Maney Publishing Long-Term Chemical and Physical Processes in Oil Paint Films Author(s): David Erhardt, Charles S. Tumosa and Marion F. Mecklenburg Source: Studies in Conservation, Vol. 50, No. 2 (2005), pp. 143-150 Published by: Maney Publishing on behalf of the International Institute for Conservation of Historic and Artistic Works Stable URL: http://www.jstor.org/stable/25487732 . Accessed: 05/12/2014 10:58 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Maney Publishing and International Institute for Conservation of Historic and Artistic Works are collaborating with JSTOR to digitize, preserve and extend access to Studies in Conservation. http://www.jstor.org This content downloaded from 160.111.254.17 on Fri, 5 Dec 2014 10:58:14 AM All use subject to JSTOR Terms and Conditions 143 Long-Term Chemical and Physical Processes inOil Paint Films David Erhardt, Charles S.Tumosa andMarion F.Mecklenburg Oil paints dryby polymerization. This 'drying'process may be substantiallycomplete and thesurface of thepaint film dry to the touch within but measurable continue slower weeks, changes for years. Other, processes also continue, primarily hydrolysis of esters. as case glyceride This produces carboxylic acid groups eitherfree fatty acids (in the of acid groups that have not reacted or acid bound to the oil matrix case otherwise) groups crosslinked (in the of acid groups that have engaged in polymerization These react with to in the case a reactions). may pigments form carboxylate salts (called soaps of fatty acid). These changes affect the the and the that conservation treatments it. physical properties of paint way affect This paper examines the extent of and in some and hydrolysis soap formation naturally aged drying oil paint films, the extractability of these materials in organic and measured and over time in solvents, predicted changes thephysical properties of naturally aged paint films. Long-term and mechanical due to are minor to or physical changes aging compared those produced by overcleaning excessive exposure to heat. or INTRODUCTION fatty diacids formed by scission reactions of the unsaturated acids. fatty The acid groups produced by Plant oils are of esters composed primarily triglyc?rides, react hydrolysis may with metal ions from pigments to of glycerol and fatty acids. In drying oils, most of the produce carboxylate salts (referred to as soaps in the case acids from which the are derived are fatty triglyc?rides of a Free are fatty acid). glycerol molecules formed only oils, and oil polyunsaturated. Drying paints compounded after all three are so glyceride bonds hydrolyzed, that from them, dry by a process of autoxidation and subse little free glycerol will be present unless extensive quent polymerization of the unsaturated fatty acid hydrolysis has occurred. The effects of cannot in hydrolysis groups the triglyc?rides. This process is quite complex be understood unless the influence of other processes is the (see, e.g., review by Wexler [1]). An oil paint film also known. In this study the bulk mechanical that is to the touch properties dry within weeks undergoes further of several paint films are followed and the extent of the reactions for decades longer. Chemically this includes hydrolysis and oxidation reactions examined. further crosslinking reactions, oxidation of unsaturated acids and hydrolysisof glyceride bonds. Hydrolysis is a a of oils significant chemical reaction in paint film even in the Hydrolysis first few years [2]. Hydrolysis may yield saturated fatty The reaction that produces the ester bonds in oils can be acids (which lack the functional that react groups during reversed. This process, called hydrolysis, yields the the unsaturated acids that crosslinking process), fatty and component fatty acids glycerin. Partial hydrolysis, of have not yet become part of the crosslinked oil matrix one or two ester bonds, di- and or yields monoglycerides, otherwise reacted, acid groups attached to the matrix respectively. If any of the fatty acids have reacted to bonds formed and short-chain by during polymerization, become of the part polymer matrix, hydrolysis yields an acid attached to group the polymeric matrix by the ReceivedMarch 2003 bond formed during polymerization. That hydrolysis of STUDIES IN CONSERVATION 50 (2005) PAGES 143-150 This content downloaded from 160.111.254.17 on Fri, 5 Dec 2014 10:58:14 AM All use subject to JSTOR Terms and Conditions 144 D. ERHARDT, C. S. TUMOSA AND M. F. MECKLENBURG occurs a extent was more in some glycerides has been well known for long time. of hydrolysis than 90% older a The hydrolysisof oils (and fats)with alkali to produce paint samples. However, they also made number of was one statements other of soaps (the fatty acid salts of the alkali) probably unsubstantiated regarding aspects of the first chemical reactions discovered. The pioneer aging, especially regarding the physical and mechanical ing chemistCarl Wilhelm Scheele firstisolated glycerin aspects of aging. It is unlikely that the evaporation of a in the late 1700s (he also discovered oxygen, nitrogen, glycerol, which has boiling point of 290?C (with a at normal chlorine, and other elements and compounds). Scheele decomposition), would be significant process a matrix such showed that glycerin and fatty acids could be produced temperatures, especially from within solid as by the action of lead oxide on fixed (vegetable) oils in the polymerized oil. In fact, their results show that was more a are the presence of water [3] (it not until than significant amounts of glycerol present in quite old was to the hundred years later that the term hydrolysis used paints. Work by the present authors showed that describe the reaction). It was realized very early in the relative amounts of glycerol increased with increasing was due to the fact that all three last century that hydrolysis of oil responsible for hydrolysis, primarily gly some ester must to free many of the changes (some good, bad) in the pro cerol bonds undergo hydrolysis yield time Boon et al. also inferred from their data that perties of paints stored for long periods of (years), glycerol [8]. water are in an ionomeric form without that the presence of promoted hydrolysis and soap the paints mostly the relative amounts of free acid formation [4], and that completely ionomeric paints presenting data showing from of versus cite studies could be prepared directly soaps hydrolyzed carboxylate present. They showing the that old have little extractable material as drying oils [5]. O'Neill and Brett, in examining paintings may at their statement that the free acids are reactions taking place in paint films, looked the supporting present as was amount of material that could be extracted from oil paint soaps; however, the study cited of previously amount ion in the cleaned solvent that had films after aging, and the of metal (i.e., treated) paintings likely little extractable material left.Work the extracted material. They concluded that paint films easily by present more that if not of the free reacted fastest with basic oxide pigments, slowly authors shows many, most, fatty even more if at molecules are as the acids rather than as with carbonate pigments, and slowly, all, acid present that the Boon et al. also mention that con with inert oxide pigments. They also concluded soaps [8]. repeatedly to the ionomeric form is for the formation of metal soaps contributes to the particular version responsible film hard and this properties of white lead and zinc white oil paints [2]. paint becoming brittle, although association has not been demonstrated in their or any the other study. To the contrary, previous work by pre Hydrolysis of dried oil paint films sent authors, summarized in this article, shows that the some free and stiffness of films in a that While hydrolysis and soap formation with strength paint change way oils occurs can be modeled and over fatty acids present in drying very quickly, mathematically, changes long films occurs over a time of can be The rate of extensive hydrolysis of paint periods aging predicted. changes concern Most oil in these slows as the span longer than is of industrially. paint properties considerably paints age, on occur in before and a film hundreds of old should not be research focused changes that paints paint years or the after much stiffer or more brittle than a film application during drying process applica paint only oil was of con decades old. If a is to become brittle, as for tion. Hydrolysis in unpolymerized great paint going instance zinc oxide it does so cern because it affects the mixing, storage and working pigmented paints do, quite on the behaviour of It is that films do properties of the paint. Research early. easily shown, though, paint focused on the oxidative become brittle after to paint after application, though, prolonged exposure high temp since and forma eratures or solvents The authors have in polymerization process, hydrolysis soap [9, 10]. present a number of tion in the dried oil were much slower. Consequently, fact prepared completely hydrolyzed paint Boon and co films from mixtures of and free acids few reports examined the process until pigments fatty These workers reintroduced the concept and showed that (derived from the hydrolysisof dryingoils). films, extensive over the museum which are now several old, are coherent and some hydrolysis could be quite years a method that are as flexible as from the time-scale [6, 7].