Solvent-mediated extraction of fatty acids in bilayer oil paint models: a comparative analysis of solvent application methods

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 ( C 17 :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 C 17 after solvent application was visualised using static Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) on cross sections, showing that 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.

Quantifying solvent action in oil paint using portable laser speckle imaging

“…However, it is known that the exposure of oil paints to organic solvents for varnish removal, or to water for the removal of surface dirt, can affect the physicochemical properties of the paint in an undesired way [3][4][5] . For example, exposure to organic solvents can lead to embrittlement of paint 6,7 and to the extraction [8][9][10][11][12][13][14] and redistribution 15 of soluble paint components. An increased rate of formation of degradation products, such as crystalline metal soaps (complexes of metal ions and long-chain saturated fatty acids), has also been reported 15,16 .…”

“…For example, exposure to organic solvents can lead to embrittlement of paint 6,7 and to the extraction [8][9][10][11][12][13][14] and redistribution 15 of soluble paint components. An increased rate of formation of degradation products, such as crystalline metal soaps (complexes of metal ions and long-chain saturated fatty acids), has also been reported 15,16 . Although a considerable amount of progress has been made 3,4,[17][18][19] , the knowledge of the influence of solvent-based cleaning on fundamental chemical processes in oil paint does not currently allow for a reliable assessment of the risks involved in cleaning.…”

“…What exactly defines the impact of solvent action on paint is a complex interplay between many different chemical and physical phenomena. After the application of solvent, solvent swelling [24][25][26] , -diffusion 27 , -evaporation 28,29 , -leaching (extraction) [9][10][11][12][13][14] and chemical reactions 15,16 occur simultaneously. It is not possible to quantitatively measure the total physicochemical influence of these combined processes during cleaning practice, and conservators often assume that minimal exposure to water or solvents limits the risks associated with cleaning.…”

Review of the use of NMR spectroscopy to investigate structure, reactivity, and dynamics of lead soap formation in paintings

“…In recent years, the interest on the potential role of the cleaning treatments as activating agents for degradation has become a field of active study. 30,[79][80][81] In this framework, unilateral NMR may be considered one of the most useful techniques to evaluate and monitor a cleaning treatment. With unilateral NMR, it is possible to measure the hydrogen depth profile, the relaxation times, and the self-diffusion coefficients of water and solvents within the paint layer in a nondestructive way.…”

Chemistry for cultural heritage: connecting past and future

“…New studies in metals and paintings cleaning are also reported: Baij et al [21] contributed to a better understanding of the impact of different methods of solvent application on oil paintings and highlighted important differences between these methods; Moretti et al [22] aimed at developing an efficient methodology for in situ assessing painting surfaces after the laser cleaning using optical coherence tomography (OCT) and reflection FT-IR spectroscopy complemented with laser induced fluorescence (LIF) spectroscopic measurements; Yiming et al [23] developed a new kind of eco-friendly gel based on PHB (poly-3-hydroxybutyrate), biodiesel (BD) and dimethyl carbonate (DMC) for the cleaning of wax-based coatings applied on indoor bronzes.…”