Varnishes and Surface Coatings: The History of Synthetic Resin Varnishes

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Author: Bradford Epley
Date: Submitted July, 1996
Compiler: Wendy Samet

General Introduction

The desirable qualities of surface films for paintings were set forth in 1930 by a committee of the Conference on the Examination and Conservation of Works of Art, held under the auspices of the International Museums Office, Rome. The identified qualities of the films were:

• providing protection of the painting from atmospheric impurities

• maintaining elasticity and cohesiveness during normal environmental changes

• retaining the elasticity of the paint film

• transparency and colorlessness

• applicability in a thin coating

• resistance to “blooming”

• ease of removability

• not being “brilliant”

(Stout and Cross 1937, 241). Acknowledging the failings of natural resins, as well as the lack of viable tested alternatives, a group convened by W.G. Constable in 1933 concluded that adequate protective and aesthetic properties could only be achieved through a two-part system consisting of a thin natural resin varnish covered with a layer of wax (Ruhemann 1968, Stout and Cross 1937). The main impetus of varnish research soon became the search for a single film material that could fulfill all the desirable protective and aesthetic qualities. The 1957 Seminar on Resinous Surface Coatings in Oberlin, Ohio, and the subsequent 1959 publication of On Picture Varnishes and Their Solvents was the culmination of much of this research. Early in the exploration of synthetic resins, however, it became clear that they, too, had their limitations. Although they could form a more durable type of coating, questions arose concerning their long-term removability and limited saturation of colors, resulting in an apparent compromise between desired protective and visual functions. As a better understanding of the deficiencies of both natural and synthetic resins progressed alongside the burgeoning synthetic coatings industry, multicomponent systems appeared to offer a solution. Based on combinations of layering and the addition of stabilizers and polymeric additives, both natural and synthetic resins appeared to allow for a reasonable balance between a varnish coating's limited protective capacities, its long-term stability and removability, and the desirable visual effects that it can produce.

The Development of Criteria for the Testing of Synthetic Resins for Varnishes

In one of the earliest attempts to evaluate surface coatings for paintings (Stout and Cross 1937), various coatings were studied and their comparative merits evaluated in such categories as transparency, permeability, cohesiveness, flexibility, and solubility. In 1950 the National Gallery of Art, Washington, D.C., appointed Dr. Robert L. Feller to the newly established Fellowship on Artists' Materials at the Mellon Institute, where he began an in-depth study of varnish properties and technology and published the results of the first extensive testing on the physical properties of both the available synthetics and the natural resins damar and mastic (Feller, Stolow, and Jones 1985). His results permitted comparison of the various resins with regard to three basic properties: solubility, viscosity grade, and hardness. Feller developed the concepts of “solubility grade,” the relative tendency of a resin to dissolve in hydrocarbon solvents, and “viscosity grade,” the viscosity in centipoises of a 20 weight percent solution of a resin in toluene at 70° F (Feller, Stolow, and Jones 1985). Though the chosen concentrations have no theoretical basis, they provided a simple way to evaluate the relative solubility and viscosity of resins in a commonly used varnish solvent–toluene. In addition to data on both Sward and pencil hardness, measurements of film flexibility were systematized by bending films cast on aluminum foil around mandrels of various diameters (Feller, Stolow, and Jones 1985). Gel permeation chromatography of various traditional and modern varnish materials, performed several years later at the Metropolitan Museum of Art, served both to confirm previously reported molecular weights for some and to establish them for others (de la Rie 1987a, 5).

Research into polymer crosslinking furthered the development of standardized techniques for determining resin properties. As early as 1956, Garry Thomson of the Scientific Department, National Gallery, London, presented his development of a simple nonquantitative “paper leaching” technique for detecting the presence of crosslinked materials in both naturally and artificially light aged samples (Thomson 1956). Concurrently, other researchers, including Feller at the Mellon Institute, were exploring both thermal and light-induced crosslinking, though each lab used different experimental conditions. Using the results of Venkataraman on the fading of dyes and the Harrison Report of the Metropolitan Museum of Art, Feller attempted to correlate the results from light aging into equivalent hours of gallery exposure (Feller, Stolow, and Jones 1985, 196). He further attempted to relate accelerated aging data with ordinary conditions by developing a paper chromatographic technique to determine the increase in average molecular weight of resins during natural aging on test panels (Feller, Stolow, and Jones 1985, 197–200). The end result of correlating artificial and natural aging data, presented in 1975 and 1978, was the definition of a series of stability classes for evaluating the photochemical stability of resins and polymers (classification/useful lifetime): Class T/less than 6 months; Class C/less than 20 years; Class B/between 20 and 100 years; Class A/over 100 years use (Feller 1975; 1978). The basis of the standards proposed was the degree of fading under visible and near ultraviolet radiation of the International Standards Organization's R105 blue wool scale (Feller 1975). In further attempts to standardize the testing of the solubility of aged varnish films, Feller and Bailie presented a series of solvent mixtures based on acetone, toluene, and cyclohexanone, (Feller and Bailie 1972; Feller 1976b) which have subsequently been utilized by de la Rie, De Witte, Lafontaine, and others. While toluene has been utilized almost exclusively as the solvent for varnishes under investigation (de la Rie and McGlinchey 1990a, 160; de la Rie 1993, 567; Lafontaine 1979a, 15; De Witte et al. 1981), recent research, which requires more study, indicates that variations of the solvent composition of a resin can affect the long-term properties of resin varnishes (Lawrence, 1990; Hansen et al., 1991). With the passage of several decades since the first use of synthetic coatings, additional projects have been initiated to evaluate gallery-aged varnishes of known application dates with respect to solubility and optical properties (Ramsay-Jolicoeur 1987).

Poly(vinyl acetate)

Chemical Synthesis and Manufacture

The polymerization of vinyl compounds may date to as early as 1835 (Mills and White 1987, 112). In 1912, poly(vinyl acetate) was reported by Klatte and Rollett (Powell 1972). Industrial production was begun in Germany and Canada in 1917 (Mills and White 1987, 112) and was well under way in the United States by the 1930s. It is prepared by free-radical polymerization, either in bulk or solution (Horie, 1987, 92), and is facilitated by radical initiators (e.g., benzoyl peroxide), ultraviolet light, and catalysts (formerly a mercury compound and more recently compounds containing palladium and copper salts) (Powell 1972; Gettens 1935).

Conservation History of Poly(vinyl acetate)

Poly(vinyl acetate) was one of the earliest of the synthetic resins to be mentioned as a potential picture varnish in the conservation literature (Gettens 1935; Stout and Cross 1936). In the 1935 article, Rutherford Gettens stated his belief that the most promising synthetics appeared to be polymerized vinyl acetate and other vinyl esters. In 1956, Garry Thomson indicated that poly(vinyl acetate) did not undergo thermally-induced crosslinking (Thomson 1956). The following year, Thomson published results for crosslinking induced by light exposure (Thomson 1957). After exposure to lighting conditions in northern Nigeria which caused natural resins to disintegrate, the poly(vinyl acetate) samples remained in good condition and retained their original solubility in organic solvents (Thomson 1957, 67). Since that time, poly(vinyl acetate) has been shown to be one of the most stable synthetic resins, resisting crosslinking under both natural and artificial light aging (Feller 1963, 174). Additionally, this resin does not appreciably crosslink or degrade in air, though it does suffer slight oxidation and other minor changes (David et al. 1970, 962). Formulas for either spray or brush application of the resin solutions have appeared in numerous conservation manuals (Ruhemann 1968, 274–9; Plenderleith 1962, 183; Bradley 1950) and appear in Section V.D., Polymeric Varnishes: Poly(vinyl acetate) and Section IX., General Application Techniques. Early in the development of poly(vinyl acetate) picture varnishes, there was discussion concerning their optical properties. Especially notable was their lower saturation of paint colors when compared to the natural resins (Gettens 1935). Both Thomson and Feller expressed the belief that the lesser degree of saturation was due primarily to both the low concentrations of varnish solution allowable and the very high viscosity it develops during drying, rather than owing to its low refractive index or an adhesion problem (Feller 1957a, 1143; Thomson 1957, 74). As the poly(vinyl acetate) solution dries, the drying resin film conforms to the irregularities of the painting beneath, resulting in a more matte surface. In 1935 poly(vinyl acetate) was shown to have a relatively high permeability to water and water-soluble gases (Gettens 1935), which was later confirmed by both Thomson and Feller (Thomson 1963, 178; Feller, Stolow, and Jones 1985, 151). Furthermore, it was suggested in the 1930s that poly(vinyl acetate) films were transparent to ultraviolet light (Gettens, 1935). Absorption of longer wavelength ultraviolet light by aged damar, as well as the transparency of poly(vinyl acetate) films to these same wavelengths, was later verified (Feller, Stolow, and Jones 1985, 148–50).

During its production, the polymerization process can be stopped at any stage, resulting in poly(vinyl acetate) resins of different molecular weights whose solutions will have different viscosities under identical conditions. Due to the increasing importance of poly(vinyl acetate), in 1958 the National Gallery of Art Research Project prepared a list of the different molecular weight grades then available in various countries (Feller 1959, 56; Feller, Stolow, and Jones 1985, 227). One of the earliest forms of poly(vinyl acetate) available, and on which most of the initial tests were performed, was Union Carbide Vinylite® A. Though since discontinued, Vinylite® A was of similar molecular weight to the later Union Carbide Bakelite® Series AYAF. The expansion of the Bakelite Series to include the lower molecular weight AYAB, and later still AYAC, allowed for applications of less viscous solutions at higher concentrations. However, it was also suggested that, due to low second-order glass transition temperatures, AYAC and AYAB should be covered with a final varnish possessing a second-order glass transition temperature well above room temperature to reduce the tendency of dirt accumulation (Feller 1984). There is evidence that Joseph Albers and his assistants applied both prepared and proprietary mixtures of poly(vinyl acetate) as final varnishes throughout the 1950s and 1960s (Garland 1983). Additionally, the use of poly(vinyl acetate) as a final varnish is mentioned at the IIC Lisbon Conference in 1972, though its preferred role as an inpainting medium is implied (Hulmer 1972). An isolating varnish utilized by Gustav Berger and sold by Conservator's Products Company is reported to contain poly(vinyl acetate) resins “harder than Hoechst Mowilith 20,” dissolved in a proprietary mixture of solvents (Berger 1990a, 8; 1990b). However, due to its low glass (second order) transition temperature, this retouch varnish must be covered with a different varnish of a higher glass transition temperature (Berger 1990b).

Acrylic and Methacrylic Polymers

Chemical Synthesis and Manufacture

Though the first acrylic and methacrylic solution resins for coatings were introduced in 1936, acrylic polymers have been commercially available in the United States since 1931 (Allyn 1971). Redtenbacker's preparation of acrylic acid via the oxidation of acrolein was described in the literature as early as 1850 (Allyn 1971). In his doctoral thesis in 1901, Otto Rohm of Darmstadt both described the liquefied condensation products obtained from methyl and ethyl methacrylate and produced solid acrylic polymers. While Dr. Rohm commercially synthesized acrylic esters in 1927 and commercial production of methacrylate monomer began in 1936, it was not until World War II that poly(methyl methacrylate) was produced in large volume as cast sheets for aircraft enclosures (Allyn 1971). The majority of acrylic polymers used as varnishes are methacrylates, derived from methacrylic acid, acrylates derived from acrylic acid, or copolymers of each group (Horie 1987, 103). The alkyl group can be chosen to produce a range of polymers (Horie 1987). In the 1940s and 1950s, the use of acrylic resin as an artist's material became widespread in an aerosol varnish utilizing methylene chloride and related solvents (Feller 1963, 172).

Conservation History of Methacrylates

Iso-Amyl Methacrylate

In 1953 an experimental poly(isoamyl methacrylate) varnish, known as 27H, was developed by Robert L. Feller at the Mellon Institute with selected film strength, high resistance to yellowing, and solubility in a petroleum solvent containing no aromatic compounds (Feller 1963, 172). In the years immediately following, samples of the resin 27H were supplied by Feller to several laboratories, including the Intermuseum Conservation Association of Oberlin, Ohio, and the National Gallery, London, for further observation and use in treatments (Feller, Stolow, and Jones 1985, xii). During experiments conducted in 1955 under the National Gallery of Art Research Project investigating the thermal depolymerization of poly(isoamyl methacrylate), Stuart Raynolds found that after heating the polymer became insoluble in hot benzene and toluene (Feller, Stolow, and Jones 1985, 195). Furthermore, a test panel coated with the isoamyl methacrylate resin, which had undergone exposure in a fadometer was found to be highly resistant to removal by solvents (Feller, Stolow, and Jones 1985, 195). This information, along with other published contemporary research, prompted Feller to explore further the effects of ultraviolet radiation and heat on a group of methacrylate polymers. Studies based on the ease of removal of artificially aged samples showed a large difference between the crosslinking tendencies of the various methacrylates (Feller, Stolow, and Jones 1985, 154–64). Poly(isoamyl methacrylate) was shown to be one of the resins most susceptible to crosslinking upon exposure in a fadometer (Feller 1957b). Additional tests were performed on naturally aged samples from the Fogg Art Museum utilizing a semiquantitative paper chromatographic technique developed to indicate any decrease in solubility due to the crosslinking of the samples (Feller, Stolow, and Jones 1985, 197–200). These tests indicated that crosslinking of the isoamyl methacrylate could occur under normal gallery exposure conditions, though the resin may remain removable in “mild” solvents for at least 28 years (Feller, Stolow, and Jones 1985, 200). The color stability of poly(isoamyl methacrylate) was also tested under extreme conditions of sunlight exposure and found to discolor insignificantly (Feller 1976a, 140). Further studies by the Research Project found that it was possible to inhibit the crosslinking tendencies of isoamyl methacrylate by inclusion of substituted benzophenones, but further research was discontinued in favor of the search for a more inherently stable resin (Feller, Stolow, and Jones 1985, 159–60).

During this same time period, Garry Thomson was examining synthetic and natural resin samples which had been exposed for six and one-half months at an outdoor exposure station in northern Nigeria. Thomson's observations on the crosslinking of isoamyl methacrylates were based on his “paper leaching” technique (Thomson 1956; 1957). Based on his findings of varying degrees of crosslinked material in the light aged samples of the isoamyl methacrylate, the use of methacrylate and other long-chain linear polymers was suspended at the National Gallery, London, pending further investigation (Thomson 1957, 67). Plenderleith echoed this conclusion in his manual on conservation (Plenderleith 1962, 184) as did Ruhemann (Ruhemann 1968, 274–9).

Butyl Methacrylates

During the early 1930s isobutyl (then Lucite® 45, now Elvacite® 2045) and n-butyl methacrylate (then Lucite® 44, now Elvacite® 2044) polymers started to be used as picture varnishes (Feller, Stolow, and Jones 1985, 125). Though use of these methacrylate varnishes increased, it was not until about 20 years' experience was gained before this use was published (Rawlins and Werner 1949; Werner 1952, 364). As with the other methacrylate polymers, the butyl methacrylates held promise as picture varnishes through their availability in modest molecular weights, resistance to yellowing, solubility in hydrocarbon solvents, flexibility, and, at least for isobutyl methacrylate, a significantly high glass transition temperature that would prevent dirt pick-up (Horie 1987, 106). However, in 1953 Drinberg and Yakovlev were the first to point out the thermal crosslinking of poly(butyl methacrylate) (Drinberg and Yakovlev 1953). In 1957 Thomson inferred the thermal crosslinking of poly(n-butyl methacrylate) based on a comparison with the thermal crosslinking of poly(isoamyl methacrylate) (Thomson 1957, 67). The formation of crosslinks of the n-butyl methacrylate under strong fluorescent light exposure was also responsible, along with poly(isoamyl methacrylate), for the cessation of methacrylate use at the National Gallery, London (Thomson 1957, 647).

By 1957, the National Gallery of Art (Washington, D.C.) Research Project had conducted several tests on the relative removability of artificially aged methacrylate samples. The results, presented at the Oberlin Conference in April 1957, indicated that poly(isobutyl methacrylate) and poly (n-butyl methacrylate) were among the methacrylate polymers most likely to become crosslinked (Feller 1957b). Following the artificially aged samples, Feller then obtained several samples of methacrylate coatings which had been aged for 10 years on the Fogg Art Museum's laboratory wall. Examination of these samples showed that poly(n-butyl methacrylate) would most likely not develop resistance to removal in methyl cyclohexanone until it received more than 28 years of “normal gallery exposure” (Feller, Stolow, and Jones 1985, 158). Subsequently, Feller stated he believed that, through monitoring the progression of insolubility along with the possibility of inhibiting the crosslinking by the addition of substituted benzophenones, there did not appear to be a need for immediate alarm, though further research was clearly required (Feller 1963, 173–4; Feller, Stolow, and Jones 1985, 158). In 1976 Feller reported that poly(n-butyl methacrylate) had been largely rejected as a final surface coating due to its low glass transition temperature and subsequent tendency to imbibe dirt (Feller 1976a, 138).

Additionally, as lower viscosity grades of the resins with better leveling power–such as Paraloid® B-67–became available, the older and higher viscosity grades, such as the Lucite® 44 and Lucite® 45, gradually fell out of favor as final varnishes (Feller 1976a). Binney & Smith Soluvar® Varnish at one time was based on a mixture of 1:1 Paraloid® B-67, isobutyl, and F-10, n-butyl methacrylate, 30% solids in a slow drying petroleum distillate (Feller 1984). In the mid-1980s, Golden Artists' Colors introduced Golden MSA® Varnishes, a proprietary mixture of isobutyl and n-butyl methacrylate in a mineral spirits/Stoddard solvent-type mixture (personal communication, Ben Gavitt of Golden Artists' Colors, 1996).

The Rohm and Haas proprietary resin Paraloid® B-67 was of interest to Feller, namely because its aging tests indicated that it was not simply pure poly(isobutyl methacrylate). While its infrared spectra, gas chromatographic analysis, and solubility were all consistent with poly(isobutyl methacrylate), it was uncharacteristically resistant to crosslinking (Feller 1976a, 144). Feller postulated that its stability derived from both its low molecular weight and the presence of an inhibitor, the latter of which was indicated by crosslinking tests utilizing turpentine as the solvent (Feller 1976a, 142–3). However the presence of an inhibitor was never officially acknowledged by Rohm and Haas, nor detected by infrared as its concentration was most likely less than 1% (Feller, personal communication, 1996). The IIC International Congress of 1972 offered a workshop to explore and further evaluate many of the synthetic varnishes used by conservators. Paraloid® B-67 varnishes were demonstrated, both as single component varnishes as well as within multicomponent layered systems (Hulmer 1976).

The evidence of crosslinking in the poly(isoamyl methacrylate) and the poly(butyl methacrylate)s prompted further research by Feller for structures in the methacrylate family that have little tendency to crosslink. In 1957, based on the artificial aging of several methacrylate samples, Feller suggested that other types of polymers would be found that did not have particularly sensitive sites on the polymer chain for the development of crosslinks (Feller, Stolow, and Jones 1985, 158). Poly(n-propyl methacrylate) was one of those tested and found to resist a change in solubility upon artificial light aging (Feller, Stolow, and Jones 1985, 159). An additional offshoot in the search for polymers lacking crosslinking tendencies was the synthesis by Stuart Raynolds of polymers of tetrafluoropropyl and octafluoroamyl methacrylate (Feller 1976a, 144). Though these polymers showed high resistance to crosslinking, withstanding 1,000 hours on aluminum foil in a carbon-arc fadometer, research was discontinued due to the special solvents required as well as problems associated with wetting and adhesion (Feller 1976a, 144).

Conservation History of Acrylics

Copolymers of certain acrylics and methacrylics were also among those tested for stability and found to resist a change in solubility upon artificial light aging (Feller, Stolow, and Jones 1985, 159). In 1963, Feller reported that Rohm and Haas' Paraloid® B-82 was highly resistant to crosslinking (Feller 1963, 175). Although the polymer required a more polar solvent for removal after aging, the change in solubility was attributed to partial oxidation rather than crosslinking, as a high percentage of soluble material remained (Feller 1963, 174). Commercially available polymers similar to Rohm and Haas Paraloid® B-82, such as Rohm and Haas Paraloid® B-72, were suggested by Feller for certain treatments which allowed for its application in xylene with diethyl benzene or all-aromatic petroleum fractions added as needed to retard evaporation (Feller 1963, 175). Slightly cloudy solutions were observed, though yielding a “satisfactory appearance” (Feller 1963, 175). In 1972, Feller stated that Paraloid® B-72 was “the most stable thermoplastic resin soluble in hydrocarbon solvents” as it did not crosslink after more than 1,000 hours on aluminum foil in a carbon arc fadometer and that it also exhibited no weight loss upon exposure to near-ultraviolet radiation (Feller 1976a, 144–5). The composition of Paraloid® B-72, however, was not known until 1978. In 1978, De Witte noted a difference between both the physical appearance and solubility of newly produced samples of Paraloid® B-72 manufactured in 1976 and 1978. He performed extensive testing, comparing refractive indices, viscosity grade, solubility, and infrared spectra, and determined that “old” B-72 was a 68/32 ethyl methacrylate/methyl acrylate copolymer and the “new” B-72 was a 70/30 ethyl methacrylate/methyl acrylate copolymer (De Witte et al. 1978). Prior to this time Rohm and Haas literature, as well as the unconfirmed results of a destructive distillation, led Feller to believe that Paraloid® B-72 was a roughly 50/50 mixture of methyl methacrylate and ethyl acrylate (Feller, personal communication, 1996). Preparation and testing of methyl methacrylate/ethyl acrylate copolymers at the Mellon Institute yielded stability results identical to that of Paraloid® B-72. Though identified as being both a methyl methacrylate/ethyl acrylate copolymer and an ethyl methacrylate/methyl acrylate copolymer in several of Feller's early publications, any identification of Paraloid® B-72 as an ethyl methacrylate/methyl acrylate copolymer in these articles prior to 1978 was purely coincidental (Feller, personal communication, 1996). Paraloid® B-72 varnishes were demonstrated during the 1972 IIC International Congress workshop, both as single component varnishes as well as within multicomponent layered systems (Hulmer 1976).

In 1981, De Witte and others attempted to synthesize a varnish combining the chemical stability of the acrylics with the color rendering of a low molecular weight varnish, such as damar. The optimum optical properties of the varnish were determined by a jury of restorers. The resulting phenyl acrylate/methyl methacrylate copolymer (30/70), in addition to a low viscosity grade and refractive index of 1.547, had aging characteristics that placed it within Feller's designated Class A materials, though further study is still necessary (De Witte et al. 1981).

(Editor's Note: In summer 1997, the North American product Acryloid® was renamed Paraloid®. The name has thus been changed throughout this volume, except where it occurs in titles and quotes.)

Ketone Resin Varnishes

Chemical Synthesis and Manufacture

Patents concerning the synthesis of ketone resins started appearing in 1930 (de la Rie and Shedrinsky 1989, 10). Synthetic low molecular weight resins based on the condensation reaction of a cyclic ketone and formaldehyde have been used as coatings for the conservation of works of art since the 1940s (Crombie 1994) or 1950s (de la Rie and Shedrinsky 1989, 11). The synthesis of ketone resins consists of a poly-condensation reaction of methyl cyclohexanone in the presence of methanolic alkali (de la Rie and Shedrinsky 1989, 10). Early ketone resins were marketed as a specialty component to provide gloss and hardness for lightfast lacquers and nonyellowing enamels (Crombie 1994). Though ketone resins were never intended industrially as the single component of a clear coating (de la Rie and Shedrinsky 1989, 9), varnishes of ketone resins in mineral spirits have been utilized as picture varnishes.

AW-2® is a polycyclohexanone invented in Germany in 1930. Production by BASF began in 1950 and ran until 1967 (de la Rie and and Shedrinsky 1989, 13). AW-2® was a condensation product of cyclohexanone and methyl cyclohexanone (de la Rie and Shedrinsky 1989, 13). MS2® was also a condensation product of cyclohexanone and methyl cyclohexanone (de la Rie and Shedrinsky 1989, 13). MS2® arose out of Howards of Ilford's existing cyclohexane product, Methyl Sectone, in the early 1950s (Crombie 1994). MS2A®, a ketone resin with all the carbonyl groups reduced to hydroxyl groups via sodium borohydride, was commercially introduced in 1961 (de la Rie and Shedrinsky 1989, 13). It was initially developed in the late 1950s at the suggestion of Garry Thomson (de la Rie and Shedrinsky 1989, 12). In 1963 Howards of Ilford changed the manufacturing process of MS2A®, based on the reduction of BASF's AW-2® rather than MS2® (de la Rie and Shedrinsky 1989, 13). The resulting MS2B® had a slightly different solubility and higher viscosity than MS2A®. MS2A® also changed when Howards of Ilford changed its MS2® process in 1963. In 1967, BASF discontinued AW-2® and subsequently Howards of Ilford discontinued MS2B®. BASF replaced AW-2® with Ketone Resin N®, a condensation product of cyclohexanone only (de la Rie and Shedrinsky 1989, 13). It has been suggested that the presence of the methyl groups in the older condensation products, MS2® and AW-2®, may have served as an internal plasticizer (de la Rie and Shedrinsky 1989, 13). Ketone Resin N® was discontinued by BASF in 1982 and replaced by Laropal® K80, a resin also based on cyclohexanone only (de la Rie and Shedrinsky 1989, 10). In the late 1960s Howards of Ilford was incorporated into the Laporte Group and MS2A® phased out. In 1993 Linden Chemicals Ltd. (now Linden Nazareth) acquired all of Laporte's interest in MS2A® and resumed its production (Crombie 1994).

Conservation History of Ketone Resin Varnishes

At the same time that Thomson revealed his findings on the crosslinking of the methacrylate polymers, he also identified many of the strengths and weaknesses of the polycyclohexanone resins (Thomson 1957). Thomson described their low solution viscosity, brittleness, and their decreased tendencies to bloom, wrinkle, and yellow relative to the natural resins (Thomson 1957, 647–74). Feller discussed the cyclohexanone resins in both 1957 and 1963, but the main thrust of his experimentation appears to have been with butyl methacrylates and other acrylics (Feller, Stolow, and Jones 1985; 1963). In 1963 Thomson discussed further research into the sensitivity of certain functional groups to photochemical oxidation, as well as the working requirements of the cyclohexanone resins such as application concentration and gloss modification (Thomson 1963). Thomson also stated that MS2A® was a superior resin to both MS2® and AW-2® (Thomson 1963, 178). It was also mentioned that wrinkling, a direct result of oxidation, may be prevented by applying a top coat of polyvinyl alcohol) (Thomson 1963, 178). In contrast to Feller, Thomson questioned the extent to which film strength should play a role. Thomson noted that cracking in cyclohexanones is equally likely due to oxidative breakdown as to brittleness and that films of high strength and adhesion may cause loss of paint during cleaning or accidental abrasion (Thomson 1963, 177).

In 1972, Herbert Lank reported his experiences with MS2A®, stating that its handling properties closely matched those of damar and mastic, with a reduced tendency to yellow. Lank described the inclusion of wax in the final coating to prevent abrasion as well as to achieve a matte finish. The addition of n-butyl acetate to enhance wetting was also mentioned, though it is not currently used by Lank. Lank also noted that the gloss of MS2A® may diminish over time (Lank 1972).

Mills experimented with the partial esterification of the hydroxyl groups present in MS2A® in the early 1960s, but these products were never fully tested (de la Rie and Shedrinsky 1989, 13). In 1989 de la Rie reported on the improved stability and flexibility of Laropal® K80 upon reduction of ketone groups and partial esterification (de la Rie and Shedrinsky 1989, 13–16). MS2A® may be effectively stabilized to light by addition of 2% resin weight Tinuvin® 292, a hindered amine light stabilizer, while Laropal® K80 could not be recommended due to its inherent instability, despite some stabilization provided by the added Tinuvin® 292 (de la Rie and McGlinchey 1990a). In 1995, however, Jerzy Ciabach reported that the flexibility of Laropal® 80 could be increased by the addition of 10% resin weight of Plexigum® P28 (polyisobutyl methacrylate) or Plexigum® PQ610 (a copolymer of butyl and higher C5-C10 methacrylates). When these methacrylates of low polymerization and Sanduvor® 3050 (hindered amine light stabilizer) were added to Laropal® K80, the resulting films exhibited increased flexibility, improved resistance to light, and improved solubility upon aging (Ciabach 1995, 86).

Synthetic Low Molecular Weight Resins

Chemical Synthesis and Manufacture

Up to the middle of this century, natural resins, such as damar and mastic, were utilized in large quantities by industry in the production and modification of paints, printing inks, and the first pressure-sensitive adhesives (McGlinchey 1990, 563). In the early 1950s demand for the natural resins declined as synthetic, low molecular weight resins, offering increased variety of molecular weight and polarity, became widely available (McGlinchey 1990, 563).

All synthetic, low molecular weight resins are either cyclic or highly branched molecules formed in polymerizations that terminate after linking a small number of monomers (McGlinchey 1990, 564). These branched and bulky small polymers, or oligomers, are amorphous and possess relatively high refractive indices (McGlinchey 1990, 564). One of the points favoring the development of low molecular weight resins is that crosslinking is unlikely to have a large influence on the solubility of the resins (Feller, Stolow, and Jones 1985;Thomson 1957, 64–5). Although the molecular weight of these resins may increase upon aging, the resulting molecular weights are quite low relative to the starting molecular weights of many of the synthetic polymers (de la Rie 1987b, 792). Additionally, the derivation of many of these resins from nonpolar starting materials, such as unmodified terpenes and other hydrocarbons (McGlinchey 1990, 564), results in resins soluble in aliphatic solvents, which in turn are the least damaging to paintings (Feller, Stolow, and Jones 1985, 47–111). Furthermore, the lack of active functional groups, such as ketone groups or carbon-carbon double bonds, insures resistance to autoxidative degradation (de la Rie and McGlinchey 1990b, 168).

Aldehyde Resin

Commercially available aldehyde resins are the condensation products of aliphatic aldehydes of low molecular weight and urea (de la Rie and McGlinchey 1990a, 169). They were developed as a replacement for the ketone resins where increased ultraviolet stability was required (McGlinchey 1990, 564). In selecting a group of synthetic low molecular weight resins for study, de la Rie tested several aldehyde resins available from BASF, but found only one noncommercially produced resin that combined solubility in aliphatic solvents of low aromaticity and resistance to photochemical degradation (de la Rie and McGlinchey 1990a, 169). The infrared spectrum of this limited production aldehyde resin indicated a possible cyclic amide (de la Rie and McGlinchey 1990a, 169). Though the amide group in the resin is capable of photoinduced alpha cleavage, testing found this particular resin quite stable (de la Rie and McGlinchey 1990a, 169–72). With up to at least 800 hours exposure in a xenon arc fadometer, de la Rie found the unstabilized aldehyde resin remains soluble in 100% cyclohexane. Thereafter, a relatively sudden decrease in solubility occurs, though the solubility change is still far less than for the natural and ketone resins (de la Rie and McGlinchey 1990a, 169–72). The aldehyde resin, stabilized by the addition of 0.5% hindered amine light stabilizers (HALS), by weight of resin, withstood failure due to embrittlement-induced cracking for up to at least 3,100 hours exposure (de la Rie and McGlinchey 1990a, 169–72). Additional evaluations, based on changes in ultraviolet absorption and in infrared spectra during aging, revealed the high stability of the resin film when stabilized with HALS at 2% resin weight, such that it may be classified as a Class A material (de la Rie and McGlinchey 1990a, 172). Since the aging simulated light filtration from normal window glass, the stability of this resin in the presence of some ultraviolet radiation marked a major improvement in stability over the natural resins. Samples of the resin were supplied to several conservators who found the saturation achieved with it comparable to that of mastic (Leonard 1990). This is notable as the aldehyde resin's refractive index of 1.494 is lower than that of mastic or damar (de la Rie and McGlinchey 1990a, 168). The solubility of the resin in both solvents of low aromaticity as well as more polar solvents, such as alcohol and acetone, indicate a possible use as an inpainting medium (Whitten, personal communication, 1996).

Hydrogenated Hydrocarbon Resins

The available hydrogenated hydrocarbon resins are derived from the oligomerization of either unsaturated C9 isomers or the C10 molecule dicyclopentadiene (de la Rie and McGlinchey 1990a; McGlinchey 1990). In both cases, after polymerization, the oligomer is hydrogenated under high temperature and pressure to eliminate unsaturation (McGlinchey, 1990). As early as 1940 the hydrogenation of unsaturated hydrocarbon resins was known to produce resins that were extremely resistant to ultraviolet radiation and atmospheric oxidation (Fleck 1945). The hydrogenated hydrocarbon resins tested by de la Rie–Arakawa Chemical's Arkon® series, Exxon's Escorez® series, and Hercules' Regalrez® series–are all soluble in both aliphatic and aromatic hydrocarbons (de la Rie and McGlinchey 1990a, 169). As with the aldehyde resin, de la Rie found that after up to at least 800 hours exposure in a xenon arc fadometer, the unstabilized hydrogenated hydrocarbon resins remain soluble in 100% cyclohexane. Thereafter, a relatively sudden decrease in solubility occurs, though the solubility change is still far less than for the natural and ketone resins (de la Rie and McGlinchey 1990a, 170–2). Insoluble material was detected in the hydrocarbon resins after 2,000 hours exposure in the fadometer, however the films could still be easily removed with a 50/50 mixture of cyclohexane/toluene (de la Rie and McGlinchey 1990a, 170–2). Most of the unstabilized hydrogenated hydrocarbon resins, as well as those ultraviolet-stabilized with 0.5% HALS by weight of resin, withstood failure due to embrittlement-induced cracking up to 2,000 hours exposure (de la Rie and McGlinchey 1990a, 170–2). Additional evaluations, based on changes in ultraviolet absorption and in infrared spectra during aging, revealed the high stability of the hydrogenated hydrocarbon resin films when stabilized with 2% HALS (de la Rie and McGlinchey 1990a, 170–2). All such stabilized resins may be classified as Class A materials, according to Feller's system (de la Rie and McGlinchey 1990a, 172). Regalrez® products, notably Regalrez® 1078 (de la Rie and McGlinchey 1990a, 172) and 1094 (de la Rie 1993, 571), stabilized with 2% HALS, by weight of resin, remain exceptionally stable after nearly 4,500 hours of exposure within a fadometer. Again, the stability of these resins in the presence of ultraviolet radiation proves them to be far more stable than their natural counterparts which deteriorate substantially under the same conditions in far less time (de la Rie and McGlinchey 1990a, 168). Following the publication of the article by de la Rie and McGlinchey, several proprietary varnishes based on hydrogenated hydrocarbon resins were introduced, namely Gamvar®, a solution of Regalrez® 1094 in proprietary hydrocarbon solvent mixture with separate Tinuvin® 292 stabilizer additive (Bob Gamblin, personal communication), and Conservator's Products Company's UVS® Varnishes, a mixture of Regalrez® 1094 (formerly utilizing Escorez® 5380) and a proprietary plasticizer, in hydrocarbon solvent, also with separate Tinuvin® 292 additive (Conservator's Products Company UVS® Varnishes literature).

Cellulose Derivatives

There is a large variety of esters derived from cellulose. The number of alkyl groups added to each glucose unit of the chain is described as the degree of substitution (Horie 1987, 124). Low degrees of substitution provide water-soluble, brittle materials, such as methyl cellulose. By increasing the substitution, both the solubility in organic solvents as well as the plasticity increase (Horie 1987, 126).

Cellulose Acetate

Cellulose acetate is manufactured by heating cellulose with acetic anhydride and a sulfuric acid catalyst (Horie 1987, 130). It became widely available during World War I as a coating for aircraft fabric (Horie 1987, 130). In 1937, Stout and Cross gave cellulose acetate an unfavorable rating as a potential varnish due to its high solubility in chlorinated hydrocarbons and glycol ethers (Stout and Cross 1937).

Cellulose Nitrate

Cellulose nitrate is prepared by soaking a cellulose pulp in a mixture of concentrated nitric and sulfuric acids (Horie 1987, 132). Cellulose nitrate has a high glass transition temperature and is usually compounded with plasticizers such as camphor or dibutyl pthalate (Horie 1987, 132). Due to its instability, as early as 1899 doubts were first cast about its permanence as well as its effect on objects (Posse 1899). Gettens recommended poly(vinyl acetate) as a substitute for cellulose nitrate (Gettens 1935). As with cellulose acetate, it has been suggested that cellulose nitrate may have been utilized as varnishes for paintings (Johnson 1976; Selwitz 1988).

Sodium Carboxymethylcellulose (CMC)

In 1974 Brenner reported that films of CMC may have the potential to serve as a protective coating on the bare canvas regions of color field paintings (Brenner 1974).

Crosslinking Polymers

Though never recommended in any modern conservation manual, there is the possibility that coatings intentionally designed to crosslink may have been applied to paintings. Sheldon Keck and Robert Feller discussed the detection of an epoxy resin coating on one painting (Keck and Feller 1964). Additionally, evidence being compiled indicates that a commercial cabinetry finish coating based on urea-formaldehyde or melamine formaldehyde may have been applied to several paintings prior to their acquisition by a number of American museums (David Miller, personal communication).

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