acid composition in archaeological pots. Even after catalytic hydropyrolysis, a significant amount of residual carbon (>50 wt.% of the total organic carbon) remained on the archaeological sherds in the form of highly condensed aromatic polymers, with much lower (6 wt.%) levels of residual carbon persisting in the control pot. This aromatic macromolecular phase on the vessel interior most probably originates from repeated use of the vessels for cooking, via gradual polymerisation/aromatisation of food residues, rather than being derived from smoke condensates produced from cooking on open wood fires. The co-existence of preserved aliphatic fatty acids and a thermally-stable aromatic macromolecular phase within the same ceramic matrix suggests that the preserved lipid components were introduced into the vessel after the formation of the bulk of the char, and so fatty acid analyses most probably provide information concerning the later uses of archaeological pots prior to burial.