First results on thermally induced porosity in chlorite cooking vessels from Merv (Turkmenistan) and implications for the formation and preservation of archaeological lipid residues

Excavations at the ancient city of Merv in Turkmenistan have yielded a significant assemblage of ceramic and stone vessel fragments from contexts dating to the Sasanian and medieval periods (5th–13th centuries AD). Many of the vessel fragments are encrusted with apparent food residue deposits offering scope for organic residue analysis and the potential to compare the use of these two very different vessel types. To enable meaningful comparison we sought to establish whether the vessel stone (a talc-chlorite schist) can absorb and preserve organic residues indicative of past use. This paper reports the first investigation to address this question using standard methods for archaeological residue analysis coupled with study of the properties of the schist fabric and basic absorption experiments. The results demonstrate that heating during cooking, even at relatively low temperatures, modifies the stone microstructure by increasing its porosity. These pores enhance the absorption capacity of the schist fabric such that chlorite cooking pots can absorb and preserve sufficient organic residue to reflect the original processing of foodstuffs, as has been previously demonstrated for ceramic fabrics. Comparison of pore capacity, archaeological residue yield and experimental absorption indicates the significance of pore-size distribution for the relative retention and preservation of organic residues in these fabrics. The results of this study indicate that a wider range of archaeological material may be suitable for absorbed residue analysis than has previously been recognised. The findings also challenge previous assumptions concerning the relationship between the properties of chlorite vessels and their mode of use in Central Asia and Near East in antiquity.