The impact of mineral composition on the yield and preservation of selected fatty acids in replicate archeological ceramics

The process by which clay influences the preservation and retention of absorbed lipids in archaeological pottery is not well understood. Although properties such as porosity and calcium content are frequently suggested as primary factors in lipid retention, their exact influence remains unclear. To investigate this, replicate clay ceramics were manufactured using varying proportions of illitic/illite-smectitic clay, mixed with two different tempers (sand and chalk), and fired at 800°C in an oxidizing atmosphere. This study aimed to assess the impact of these compositional variations on the preservation and retention of selected fatty acids, palmitic acid (C16:0) and oleic acid (C18:1), often discovered in archaeological lipid residue analysis, in both heat-degraded (100°C for 14 h) and undegraded forms. Gas chromatography-mass spectrometry (GC-MS) was used for quantification, employing calibration curves generated from standard solutions of fatty acid methyl esters (FAME). Two methylation techniques were utilized: acid-catalyzed methylation (ACM) and solvent extraction using a mixture of dichloromethane (DCM) and methanol (MeOH), followed by m-(trifluoromethyl)phenyltrimethylammonium hydroxide (TMTFTH) methylation. A comparison of the two techniques revealed that prior solvent extraction using a combination of DCM-MeOH may have limited the amount of C16:0 and C18:1 methylated by TMTFTH, causing inconsistent results in quantification compared to ACM. From the ACM technique, results showed that sand-tempered briquettes, whether degraded (for C16:0) or non-degraded (for C16:0 and C18:1), have minimal influence on fatty acid yield while degradation significantly affects the yield of C18:1. In contrast, C16:0 and C18:1 recovery from chalk-tempered briquettes is proportional to clay content and inversely proportional to the chalk content. This holds both for degraded and non-degraded briquettes. This work highlights the importance of considering and investigating the mineralogical composition of clay and non-clay minerals for predicting the lipid yield from ancient ceramics.