GC-MS characterization of proteinaceous and lipid binders in UV aged polychrome artifacts

Abstract

This paper presents the most significant results obtained from the characterization of protein and lipid binders in a broad range of reference paint materials prepared and stored at Opificio delle Pietre Dure in Florence (Italian Ministry of the Cultural Heritage, Italy). The amino acid distribution for protein binders and the fatty acid distribution for lipid binders were determined by gas chromatography-mass spectrometry before and after being artificially aged through exposure to UV light at 254 nm or 366 nm for different time periods in a climatic chamber (20°C and 80% RH). The results were compared with those relevant to old paintings of different ages, and showed that UV light aging does not significantly affect the amino acid profile of protein binders. Consequently, protein binders in old paintings can be reliably identified by comparing the amino acid composition with that of reference paint materials which have not been aged. However, the composition of lipid binders is substantially affected by UV irradiation, leading to a lowering of oleic acid and the formation of azelaic acid and other dicarboxylic acids including oxalic acid. An oleic to stearic acid ratio of less than 0.5 was observed in all the samples of works of art, and this parameter can be used to evaluate the extent of the artificial aging process. The formation of oxalic acid was also observed starting with pure unsaturated fatty acids, thus supporting the chemical origin of oxalate patina.