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 multi-faceted 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
organic residue analysis specialists to carefully consider the impact of
post-firing coating before interpreting residues from archaeological
ceramics and to implement a multi-faceted ORA approach to aid their
detection.