Solubility picture varnish

Methyl and ethyl methacrylate polymers, although extensively used in Industry, do not possess the solubility characteristics (low polarity) that would make them appropriate for use over traditional oil paintings and other organic-based museum objects that might be sensitive to polar solvents such as alcohols, ketones and esters. Poly(n-butyl methacrylate), offered as an artists varnish in the late 1930 s, did not become widely accepted in the war-disrupted decade that followed. Accordingly, early in 1951, our laboratory began a detailed study of the higher alkyl methacrylate polymers for potential use as picture varnishes (1). 

The first synthetic polymers to be used as paint varnishes were acrylic and vinylic resins. Poly(vinyl acetate) (PVAc), commercialized under the name Mowilith by Hoechst and Vinylite by Union Carbide, has been used in conservation as an adhesive since 1932 and in 1937 it was proposed as a picture varnish by Stout and Cross [63]. PVAc was soon rejected as a varnish because, despite its light stability and good solubility in organic solvents, it demonstrated poor optical properties in terms of colour saturation and the tendency to pick up dirt due to its low glass transition temperature. 

Poly(butyl methacrylate) (PBMA) began to be used as a picture varnish in the early 1930s. It encountered a considerable success because of its resistance to yellowing, adequate flexibility, no dirt pick-up and good solubility in nonpolar hydrocarbon solvents. Products based on PBMA, such as Elvacite 2044 and Elvacite 2045 by Du Pont, were abandoned when it was discovered that under light exposure they cross-link to an unexpected extent becoming insoluble [64]. From this point of view acrylic copolymers based on methyl and ethyl acrylates/methacrylates show a much better long-term stability. 

Most soluble resins that are being used as consolidants in conservation today are also used in picture varnish. Thus these resins have a history of satisfactory use that, in the case of poly(vinyl acetate), extends over almost 60 years (6). Consolidants can cause some darkening of wood surfaces much in the way wetting them with water deepens their color, but with proper technique they will not leave a shiny surface film (7). Because excessive... 

FIGURE 2.19 Teas plot of solvent mixtures of cycldiexane (1), toluene (5) and acetone (11) for assessing the solubility of picture varnishes (see Table 2.5 and Section 3.2). Sources Feller (1976, 1978). 

Smith, G. D., Johnson, R. (2008). Strip Teas - solubility data for the removal (and application) of low molecular weight synthetic resins used as inpainting media and picture varnishes. Western Association for Art Conservation Newsletter, 50(1), 11—19. 

Paraloid B-67, a PiBMA (Chiantore and Lazzari, 1996), is used as a picture varnish (Figure A3.14). Although it cross-links (Feller, 1976), especially under severe UV exposure (Chiantore and Lazzari, 2001), it appears to be peculiarly stable and of Class A standard (Feller, 1983). However, this stability appears to derive from an unknown composition, which creates an extended induction period before cross-linking starts. It is soluble in less polar solvents than B-72. When applied to marble, 15% rapidly becomes insoluble, which increases on accelerated ageing to 70% (Favaro et al., 2006). 

Cellulose acetate in an acetone-soluble form became widely available during World War 1 as a coating for aircraft fabric, dope . CA displaced cellulose nitrate, which was too inflammable. CA was first used, apparently without a plasticizer, in a solution in acetone for fixing pigments on paper and stone (Scott, 1923) and as a consolidant for cloth and other materials (Plenderleith, 1934). It was rejected in 1937 as a possible picture varnish (Stout and Cross, 1937). CA has been largely replaced for conservation uses by modem materials. 

---