Organic Residue Analysis - Sampling and Methods
Figure 1. Sequence of the sampling strategy for organic residue analysis © Julie Dunne
Organic residue analysis is a minimally destructive method. To ensure a maximum success rate and in order to generate enough data for robust hypothesis testing we prefer samples to meet the following criteria:
Rim sherds are preferable. This is because, during cooking, fat floats to the surface, thereby absorbing into the clay matrix of the rim.
We require a minimum of 30 sherds per chronological phase or vessel type.
Ceramics can be washed, although freshly excavated potsherds are ideal. We have also obtained good results from museum collections even when they have been stored for several decades. We are also hoping that improved methods will allow us to sample surface material, although sand blasting in the Sahara does often have a detrimental effect on lipid preservation.
Once we receive the sherds, we will remove a piece approximately 1.5cm in diameter (the size of a 1€ coin). We will then return the remaining sherd to you. We are happy to discuss the sampling location in detail in order to avoid any surface decoration or interesting features.
In order to remove any contaminating compounds from the burial environment and/or handling we undertake a thorough mechanical removal of the sherd surface before powdering the remaining fabric. Lipids are then extracted from the powdered fabric using a recently introduced protocol, which involves the direct hydrolysis and methylation of lipids generating both high-throughput and higher recoveries of lipids (Correa-Ascencio and Evershed 2014). The extract is then placed in a GC (gas chromatograph) which tells us whether any lipids are present. It is then analysed on a GC-MS (gas chromatograph-mass spectrometer) to identify the components present. We then use GC-C-IRMS (gas chromatography-combustion-isotope ratio mass spectrometry) to determine the stable isotope values of individual fatty acids. This means we can identify the differences between dairy fats and ruminant and non-ruminant carcass fat processing. The cooking of fish and plant waxes can also be identified.