Beyond the surface: untangling molecular complexity in organic residue analysis of coated archaeological ceramics

Mixed molecular and isotopic signals are readily encountered in organic residue analysis (ORA) of archaeological ceramics, which can impact our ability to identify the contents/use of the vessel. One reason for these mixed signals that is rarely considered is the mixture of organic products used to coat vessels and the organic products that were later contained in the ceramic vessel. Here, we apply a multifaceted ORA approach to experimental cooking vessels that were coated with a range of organic products during post-firing treatment and then used to cook a variety of organic contents. Our results show that the visibility of the coating agent or cooking commodity varies depending on the type of extraction or analytical method applied. We show that it is important to use a combination of extraction methods (acidified methanol and solvent extraction) and that both molecular and isotopic data must be considered. For the first time, we have also shown that thermal transformation markers could reflect the post-firing coating as opposed to cooking. We call on ORA specialists to carefully consider the impact of post-firing coating before interpreting residues from archaeological ceramics and to implement a multifaceted ORA approach to aid their detection.