Microscale distribution and concentration of preserved organic molecules with carbon-carbon double bonds in archaeological ceramics: relevance to the field of residue analysis

Biologically produced compounds preserved in ancient ceramics can provide invaluable information on the vessel contents. Analysis and interpretation of these so-called archaeological “residues” is therefore important for understanding and reconstructing aspects of social and cultural behaviors of ancient societies. Based on the reaction of unsaturated compounds with iodine, we developed and apply two new methods. The first is a simple and relatively rapid method for assessing the amounts of unsaturated compounds in archaeological ceramics using X-ray fluorescence (XRF). We show that this method is a reliable indicator for assessing the general preservation state of the organic material and therefore a potential pre-screening method for identifying ceramic samples suitable for organic residue analysis. The second complementary approach, based on the same iodine reaction, makes it possible to map the unsaturated molecules on ceramic surfaces at a scale that enables to correlate organic matter distribution and the underlying mineral grains using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) detector. With this method we show that the extent of adsorption of lipids onto ceramic surfaces varies with the surface properties of the different minerals in the ancient ceramic, with calcium containing minerals showing the highest affinity for compounds with unsaturated bonds. The ceramic substrate therefore influences the types of organic compounds bound and hence preserved in the ceramic. Fundamental information obtained using this method is essential for better interpreting molecular assemblages extracted from archaeological ceramics.