Chemical processes in aged oil paints affecting metal soap migration and aggregation
The nature of the surface of the painted picture is determined to a large extent by chemical and physical processes underneath the surface of the paint. The binding medium changes from a chemically air drying viscous mass of polyunsaturated triglycerides Â“the oilÂ” via slow hydrolytic processes to a metal bound ionomer. This network system in turn appears to be vulnerable to further environmental attack especially by acidification that is postulated to disrupt the ionomeric structure. As a result monocarboxylic fatty acids can be mobilized and reorganize in the form of liquid crystalline metal soap masses within the paint layers. When they expand beyond the paint layer, they distort the paint and may erupt at the surface where they protrude or even extrude. These deformations are now recognized in thousands of paintings. Many of these metal soap masses furthermore mineralize forming minium (lead orthoplumbate) and/or lead-hydroxy/chloride-carbonates in lead soap or zinc carbonates in the case of zinc soaps. Generally the volume changes due to these processes are minimal although the metal soap may grow into larger masses of 100-200 micron diameter. This theory could be developed by an integrated molecular level approach involving GCMS (gaschromatography mass spectrometry) and DTMS (Direct Temperature resolved mass spectrometry) work and various forms of chemical microscopy using imaging SIMS (secondary ion mass spectrometry), FTIR (Fourier Transform Infrared spectroscopy), Raman spectroscopy and Scanning electron microscopy with energy dispersed Xray analysis (SEM-EDX).
|H. Mar Parkin|
Boon, J. J, Hoogland, F. G, & Keune, K. (2007). Chemical processes in aged oil paints affecting metal soap migration and aggregation. In H Mar Parkin (Ed.), AIC paintings specialty group postprints : papers pres. at the 34th annual meeting of the AIC of Historic & Artistic Works providence, Rhode Island, June 16-19, 2006 (pp. 16–23). AIC.