Organic residue analysis in archaeological pottery. Heating markers in non-cooking vessels from Etruscan Tarquinia (Italy): a sampling problem?

The organic residues in the material of a series of pottery vessels from a votive deposit (3rd - 2nd century B.C.) in the excavations of the Etruscan city of Tarquinia (Italy) were analyzed by gas chromatography – mass spectrometry (GC-MS). Long-chain (31, 33 and 35 carbon atoms) ketones were initially detected in the lipid extracts of all the ceramic samples examined, irrespectively of the characteristics of the vessels in terms of shape and texture and of the possibility they had been used for cooking, recognized in several studies as the main cause of the formation of these compounds. Considering that some cases have also been reported in the literature in which the presence of these ketones seemed unexpected for a given ceramic container, it was decided to examine the possible correlation of their occurrence, for the set of vessels examined in the present work, with the sampling mode used to obtain the ceramic powder before extraction with solvents for the recovery of the organic residue. It was thus possible to demonstrate that the use of a drill for sampling, a modality not unusual in this kind of study, can give rise to the formation of these compounds in an archaeological ceramic material in which fatty acids are absorbed, particularly if it contains calcium carbonate. A laboratory experiment based on a fragment of modern pottery in which palmitic and stearic acids were absorbed from a solution, and from which powder samples were then obtained both by drilling and by manual scraping of the pottery surface, allowed to validate the hypothesis above. It was therefore possible to highlight the importance of careful consideration also of the sampling phase in interpreting the results of the analysis of organic residues in archaeological pottery.