Lambert Baij

Paintings conservators clean paintings to enhance the readability and extend the lifetime of oil paintings. Important aspect of cleaning are the removal of old or discoloured varnish layers or other unwanted conservation materials like overpaints. For varnish removal, organic solvents such as acetone, ethanol and hexanes are commonly used, whereas aqueous solution with additives are mostly used to remove surface dirt. However, scientific investigations have shown that the exposure of oil paintings to organic solvents can affect the chemical and physical properties of oil paintings in an undesired way. For example, exposure to organic solvents can lead to embrittlement of paint, the extraction and redistribution of soluble components between paint layers, and an increased rate of metal soap formation in models paint systems. The formation of metal soaps (metal complexes of long-chain saturated fatty acids) is a serious problem affecting the appearance and structural integrity of many oil paintings. Although cleaning actions are carried out with great care and attention for detail, it remains difficult to assess the effects that cleaning might have on the internal chemistry and long-term stability of a painting. To assess the effects of solvents on the molecular mechanisms of paint alteration, a thorough understanding of the chemistry of oil paint is needed.

When zooming in on a painting to the molecular scale, an oil painting is a triglyceride-based polymer where (in)organic pigment particles are suspended in a crosslinked oil polymer. The chemistry of such a polymer is complex because the starting materials consist of a mixture of natural drying oil and (in)organic pigments, which becomes exceedingly complex due to the subsequent drying, ageing and degradation of these paint materials. Even without considering the possible influence of solvent-based cleaning interventions, it is a great challenge to unravel the chemical mechanism by which such a paint polymer alters over time. The added complexity that solvent exposure may induce to the ongoing alterations in paint, renders scientific investigations into the effects of cleaning of oil paint particularly challenging. What happens when certain materials are extracted or displaced between paint layers, and how does that effect the stability of the object on longer timescales? Such questions formed the foundation for the research presented in this PhD thesis.

Given the adverse effects associated with solvent-based cleaning treatments described above, solvent exposure is ideally kept to a minimum. However, the required extent to which solvent exposure should be minimised may vary considerably for different paintings. Therefore, a strategy is required to estimate the risks associated with cleaning. For a proper risk assessment, one should express the the risks associated with cleaning in term of measurable parameters. To demonstrate the progress that was made during this research, a summary of the content of this thesis will be presented next.

After discussing the composition of relevant paint materials in CHAPTER 1, CHAPTER 2 describes the use of a tailored, non-porous model system for aged oil paint to measure paint swelling and solvent diffusion for a wide range of relevant solvents and for three cleaning gels. Using Dynamic Mechanical Analysis (DMA), the glass transition temperature of our model systems was found to be close to room temperature. Subsequently, using a custom sample cell and time-dependent Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, we were able to accurately measure swelling and diffusion processes in the polymer films. To quantify the spectroscopic data, we developed a model that describes solvent transport with a swelling-dependent diffusion coefficient. The relation between solvent properties, the diffusion coefficient, and the swelling capacity proved to be complex and could not be predicted using pure solvent properties. However, it was found that strongly swelling solvents generally diffuse faster than weakly swelling solvents, and that pigmentation does not significantly influence solvent diffusion. Additional solvent retention caused by cleaning gels was not found. These results contribute to a better understanding of transport phenomena in paintings and support the development of improved paint cleaning strategies.

CHAPTER 3 builds on the methods developed to measure solvents diffusion using time-dependent ATR-FTIR and describes the diffusion of palmitic acid and subsequent metal soap crystallisation in ionomeric binding media. The simultaneous presence of free saturated fatty acids and polymer-bound metal carboxylates leads to rapid metal soap crystallisation, following a complex mechanism that involves both acid and metal diffusion. Solvent flow, traces of water and both reactive and inert pigments are shown to enhance metal soap crystallisation. Subsequent imaging ATR-FTIR experiments on embedded cross-sections show that the distribution of crystalline zinc soaps is influenced by the presence of both inert and reactive pigment particles. Two combined effects of reactive ZnO pigments are distinguished: (1) ZnO particles can break down during HPa exposure, either by the reaction of HPa directly at the ZnO surface or by providing zinc ions to empty carboxylic acid groups in the network and (2) ZnO particles can serve as nucleation site for zinc soaps to crystallise, thereby enhancing the rate of nucleation, crystallisation, or both. These results contribute to the development of improved paint cleaning strategies, a better understanding of oil paint degradation and highlight the potential of time-dependent ATR-FTIR and imaging ATR-FTIR spectroscopy for studying complex dynamic processes in polymer films.

CHAPTER 4 describes the formation of carboxylic acid (COOH) groups in the polymerised oil medium by tracking the formation of amorphous zinc carboxylates. Although the concentration of COOH groups is crucial to understand oil paint chemistry, analytical challenges prevented COOH quantification in complex polymerised oil samples thus far. The concentration of COOH groups is important in understanding oil paint degradation because it drives the breakdown of reactive inorganic pigments to dissolve in the oil network and form metal carboxylates. The metal ions in such an ionomeric polymer network can exchange with saturated fatty acids to form crystalline metal soaps, leading to serious problems in many paintings worldwide. We developed two methods based on ATR-FTIR spectroscopy to accurately estimate the COOH concentration in artificially aged oil paint models. Using tailored model systems composed of linseed oil, ZnO and inert filler pigments, these dried oil paints were found to contain one COOH group per triacyglycerol unit. Model systems based on a mixture of long chain alcohols showed that the calculated COOH concentration can be fully explained by side chain autoxidation at low relative humidity (RH). The influence of increasing RH and ZnO concentration on COOH formation was studied and high relative humidity conditions were shown to promote the formation of COOH groups. No significant ester hydrolysis was found under the conditions studied. Our results show the potential of quantitative analysis of oil paint model systems for aiding careful (re)evaluation of conservation strategies.

In CHAPTER 5, detailed investigations into the molecular structure around zinc ions in oil paint binding media are presented. Here, we demonstrate that zinc carboxylates formed in paint films containing zinc white pigment adopt either a coordination chain or an oxo-type cluster structure using several types of infrared (IR) spectroscopy. The presence of water governs the relative concentration of these two types of zinc carboxylate coordination. Using small angle X-ray scattering (SAXS), we show that certain ion-rich domains within the model systems contain clusters with a radius of 1–1.5 nm. Although a direct link between the molecular coordination and the SAXS response could not be established, we found that the major part (65–98%) of the zinc ions did not cluster and the concentration of clusters decreased at high temperature. These results pave the way for a molecular approach to paintings conservation and show the relation between molecular (IR) and supramolecular (SAXS) information on polymer structure.

In CHAPTER 6, the extraction of a saturated fatty acid (SFA) marker and the formation of zinc soaps are monitored to measure the impact of solvent cleaning on tailored bilayer model systems for aged oil paint. Three methods of solvent application are 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 (C17:C16) 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 C17 after solvent application was visualised using static Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) on cross sections, showing that C17 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.

In CHAPTER 7, we use a newly developed portable Fourier Transform laser speckle imaging (FT-LSI) setup as a highly resolved motion detection instrument. We employ FT-LSI to probe pigment motion, with high spatiotemporal resolution, as a proxy for the destabilising effects of cleaning solvents. In this way, we can study solvent diffusion and evaporation rates and the total solvent retention time. In addition, qualitative spatial information on the spreading and homogeneity of the applied solvent is obtained. We study mobility in paint films caused by air humidity, spreading of solvents as a result of several cleaning methods and the protective capabilities of varnish. Our results show that FT-LSI is a powerful technique for the study of solvent penetration during oil paint cleaning and has a high potential for future use in the conservation studio.

In CHAPTER 8, an overview of the technical cleaning literature since the 1950s is given. We define the physicochemical processes that occur simultaneously during cleaning as solvent action and investigate how these processes vary with the polymeric structure of the oil binding medium. The results presented in Chapter 2–7 are placed in context of historical publications and possible future research. The sections are divided into solubility, swelling and diffusion, leaching, solvent evaporation and retention and solvent-mediated chemical reactions. Models that predict varnish solubility, such as the Teas diagram, are discussed and placed in the context of recent developments. Technological developments in the field of modern materials for solvent- and water-based cleaning are also discussed. Finally, an outlook for the field of cleaning science is presented.