Animal glues in mixtures of natural binding media used in artistic and historic objects: identification by capillary zone electrophoresis

Abstract  Animal glues were often used in historic and artistic objects, e.g. as paint ground, as binders for pigments, or as adhesives. The sources were egg, casein, or different collagens. For restoration and conservation purposes it is important to know which kind of animal glue a museum object contains. Capillary electrophoresis can deliver such information, because it enables differentiation among the three proteinaceous glue classes according to their different amino acid patterns after hydrolysis. This work deals with the most relevant problem in practice, whether this identification is obstructed by the presence of other binders, with which they are mixed in many real samples; in particular, interference from plant gums and drying oils was investigated. Capillary electrophoresis of the hydrolysates (after reaction with 6 mol L–1 HCl) was performed with an acidic background electrolyte consisting of chloroacetic acid (51.9 mmol L–1) adjusted with LiOH to pH 2.26. The underivatised analytes were detected with a contactless conductivity detector. It was found that the constituents of the plant gums (monosaccharides) or drying oils (long-chain fatty acids and short-chain dicarboxylic acids) never interfered with identification of the animal glues, as shown for artificial mixtures of the different binders even at tenfold excess over the animal glue, and for egg tempera samples. The method was used to identify the filling material from a statue from the eighteenth century.