Formation of long-chain ketones in archaeological pottery vessels by pyrolysis of acyl lipids

Abstract

Studies of organic residues preserved in unglazed archaeological pottery have revealed the presence of homologous series of long-chain ketones containing 29–35 carbon atoms. The C 31, C 33 and C 35 ketones are particularly abundant and exhibit a distinct monomidal distribution. The presence of long-chain ketones in potsherds is usually ascribed to the absorption of epicuticular waxes into the pottery fabric during the cooking of leafy vegetables. However, compound specific stable carbon isotope ( δ 13C) analyses of the individual lipids present in the potsherd extracts, in combination with detailed structural information, indicates that these ketones do not derive from plant waxes. Isotopic and structural analysis of the fatty acids, which always co-occur with the ketones, suggest that a precursor-product relationship exists. Micro-scale pyrolysis of a range of free fatty acids and triacylglycerols in the presence of various inorganic matrices was undertaken in exploring the possibility of an abiological route to the formation of the ketones. Depending on the pyrolysis conditions employed, substantial yields of long mid-chain ketones were formed which were structurally and isotopically congruent to those observed in the ancient potsherds. The ketones are formed by ketonic decarboxylation (a type of head to head condensation reaction), probably involving fatty acid metal salts as intermediates, the metal being provided by the inorganic matrix. Apart from the abundant long mid-chain ketones various other products such as methylketones, methyl esters, alkanes, alk-1-enes and homologous series of minor ketones are formed as secondary pyrolysis products. These latter products are not found in the pottery probably due to less vigorous thermal conditions achieved during the original use of the vessel compared with those attained in the laboratory pyrolysis experiments. Evidence for this comes from the formation of the fatty acid methyl esters which are only produced under the most forcing of pyrolysis conditions.