gems, museum conservation

Perhaps one of the most taxing problems for a museum analyst to face is that of attempting to characterize adhesives and paint media based on proteinaceous material. In the case of European panel paintings, much useful information can be derived from an assay of the relative amounts of fatty acids present in the accompanying glycerides [1,2,3,4]. If we consider egg tempera paint, although there are no fatty acid components which are specific to the source, nevertheless a pattern of acids that is low in stearic, but with a pronounced presence of palmitic and azelaic acid, together with a modest proportion of oleic acid would suggest to the experienced analyst that an egg binder is the principal adhesive of the sample. Apart from cholesterol, which would appear to offer the possibility of a reliable indicator for egg tempera [5], there is no other specific lipid indicator that points unambiguously to one egg source. Furthermore, with a degree of experience, the analyst can deduce from the overall quantity of non-drying lipids within the sample, the presence of non-fatty binders such as animal glue, especially when combined with simple solubility tests. So a careful examination of the fatty constituents can be a useful diagnostic when taken in context and assessed with caution. Nevertheless, it has been apparent for some time, that there is very real need for an adequate analytical technique for the characterization of the nitrogen-containing components of which the bulk of a glue binder or tempera paint is composed. These are chiefly proteins, of course, which are constructed of amino acid units joined one-to-another through the peptide linkage (–CO–NH–) and represent the main binding components in glues and tempera, following the process of dehydration.