Abstract
The impact of solvent exposure on oil paintings and the differences between solvent application methods are longstanding topics in cleaning studies. Solvent exposure is ideally kept to a minimum, because solvent swelling can lead to the extraction and displacement of reactive paint components. In particular, important concerns are fatty acids displacement resulting in metal soap formation and embrittlement of paint due to solvent exposure. In this study, the extraction of a saturated fatty acid (SFA) marker and the formation of zinc soaps were monitored to measure the impact of solvent cleaning on tailored bilayer model systems for aged oil paint. Three methods of solvent application were compared: cotton swab, rigid gel and Evolon tissue (with different solvent loading). The samples were analysed by surface acoustic wave nebulization mass spectrometry (SAWN-MS) and thermally-assisted hydrolysis and methylation pyrolysis gas chromatography mass spectrometry (THM-Py-GC/MS) by comparing the calculated margaric:palmitic acid (text {C}_{17}{:}text {C}_{16}) ratio determined in the extracts (taken from the swab, gel or Evolon tissue). We conclude that both swab cleaning and squeezed Evolon tissue application result in comparable SFA extraction. The rigid gel and Evolon with controlled solvent-loading limit the amount of SFA extraction. The distribution of text {C}_{17} after solvent application was visualised using static Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) on cross sections, showing that text {C}_{17} redistribution took place in all cases where solvent was applied. Crystalline zinc soaps formation was not observed after 5 min of ethanol exposure in the embedded cross-sections with imaging ATR-FTIR, indicating that solvent exposure does not immediately trigger the formation of crystalline metal soaps. However, significant zinc soap formation was found after 30 min of ethanol exposure using Evolon tissue without controlled loading. This study contributes to a better understanding of the impact of different methods of solvent application on oil paintings and highlights important differences between these methods.
Highlights
Scientific research focusing on the cleaning of oil paintings has evolved greatly since the early investigations in the 1950s [1, 2]
Our results stress that the method of solvent application and the duration of solvent exposure can directly influence saturated fatty acid (SFA) migration and zinc soap formation in oil paint
We studied the impact of solvent cleaning using the relative amount of margaric acid (C17) extraction and zinc soap formation as indicators
Summary
Scientific research focusing on the cleaning of oil paintings has evolved greatly since the early investigations in the 1950s [1, 2]. These bilayer model systems (denoted pLOC17−Znpol , see Fig. 1) contain both SFAs and zinc carboxylates in different layers, but only form zinc soaps upon solvent exposure. Mass spectrometric analysis of extracts SAWN-MS and THM-Py-GC/MS in negative ionisation mode were used to determine the C17:C16 ratio in ethanolic extracts from the cotton swab, the Evolon tissue (with different loading) and the rigid gel.
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