| Abstract: | SUMMARIES. Oil paints dry by polymerisation. This 'drying' process may be substantially complete and the surface of the paint film dry to the touch within weeks, but measurable changes continue for years. Other, slower processes also continue, primarily hydrolysis of glyceride esters. This produces carboxylic acid groups as either free fatty acids (in the case of acid groups that have not reacted otherwise) or acid groups bound to the crosslinked oil matrix (in the case of acid groups that have engaged in polymerization reactions). These may react with pigments to form carboxylate salts (called soaps in the case of a fatty acid). These changes affect the physical properties of the paint and the way that conservation treatments affect it. This paper examines the extent of hydrolysis and soap formation in some naturally aged drying oil and paint films, the extractability of these materials in organic solvents, and measured and predicted changes over time in the physical properties of naturally aged paint films. Long-term physical and mechanical changes due to aging are minor compared to those produced by overcleaning or excessive exposure to heat.
CONCLUSIONS. The existence of long-term processes in oil paints is readily demonstrated by some simple experiments. Plots of weight over time show that the process of weight gain is quite slow after two years, but if the data were extrapolated the theoretical maximum weight gam (derived from the weight gain of pvire linseed oil and correcting for the weight ot pigment present) would not be reached tor hundreds of years, if at all. Hydrolysis ot the glycende ester bonds is a slow process relative to the initial polymerization reactions, and even samples hundreds of years old may have substantial portions of the glyceride ester bonds intact. Plotting data for the changes in mechanical behaviour also shows that the major changes occur within years or decades, but also predicts that measurable changes occur for well over 50 years and approach a limiting value after about 150 years. The major processes in oil paints can be grouped into polymerization, hydrolysis, oxidation, and soap form¬ation (it a pigment capable of reacting to form soaps is present), along with accompanying changes in the mechanical properties. Within the first few years the amounts of extractable materials decline as the smaller molecules continue to crosslink. In addition, the amount of soluble (tree) tatty acids increases due to hydrolysis of the triglycerides. A portion of this may react with metal ions present in many pigments to form insoluble metal soaps. Hydrolysis reduces the number of crosslinks between the glycerides. However, other crosslinking reactions must occur since the total amount of extract-able material decreases over time. Since the free fatty acids tend to keep the paint film flexible, any stiffening of the paint film must come from further non-ester crosslinking and/or possibly from soap formation as well. One possibility is that crosslinks occur through multivalent metal ions. Azelaic acid groups still bound to the glyceride by the original glycende ester bond can torm salts through the free acid group formed by oxidation. A multivalent metal that formed a salt with more than one such acid group could function as a crosslink, and possibly increase the stiffness of the paint. However, in a study of synthetic polymers containing acrylic acid groups, it was found that multivalent metal ions had no more effect on physical properties than the equivalent amount of monovalent ions, and that the change in physical properties was solely a function of percent conversion to ionomer, and not of the type or valency of the metal ion [17]. Whatever the case, these processes do not have a major effect on the mechanical properties, since tests of untreated paint films kept under moderate environmental conditions show that changes in the mechanical properties have already slowed significantly within the first few decades. The implications for conservation follow, in part, from previously reported work on calculating ranges of relative humidity appropnate for the museum environ¬ment [18. 19]. If paints retain a reasonable elastic region (the amount they can be deformed without permanent change) throughout their lifetime, then the allowable RH fluctuations calculated using data from the available paint samples up to 20 years old will apply to paint throughout its normal aging process. The amount of dimensional change that will occuvir in unrestrained glue or gesso (the materials in canvas paintings most respons¬ive to RH changes) or tangential wood (the material in panel paintings most responsive to RH changes) in a range of ±15% RH about a median of 50% RH is about 0.004, or 0.4%. It can be seen from Figure 5 that even 250-year-old paint is predicted to require about four times this much distortion before it breaks, while the elastic limit is similar to newer paints (about 0.4%). and would not be exceeded by such RH fluctuations even assuming worst case conditions (attachment of the relatively RH-unresponsive paint to a highly responsive substrate that is exposed to the RH extremes long enough to respond fully). It was shown previously that oil paint films retain their elastic properties even after extreme solvent treatment (24-hour immersion). While the loss of part or all of the plastic region upon aging, heating or solvent treatment does have implications for handling and transport (a paint film would be more likely to break rather than permanently distort if mistreated), it does not affect the choice of appropriate environmental conditions. |
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