Varnishes and Surface Coatings: Polymeric Varnishes

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Authors: Jill Whitten, Barbara A. Buckley, Helen Houp, Lydia Vagts, Harriet Irgang, Mark van Gelder, Susan S. Blakney, Nina A. Roth-Wells, Gianfranco Pocobene
Date: Submitted September, 1997
Compiler: Wendy Samet

Introduction: General Characteristics of Polymer Resins for Picture Varnishes

The characteristics of polymers are determined by differences in molecular weight.

The molecular weight of polymers for picture varnishes ranges from 10,000–500,000 (de la Rie 1987, 4). Although the polymers used for picture varnishes are of different chemical classes, they share certain characteristics.

The polymers used in picture varnishes are all linear, long-chain polymers composed of repeating units called monomers. The monomers are joined through different chemical reactions to form chains. The molecular weight of the polymer is determined by the number of monomer units. As the molecular weight of linear polymers increases, changes in physical characteristics are noted: the softening temperature increases, tensile strength increases, solution viscosity increases and brittleness decreases (Feller, Stolow, and Jones 1985, 126). Polymeric picture varnishes are thermoplastic.

Most polymeric picture varnishes are initially soluble in 100% aromatic hydrocarbons and most are also soluble in acetone and alcohols (B-67 will dissolve in hydrocarbons with 11–20% aromatics). Polymers form viscous solutions even at very low resin concentrations. Because of the high solution viscosity, there is still a great deal of solvent in the varnish at the “no flow point.” This causes polymers to conform to microscopically rough surfaces of paint, ground, and support as they dry (Feller, Stolow, and Jones 1985, 140). The result is a final film with a certain degree of roughness which will reflect light in a diffuse manner and will not saturate colors as well as low molecular weight resins.

Because of their high molecular weight, if only a few crosslinks are formed, polymers that are susceptible to crosslinking will become less soluble (or insoluble) over time (Chris Maines, 1997, personal communication). Some polymers degrade in the presence of ultraviolet light but yellowing does not occur to an appreciable degree. Some polymeric films turn gray over time. If the polymer has a low glass transition temperature the films may imbibe dirt. Polymers generally have a lower refractive index than low molecular weight resins.

References

de la Rie, E.R. 1987. The Influence of varnishes on the appearance of paintings. Studies in conservation 32(1):1–13.

Feller, R.L., N. Stolow, and E. Jones. 1985. On Picture varnishes and their solvents. Washington, D.C.: National Gallery of Art.

Maines, C. 1997. Personal communication.

Acrylic Resin Varnishes

Paraloid® B-72

[copolymer of ethyl methacrylate and methyl acrylate]

Important Note: At the time of the preparation of this section the name of this varnish in North America was Acryloid® B-72. During Summer 1997, the name was changed, hence it has been changed here throughout as well.

Historical Background

(1) INDUSTRIAL USE

Paraloid® B-72 is used commercially as a medium for flexible ink and aerosol sprays. It is used in general purpose clear and pigmented coatings for masonry and cloth and for finishing metal furniture and cabinets. Because of its durability and nonyellowing characteristics, B-72 is used industrially with phosphorescent and luminescent pigments.

Paraloid® B-72 is compatible with other film-forming materials such as vinyls, cellulosics, chlorinated rubbers, and silicones and can be used in combination with them to produce coatings with a wide variety of characteristics (Rohm & Haas 1975).

(2) CONSERVATION USE

Paraloid® B-72 is considered one of the most stable varnish resins available in conservation. As a varnish, it should remain colorless and soluble in the solvents in which it was originally dissolved for over 200 years (Feller, Curran, and Bailie 1981).

In addition to its use as a varnish, Paraloid® B-72 is used in conservation as a consolidant for paint both locally and for overall impregnation of oil paintings and wall paintings. It is used as an inpainting medium. It is also used in conservation as a consolidant for wood, plaster, stone, and ethnographic objects; an adhesive for ceramics and glass; and a fixative for pencil, charcoal, and chalk drawings and pastels.

Source

(1) PHYSICAL FORM

Paraloid® B-72 is manufactured by Rohm & Haas as a dry resin in the form of small transparent beads. B-72 also comes in solution, 50% solids in toluene, from the manufacturer. The old form of B-72, made prior to the mid-1970s, was manufactured in the form of white irregular lumps (De Witte 1978).

(2) ORIGIN AND MANUFACTURE

The copolymer of ethyl methacrylate and methyl acrylate is manufactured by Rohm & Haas under the trade name of Paraloid® B-72 in Europe and Canada and Paraloid® B-72 in the United States.

Acrylic polymers were first available commercially in the United States in 1931 and acrylic and methacrylic solution resins were introduced in 1936 (Allyn 1971; see also Bradford Epley, History of Synthetic Resin Varnishes, Section III, p. 35). As early as 1950, Paraloid® B-72 was being used in the conservation field as a coating for metal objects in a traveling show to U.S. museums from Vienna (The Rohm & Haas Reporter 1950).

Around 1975, the proportion of ethyl methacrylate to methyl acrylate was changed from 68:32 to 70:30 (De Witte et al. 1978). The change in proportion produced an increase in refractive index, from 1.479 to 1.481. It also caused changes in solubility; the new B-72 was less soluble in nonpolar hydrocarbons, like mineral spirits, but solubility in polar solvents like ethanol was increased (Carlyle and Bourdeau 1994). The new B-72 became soluble in 95% ethanol at room temperature whereas the old B-72 was only soluble in ethanol warmed to 65°C but precipitated upon cooling (De Witte et al. 1978).

Proprietary products formulated from a copolymer of ethyl methacrylate/methyl acrylate include:

(a) Lascaux Fixativ®

A fixative spray commercially available from Lascaux Colours & Restauro. The fixative spray contains Paraloid® B-72 suspended in a mixture of solvents (Trautwein 1997). Please note that this product is not intended for use as a varnish.

(b) Univar® varnish

This varnish was commercially available from Martin F. Weber Company from the 1960s until the early 1990s (Flax 1996).

(c) Krylon® Crystal Clear No. 41301

Sold in a spray can, which is reported to have been Paraloid® B-72 (Rohm and Haas Reporter 1961). No current information has been obtained regarding this product or whether it is still available. Krylon® Crystal Clear No. 41301 is noted as a copolymer of methyl methacrylate and n-butyl methacrylate (Williams 1992).

(d) Grumbacher Tuffilm Fixative Spray, Gloss (#643)

Appears to be predominantly ethyl methacrylate and methyl acrylate but appears to also have an undetermined ingredient. It has been noted that it does not behave the same as Lascaux Fixativ® or other Paraloid® B-72 solutions (Williams 1992).

(3) MANUFACTURERS AND VENDORS

(a) Rohm & Haas

i) Rohm & Haas, U.S.

Independence Mall West Philadelphia, PA 19105

ii) Rohm & Haas, Canada

2 Manse Road Scarborough, ONT M1E 3T9

(b) Lascaux Fixativ®

i) Lascaux Farbenfabrik

Zurichstrasse 42 CH-8306 Bruttisellen, Switzerland

(c) Conservation suppliers in the U.S. such as Conservation Support Systems and Talas. (See Appendix II, Directory of Vendors.) Also available at art supply stores and through catalogs.

Chemical and Physical Properties

(1) CHEMICAL CLASSIFICATION

A copolymer of ethyl methacrylate and methyl acrylate.

(2) CHEMICAL FORMULA/STRUCTURE

The structure is a linear copolymer of the two monomers which combine by breaking the double bonds. Monomers A and B are randomly arranged along the chain (Hackney 1994).

(3) MOLECULAR WEIGHTS

Weight average molecular weight: 65,128 Mw

Number average molecular weight: 11,397 Mn (de la Rie 1987a)

(4) VISCOSITIES OF STANDARD SOLUTIONS

40% solution at 25°C, in cps

in acetone approx. 200

in toluene approx. 600

in xylenes approx. 980

(Lascaux 1996)

(5) REFRACTIVE INDEX

The literature notes a difference in the refractive index measured by different researchers. The most commonly referred to is 1.487 (de la Rie 1987a). In 1978, De Witte notes the refractive index as 1.481 (De Witte et al. 1978). Hackney notes the refractive index as 1.483 (Hackney 1994).

(6) SOLUBILITY

Shell Odorless Mineral Spirits/Shell Sol® 71	I
Stoddard Solvent/Shell Sol® 340 HT	I
Shell Mineral Spirits 145	I
Petroleum Benzine	I
Turpentine	PIG
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	S

Xylenes	S
Toluene	S
Isopropanol	PSG
Ethanol	PSG
Acetone	S
Arcosolv® PM/1-Methoxy-2-propanol	S

Terms

S	Soluble	clear solution
SC	Soluble Cloudy	completely dissolved, slightly cloudy solution
SV	Soluble Viscous	clear, highly viscous solution
SR	Soluble Residue	residue on the bottom
PSG	Partially Soluble Gel	resin is dissolved in a clear soft gel in a two-phase mixture with solvent on top
PIG	Partially Soluble Immobile Gel	resin is dissolved in a hard gel in a two-phase mixture with solvent on top
IM	Insoluble Milky	insoluble but a minute fraction has swollen and whitened, solvent looks milky
IC	Insoluble Clear	insoluble but a small portion of the resin has swollen to the point that it is stuck to the bottom of the jar, solvent is clear
I	Insoluble	insoluble, undissolved resin sits in clear solvent

Examples: S to PSG = mostly soluble; PIG to I = mostly insoluble

Rohm & Haas, Technical Leaflet C-379, 1975 also lists heptane, isobutanol, and ethylene glycol as insoluble. A solvent selection table with viscosity values in cps. at 25°C for a 40% solids solution is also given.

Note: Lascaux reports B-72 as soluble in toluene and acetone; dilutable with xylene, Shell Sol® A, isopropanol, ethanol, and Dowanol PM; and insoluble in White Spirit, VM&P naphtha (Lascaux 1996).

See also Alan Phenix, Solvents for Paraloid® B-72, Conservation News (UKIC) Nos. 48, 49, and 50, 1992–93, for further discussion of solvents for B-72 and less toxic alternatives.

See also Appendix I, Solubility Testing Description and Solvents Used in Testing.

(7) GLASS TRANSITION TEMPERATURE (Tg) - 40°C (104°F)

“Acrylic resins with Tg values above room temperature are characterized as having high tensile strength … whereas acrylic resins with Tg values below room temperature have low tensile strength …” (Brendley 1973).

Softening point - approx. 70° C

Melting point - approx. 150° C

(Lascaux 1996)

(8) BRITTLENESS AND FLEXIBILITY

B-72 is relatively flexible. To put it in context with other resins, Hackney notes that B-72 is more flexible than damar and MS2A® (Hackney 1994). When a B-72 film is bent around a mandrel, it cracks at 0.1 mm, whereas damar cracks at a radius of 2.7 mm (Feller, Stolow, and Jones 1985, 124).

(9) HARDNESS

Paraloid® B-72 is considered a medium-hard thermoplastic acrylic resin (Lascaux 1996). For a comparison of Sward hardness at 21°C see Figure 5–3 in Feller, Stolow, and Jones 1985, 132.

Preparation/Formulation

(1) TYPICAL BRUSH SOLUTIONS

(a) A typical brush solution is in the range of 5–20% B-72 solids (weight/volume) in xylenes or toluene made from the stock solution. The most commonly used dilutions are in the 10% range. The percentage chosen depends upon the final effect desired as well as the working properties desired by the practitioner.

To prepare a working solution, the resin beads can be suspended in cheesecloth but can also be put directly in a jar with a tight seal on the lid. With gentle agitation, or periodically turning the jar upside down, it generally takes 2–3 days to make a gallon jar. A stock solution is generally made 33% w/v or is available from Rohm & Haas as 50% solids in toluene.

(b) Recipe for 33% stock solution

33 g B-72 resin beads and 100 ml xylenes

(c) Useful B-72 stock solutions

30% weight/volume in xylenes or toluene diluted 12% (1:1.5 stock: solvent) for brushing, and 7.5% for spray (1:3 stock:solvent) 40% weight/volume in xylenes or toluene diluted 16% (1:1.5) for brushing and 10% for spray (1:3) (Kushel 1991).

(2) TYPICAL SPRAY SOLUTIONS

(a) Spray solutions range from 3–15% B-72 (weight/volume) in xylenes or toluene made from the stock solution as described above in (1)(a)-(c). The most commonly used solutions are in the 10% range.

(3) ADDITIVES

(a) Solvent additives

i) The addition of slow evaporating aromatic solvents such as CycloSol® 100 (Shell), diethyl benzene, or Aromatic® 100 (Exxon) to working solutions will extend the drying time, thus allowing for greater saturation and increased working time. Some feel it also allows for a limited degree of leveling of the varnish layer. See also “Tricks of the Trade” and cautionary note in Section e)(2), Brushing, below.

(b) Matting agents

i) Fumed silica (Cabosil or Davison Syloid 244 or 308) 0.5–1% can be added to the varnish stock solution and stirred (Kushel 1991).

ii) Curran measured the matting effect of the addition of Victory microcrystalline wax from 1–10%. The appearance of a 10% wax varnish was too waxy and it was judged that it would be rare to use a mixture with greater than 3–4% wax. Although it was also found that above 3.5%, the varnish could be readily buffed when rubbed with a polishing cloth (Curran 1975).

iii) 2–4% of a microcrystalline wax such as Witco Multiwax W445 can be added to the stock solution of varnish (Kushel 1991).

iv) Many conservators find that a flattening agent is optically interfering and prefer to adjust the solvents used and/or the spray parameters.

(c) Ultraviolet Stabilizers

(See also Section VII., Phenolic Antioxidants, Stabilizers, and UV Absorbers, and Section IX., General Application Techniques.)

The addition of an ultraviolet stabilizer is usually for the purpose of stabilizing a natural resin varnish below. A recipe for an ultraviolet barrier top coat is as follows (Carlyle and Bourdeau 1994, 46):

Paraloid® B-72	45.00 g (13%)
solvent	300.00 g (xylenes)
Tinuvin® 327	1.35 g (3% ultraviolet absorber w/w to polymer)

(4) STORAGE/SHELF LIFE

The shelf life of B-72 is indefinite in dry bead form; it retains a discrete bead form and does not cold flow. Based upon the personal experience of conservators, a stock solution of B-72 appears to be good for a long time but scientists caution against keeping any stock solution for too long. A recent attempt to put B-72 resin beads purchased in 1972 into solution with Shell CycloSol® 100 resulted in a cloudy solution (Whitten 1997).

Working Characteristics and Practical Properties

(1) APPEARANCE

(a) Paraloid® B-72 is a medium gloss varnish that has the potential for providing good saturation on many paintings. B-72 may not provide satisfactory saturation on “Old Master” paintings or those that are dark in value, have very damaged paint surfaces, or have been overcleaned. In these cases, practitioners often use an interlayer of a more saturating resin.

(2) BRUSHING

(a) Open working time

The open working time of B-72 in xylenes or toluene is of short duration. Open working time may be extended somewhat by the addition of slower evaporating aromatic solvents such as diethylbenzene, Aromatic® 100, or Cyclo Sol® 100. In special cases, diacetone alcohol or benzyl alcohol have been added to the varnish solution; this practice should be cautioned against, however, because the use of highly polar solvents creates the potential for varnish/paint interaction possibly causing the paint to become more soluble (Carlyle and Bourdeau 1994, 44; Tsang and Erhardt 1992, 90).

(b) Drying time

In xylenes, B-72 that is brush-applied under normal conditions is generally dry to the touch within 5–10 minutes. The addition of slower drying solvents will lengthen the drying time, and conversely the addition or use of B-72 in a faster drying solvent, such as toluene, or with the addition of acetone will shorten the drying time. As with any varnish, temperature and relative humidity will also affect drying time.

(c) Usual application

B-72 is generally applied with a stiff natural bristle brush. As with most varnishes, the methods of application vary and ultimately depend upon the choice of the practitioner for the particular varnishing situation being encountered. Methods of brush applications are those that are in general use for most normal applications, and are further discussed in Section IX., General Application Techniques. The practitioner must work with concentration and fairly quickly in order to wet all areas of the painting before drying. Larger paintings will require that the practitioner work in smaller sections that are joined to the previous section before drying. A scrubbing motion with a low angle to the brush is often recommended. It is not generally recommended that B-72 be worked back into before it is dry to the touch because streaking and unevenness will generally result. Yet, there are some practitioners who have success by going back into the wet varnish with soft brushes to continue blending and/or to achieve a satin or matte finish. Hackney notes that B-72 becomes viscous quickly and continued brushing will unevenly distribute the varnish, and has demonstrated this on cross-sections. B-72 does not have the leveling properties of low molecular weight resins.

If the varnish coat appears to be too thin and not saturating enough, a second thin coat can be brushed on top of the first coat after drying. Although some practitioners prefer to wait at least a day before adding further layers of varnish, others proceed after the varnish is dry to the touch. If the varnish is too thick, solvent can be brushed through the varnish to thin it out by dipping a clean dry brush into solvent (i.e., xylenes if the varnish coat was dissolved in xylenes) and reworking the varnish in both horizontal and vertical directions (Thomas 1995).

(3) SPRAYING

The appearance of spray coatings of B-72 varies greatly with the use of different solvents and application methods. In combination with brush coatings of different resin films, the B-72 spray coatings can create a surface which closely replicates natural resin films, or conversely, which give the effect of an unvarnished paint surface.

The principal solvents used for dilution of B-72 are xylenes or toluene. Toluene, which evaporates faster than xylenes, gives a more matte effect in both brush and spray applications of B-72, while xylenes gives a longer working time and greater gloss in both brush and spray applications. While it is possible to achieve reasonably matte or glossy films using either solvent with a suitable application method, most conservators choose the diluent for B-72 based on the general trend of xylenes-based films to be relatively glossy and toluene-based films to be more matte. Dilutions in either solvent are generally in the 3–15% weight/volume range. The typical dilution with either solvent is 10%. For variations of these parameters, see (4) below, “Tricks of the Trade.”

(a) Open working time

There is virtually no working time for manipulation of the varnish layer beyond the actual spraying process for normal applications of B-72.

(b) Drying time

A spray application of B-72 is dry to the touch in about 5–10 minutes.

(c) Usual application

As with any spray application of varnish, the greater the distance of the gun from the object, the colder the varnish solution, the faster the evaporation rate of the solvent, and the higher the pressure, the more matte the resultant varnish film will be. A light spray of B-72 in toluene, especially at a low concentration and at a distance, can give an almost unvarnished appearance while still offering some protection to the paint film. When the pressure of the compressor is lowered, and the gun is brought in close to the object, a “wet” spray of B-72 in xylenes can be achieved which provides the saturation of a brush coat and imparts a glossy sheen to the varnish film. Many degrees of gloss and saturation are possible between these two extremes, especially if one interlayers films in toluene and xylenes, or varies the pressure or distance of subsequent films of the same solvent.

When a painting has areas of wrinkled or finely traction crackled paint, or “sunken” matte areas, it is possible to locally spray B-72 in xylenes in order to increase gloss and saturation in these areas. It is also possible to dull down glossy spots with similar local sprays of B-72 in toluene.

Finally, many conservators use B-72 spray to add a tougher final protective layer over ketone resins. Light overall B-72 sprays are frequently useful for slightly reducing the gloss of lower molecular weight resin films as well. B-72 may also be used over resins which are more likely to retain dirt.

(4) MODIFICATIONS FOR SPECIAL APPLICATIONS AND EFFECTS: TRICKS OF THE TRADE

(a) Adjusting matte/gloss effects

i) CycloSol® 53 (Whitten 1997)

B-72 can be mixed in Shell Cyclo Sol® 100 (formerly Cyclo Sol® 53) to slow down the evaporation rate for brush varnishing. This will increase working and leveling time and yield a more saturating varnish. Xylene evaporates four times more quickly than Cyclo Sol® 100 which is an aromatic solvent like xylene and toluene but with larger side groups. It is less polar than diacetone alcohol and benzyl alcohol which have been used in the past to slow the evaporation of B-72 mixtures. On two occasions, when older B-72 was used (one batch was purchased in 1972), the mixture was cloudy (Whitten 1997).

ii) The addition of Aromatic 100 (Exxon) to the working solution is used to increase saturation and working time (Tucker 1996).

iii) A 6% spray solution of Paraloid® B-72 may be applied to decrease gloss. The recipe is as follows:

80 ml toluene

20 ml xylenes

6 g Paraloid® B-72

For a more glossy varnish the solvent mix can be reversed: 20: toluene, 80:xylenes. This method uses a high volume low pressure deVilbiss® spray gun at approximately 30–40 PSI using a detail or finishing head which is similar to an airbrush, but with a wider fan (Bernstein 1996).

iv) Conversely, a 5% weight/volume solution in 1:1 xylenes:toluene can be used to increase saturation. The varnish is applied with a Binks® spray gun with the pressure set very low so that the varnish is applied more as a mist or “cloud” rather than pulsing or “hitting” the paint surface. This method is used as an attempt to achieve the characteristics of a brush coat by building up thin layers of varnish. This method has been especially successful on 19th-century paintings and on traditional 20th-century paintings (Radecki 1995).

v) Rubbing in B-72 in the same proportion as above in (iv) with silk-covered cotton is a method used to produce a thin saturating layer of varnish. It is also useful for local applications of varnish (Radecki 1995).

vi) For maximum gloss for brushing, dilute 40% stock 1:1 with mix of diethyl benzene and xylenes or diethyl benzene alone (Kushel 1991).

vii) A shot glass of diethyl benzene added to the working solution (approximately a pint) will increase the saturation of B-72 (stock: 1/3 B-72: 2/3 xylenes, v:v; working solution: 1/3 stock solution: 2/3 solvent) (Stoner 1996).

viii) A 10% solution of B-72 in approximately 20:1 xylenes:benzyl alcohol has been used for increased saturation under special circumstances (McGinn 1994).

ix) One practitioner claims good and consistent effects of saturation and gloss using pure m-xylenes, boiling point 139.3°C (“xylenes, which is a mixture of o-, m-, and p-xylenes has a boiling range of 137°C to 144°C”) (Carlyle and Bourdeau 1994, 45).

x) Gently warming the surface of the painting with quartz retouching lamps after varnishing (approximately 35–40°C) will increase the gloss of the varnish layer (Carlyle and Bourdeau 1994, 45).

xi) The diluted stock solution (20% weight/volume) can be brush applied, allowed to become almost dry to the touch, and then quickly brushed out in all directions using a dry brush to mitigate uneven gloss and produce a more “matte” sheen (Houp 1995).

xii) To dull the look of a varnish, spray as normal. Quickly clean the spray cup and the gun by spraying through with toluene. Fill the spray cup with heptane (sometimes petroleum benzine is added) and generously spray the surface of the painting while still tacky. The heptane quickly drops the gloss. Moisture can be a problem with this technique, so be especially mindful of surrounding humidity levels (Bernstein 1996).

(b) Combination Recipes

i) Damar or low molecular weight, synthetic resin varnishes may be used as an initial layer to saturate the painting and be followed by a top coat of B-72. The addition of an ultraviolet light absorber such as 3% weight/weight resin of Tinuvin® 327 or 4% weight/weight resin of Tinuvin® 1130 + 3% weight/weight resin Tinuvin® 292 can be added to retard yellowing of the damar coat (Carlyle and Bourdeau 1994, 45–6). (See also Section IX., General Application Techniques.)

ii) To reduce the “sinking in” effect and make gloss more uniform on uneven oil paint surfaces, poly(vinyl acetate) interlayers can be used once a layer of B-72 is applied. Some standard applications of poly(vinyl acetate) are:

PVA/AYAA	8% in toluene (wt/v)
PVA/AYAA:AYAC	9% (50:50 mixture) in toluene (wt/v)
PVA/AYAC	10% in toluene

The PVA layer can be finished with a top coat of B-72 (Carlyle and Bourdeau 1994, 45, 54).

iii) An application of brush applied B-72 isolating varnish followed by a spray application of Soluvar® Gloss varnish (Liquitex/Binney & Smith) has been found to help in evening out an uneven sheen (Parkin and Zucker 1995).

iv) When it is preferred to have the more stable acrylic varnish B-72 next to the paint surface, poly(cyclohexanone) coatings can be applied over the B-72 which can be removed with mineral spirits without disturbing the B-72 varnish layer. Laropal® K80, MS2A®, and Regalrez® can be brush applied as the final varnish layer (Carlyle and Bourdeau 1994, 45).

v) B-72 has also been applied next to the paint layer under a more saturating final brush coat of damar varnish (Tucker 1996).

vi) Winton® Gloss Varnish (Winsor & Newton) diluted 50% in Stoddard Solvent is brushed onto a layer of B-72 and then wiped thin with a Kim-wipe® to remove all excess varnish and keep the layer as thin as possible. This adds the saturation sometimes lacking with B-72 while keeping the varnish layer as thin as possible. It is also useful for evening out gloss. After inpainting, a final spray coat of B-72 is applied to prevent the Winton layer from scratching and collecting dust. This same method can also be used for other poly(cyclohexanones) resins such as Laropal® K80 or MS2A® (Bockrath 1995).

vii) In the 1970s, Paraloid® B-67 (Rohm & Haas) was often used as an initial isolating layer of varnish or as an interlayer, both for its saturating properties and for its ability to even out the gloss of uneven surfaces. B-67 was often finished with a spray coat of Paraloid® B-72. This system fell out of favor in the 1980s when there was some discussion regarding the stability of B-67.

(5) SPECIAL CONSIDERATIONS

(a) B-72 is not a forgiving varnish and works best with an evenly cleaned surface. Uneven residues of varnish will not be readily masked with the use of B-72. In those cases where a painting has been difficult to clean and residues of varnish remain, an interlayer of another resin is often used to correct this problem.

(b) Despite the known limitations with regard to saturation, many conservators strongly prefer to use B-72 to natural resin varnishes because of their reluctance to subject paintings to the cleaning cycle. The solvents used to remove B-72 as opposed to those used for natural resin varnishes are less likely to damage paint films during the cleaning process. Conservators who use B-72 know that it is not an easy varnish to use but feel that practice can produce an aesthetically pleasing varnish layer. Many conservators also feel that their success in its use relies upon their cleaning methods. Cleaning materials are chosen which will leave the cleaned surface without a dry or leached appearance.

(c) Paraloid® B-72 does not have the leveling properties of low molecular weight resins. But because of this, it is felt that the brushwork of a painting is accentuated with the use of B-72.

(d) Hackney compares the refractive index of dried oil paint as measured by Joyce Townsend to be about 1.50 to the refractive index of B-72, which is 1.487 (1.481–1.487; see also section (5) above, Refractive Index,). It is argued that perhaps B-72 comes closer in appearance to a dried oil paint layer than other resins currently in use (Hackney 1994). This could, perhaps, have advantages when an unvarnished appearance is desired.

Aging Characteristics

(1) CHEMICAL PROCESS. SEE BELOW.

(2) RESULTANT CHEMICAL AND/OR PHYSICAL ALTERATIONS

B-72 is considered one of the most stable resins available for conservation use. Photochemical aging tests indicate that B-72 will not show any yellowing or changes in solubility for nearly 200 years under normal museum conditions (Feller 1975). Howells et al. (1984) report changes in the Fourier transform infrared and pyrolysis gas chromatographic analysis of aged Paraloid® B-72. The nature of the change was not reported and may have been due to contamination of the samples. Ciabach (1982) reported a decrease of intrinsic viscosity upon exposure to ultraviolet which indicated a decrease in molecular weight owing to chain-scission (or chain breaking) rather than crosslinking (Williams 1992).

(3) IMPACT UPON VISUAL APPEARANCE/SOLUBILITY AND REMOVABILITY

B-72 has been observed in the laboratory to show an increase in gloss from about 100–350 hours followed by a decrease at about 300–400 hours of aging with an air-cooled xenon lamp over time from ultraviolet chain breaking and subsequent loss of degradation products (De Witte 1975). It may be found that ultraviolet filtration can diminish this change (Ciabach 1982).

Keeping in mind the diluent used for the resin coating and the solubility of the paint layer upon which the resin was applied, B-72 should be removable in xylenes or toluene (Feller 1975). Reportedly, if diacetone alcohol is added to an ethanol-resin mixture to improve the dissolution of the polymer, the diacetone alcohol could cause the resin to penetrate the paint layer to the point of irreversibility (Carlyle and Bourdeau 1994, 44).

(4) ATTRACTION AND RETENTION OF DIRT AND GRIME

B-72 does not absorb dirt as do the poly(vinyl acetates)s or poly(n-butyl methacrylate) owing to cold flow because it is above its glass transition temperature (Tg) at room temperature (Hackney 1994).

A common problem with synthetic resin varnishes and waxes is their tendency to store an electrostatic charge because they are poor conductors of electricity. As a result, stored charges tend to attract dust particles. This is particularly true for sprayed varnishes which have collected static during the spraying operation and, if matte, have a higher surface area on which to store a static charge (Hackney 1994; Hough 1995). As a result, Paraloid® B-72 surface coatings should be regularly dusted and/or will require occasional surface cleaning. Surface cleaning a B-72 varnish coating is a straightforward process.

(5) THEORETICAL LIFETIME

The theoretical lifetime of B-72 is 100–200 years under normal museum conditions (Feller 1981). Feller relates B-72 as a class A resin, that is, a material intended for over 100 years' use (Feller 1978).

(6) OTHER

From actual case studies of Paraloid® B-72 at the Tate Gallery in London, Hackney discussed two paintings that were varnished in 1964 and 1965 with B-72. The varnish layers remain soluble in xylenes and are clear and transparent (Hackney 1994).

Health and Safety

(The following information listed 1)–6) below is taken directly from the Rohm & Haas Technical Data Sheet.)

(1) PARALOID® B-72 100% SOLIDS

(a) Slight toxicity

(b) Slight fire hazard

(2) PARALOID® B-72 50% RESIN IN TOLUENE

(a) Flash point: 7°C/444, 45°F PMCC

(b) Auto ignition temperature: 480°C, 896°F (est.)

(c) Lower explosion limit: 1.2 est.

(d) Upper explosion limit: 7.1 est.

(e) Extinguishing media: alcohol foam, dry chemicals, carbon dioxide, water spray

(f) Special fire fighting procedures: wear self-contained breathing apparatus (pressure demand-MSHA/NIOSH approved) and full protective gear. Use water spray to cool containers.

(g) Unusual fire and explosive hazards: Vapors can travel to a source of ignition and flashback. Material can form explosive vapors with air.

(3) HEALTH HAZARD INFORMATION

(as related to Paraloid® B-72 50% resin in toluene)

(a) Inhalation: Solvent vapor or mist can cause headache, dizziness, nausea, loss of consciousness, irritation to nose, heart, lungs.

(b) Skin contact: Irritation to skin upon repeated or prolonged contact. Liquid substance can be absorbed through contact with skin in harmful amounts.

(c) Eye contact: Can cause irritation and transient cornea injury.

(d) Delayed effects: Repeated overexposure to toluene can cause central nervous system effects.

(e) Emergency first aid procedures: Move subject to fresh air. Give artificial resuscitation.

(f) Ingestion: If swallowed, give two glasses of water to drink. Never give anything by mouth to an unconscious person.

(g) Storage: Ground all containers when transferring material. Store in approved areas. Material can burn. Limit indoor storage to approved areas equipped with automatic sprinklers. Avoid all ignition sources. Minimum recommended storage temperatures are 18°C/0°F. Maximum recommended storage 49°C/120°F.

(h) Reactivity information: This material is considered stable. However, avoid temperatures above 260°C/500°F, the onset of polymer decomposition. Thermal decomposition is dependent on time and temperature.

(4) HAZARDOUS DECOMPOSITION PRODUCTS

none

(5) HAZARDOUS POLYMERIZATION

Product will not undergo polymerization

(6) TOXICITY INFORMATION

(a) Acute Data: Oral LD50 rat: 15000 mg/kg

(b) Dermal LD50 rabbit: 13000 mg/kg

(c) Eye irritation rabbit: moderate irritation

(d) Skin irritation rabbit: slight irritation

(e) Reproductive/Teratology Data: Toluene has been demonstrated to be embryofetotoxic and teratogenic in laboratory animals.

Disposal

It is recommended that for disposal, this material be incinerated at a facility that complies with local, state, and federal regulations (Rohm & Haas Technical Data Sheet).

References

Allyn, G. 1971. Acrylic resins. In Federation Series on Coatings Technology, Unit 17. W.H. Madson, ed. Philadelphia: Federation of Societies for Coatings Technology.

Anon. 1950. Acryloid helps preserve art treasures. Rohm and Haas REPORTER 8(3):14.

Anon. 1961. Push button spray coatings. Rohm and Haas REPORTER 19:12.

Bernstein, J. 1996. Personal communication to B. Buckley.

Bockrath, M. 1995. Unpublished.

Brendley, W.H., Jr. 1973. Fundamentals of acrylic polymers. In Paint and varnish production. Name and Place of publisher: pages.

Carlyle, L. and J. Bourdeau. 1994. Varnishes: Authenticity and permanence. Workshop Handbook. Ottawa: Canadian Conservation Institute.

Ciabach, J. 1982. Investigation of the cross-linking of thermoplastic resins effected by ultraviolet radiation. In Resins in Conservation. Proceedings of the Symposium. Edinburgh. 5.1–5.8.

Curran, M. 1975. Scattering of light over a black background by matt varnishes based on Paraloid® B-72. In Preprints of the 4th Triennial Meeting of the ICOM Committee for Conservation, Venice, 13–18 October 1975. Paris: International Council of Museums:75/22/3/1–5.

de la Rie, E.R. 1987a. The Influence of varnishes on the appearance of paintings. Studies in conservation. 32(1):1–13.

De Witte, E. 1975. The Influence of light on the gloss of matte varnishes. In Preprints of the 4th Triennial Meeting of the ICOM Committee for Conservation, Venice, 13–18 October 1975. Paris: International Council of Museums:75/22/6/1–9.

De Witte, E., et al. 1978. The Structure of “old” and “new” Paraloid B-72. In Preprints of the 5th Triennial Meeting of the ICOM Committee for Conservation, Zagreb, 1–8 October 1978. Paris: International Council of Museums:78/16/3/1–9.

Feller, R.L. 1957a. Factors affecting the appearance of picture varnish. Science 125(3258): 1143–4.

Feller, R.L. 1975. Studies on the photochemical stability of thermoplastic resins. In Preprints of the 4th Triennial Meeting of the ICOM Committee for Conservation, Venice, 13–18 October 1975. Paris: International Council of Museums:75/22/4/1–11.

Feller, R.L. 1978. Standards in the evaluation of thermoplastic resins. In Preprints of the 5th Triennial Meeting of the ICOM Committee for Conservation, Zagreb, 1–8 October 1978. Paris: International Council of Museums:78/16/4.

Feller, R.L., M. Curran, and C. Bailie. 1981. Photochemical studies of methacrylate coatings for the conservation of museum objects. In Photodegradation and photostabilization of coatings. American Chemical Society Symposium Series 151. 179th Meeting of the American Chemical Society: 183–96.

Feller, R.L., N. Stolow, and E.H. Jones. 1985. On Picture varnishes and their solvents. Revised and enlarged ed. Washington, D.C.: National Gallery of Art.

Flax, E. 1996. Vice-president of Martin/F. Weber. Personal communication to M. Bockrath.

Hackney, S. 1994. Paraloid B-72 as a painting varnish. Unpublished paper read at CCI varnish conference: Varnishes:Authenticity and Permanence.

Houp, H.A. 1995. Unpublished.

Hough, Mary Piper, column editor. 1995. [Review of a talk by Stephen Hackney at the September 1994 Ottawa Conference “Varnishes:Authenticity and Permanence”]. Western Association for Art Conservation (WAAC) Newsletter 17(1):32.

Kushel, D. 1991. Unpublished notes. State University College at Buffalo.

Lascaux Technical data sheet R 138/1996.

Lomax, S.Q. and S.L. Fisher. 1990. An Investigation of the removability of naturally aged synthetic picture varnishes. Journal of the American Institute for Conservation 29(2):18–91.

Martens, C.R., ed. 1968. Technology of paints, varnishes, and lacquers. Huntington, NY: R.E. Kreiger.

Mayer, R. 1970. The Artist's handbook of materials and technigues. New York: Viking Press.

McGinn, M.T. 1994. Unpublished treatment report, Pennsylvania Academy of the Fine Arts.

Padfield, S., ed. 1993. Varnishing: Theory and practice. The Association of British Picture Restorers Fiftieth Anniversary Conference.

Parkin, H.M. 1995. Personal communication to H. Houp.

Phenix, A. 1992. Solvents for Paraloid B-72. Conservation News 48:21–3.

Phenix, A. 1992. Solvents for Paraloid B-72. Conservation News 49:23–5.

Phenix, A. 1993. Solvents for Paraloid B-72. Conservation News 50:39–40.

Phenix, A. 1993. Artists' and conservation varnishes: An historical overview. In Varnishing: Theory and practice, S. Padfield, ed. The Association of British Picture Restorers Fiftieth Anniversary Conference.

Radecki, M. 1996. Personal communication to B. Buckley.

Rees-Jones, S. 1993. A note on the transfer of light into and out of paintings. Studies in conservation 38(4):174–9.

Rohm & Haas Technical Data Sheet C-379. 1975. Characteristics of Acryloid thermoplastic acrylic resins.

Seve, R. 1993. Problems connected with the concept of gloss. Color Research and Application 18 (4): 241–52.

Stoner, J.H. 1996. Personal communication to B. Buckley relaying technique used by John Brealy at the Metropolitan Museum in the 1980s.

Thomas, G. 1995. Personal communication to H. Houp.

Trautwein, K. 1997. Chemist/Laxcaux. Personal communication to B. Buckley.

Tsang, J.S., and D. Erhardt. 1992. Current research on the effects of solvents and gelled and aqueous cleaning systems on oil paint films. Journal of the American Institute for Conservation 31: 87–94.

Tucker, M. 1996. Personal communication to W. Samet and B. Buckley.

Whitten, J. 1997. Personal communication to B. Buckley.

Williams, R.S. 1992. The Composition of commercially prepared artists' varnishes and media. In Varnishes: Authenticity and permanence. Workshop Handbook. Appendix IV.

Zucker, J. 1995. Personal communication to H. Houp.

Paraloid® B-67, Elvacite® 2045 - poly(isobutyl methacrylate)

[an acrylic resin of the poly(butyl methacrylate) family]

Names: poly(isobutyl methacrylate) (PiBMA), Acryloid® B-67, Elvacite® 2045, Paraloid® B-67 (in Europe; see also note below).

Important Note: At the time of the preparation of this section the name of this varnish in North America was Acryloid® B-67. During Summer 1997, the name was changed, hence it has been changed here throughout as well.

Historical Background

(1) INDUSTRIAL USE

According to Tom Corle of ICI Chemicals, credit for the industrial development of PiBMA is given to Otto Röhm of Darmstadt (see also Gettens and Stout 1966), who began experimenting with acrylates in the late 19th century (Tom Corle, personal communication August 1995). The acrylic resins as we know them today were invented in 1931 and their use as an artist's material, especially as an aerosol varnish, became widespread in the 1940s and 1950s (Feller, Stolow, and Jones 1985). They are used today for a wide variety of purposes, including inks, coatings, adhesives, bus and airplane windows, injection molds, and artificial eyes and limbs.

(2) CONSERVATION USE

It appears that the various acrylic resins were first experimented with for conservation purposes in the 1930s at the Fogg Art Museum and gained fairly widespread use in the decades following, although not much has been written about their use (Rawlins and Werner 1949; Werner 1952; Feller 1971; Feller, Stolow, and Jones 1985).

Source

(1) PHYSICAL FORM

B-67 - Clear, colorless, extruded pellets

Elvacite® 2045 - Clear, colorless microspheres

Solutions of the resins in aliphatic and/or aromatic hydrocarbon solvents are sold by the manufacturers.

(2) ORIGIN AND MANUFACTURE

Originally derived from methacrylic acid, Elvacite® 2045 was known initially under the trade name Lucite 45® (Horie 1994), PiBMA is manufactured today by taking a monomer synthesized mainly from acetone and methanol and then polymerizing it in suspension in water, thus producing spheres of the resin ranging in size from 90 to 300 microns (Tom Corle, personal communication, August 1995).

(3) MANUFACTURERS AND VENDORS

PiBMA is currently manufactured as Paraloid® B-67 by Rohm & Haas, Philadelphia, Pennsylvania, as Paraloid® B-67 by Röhmtec, in Germany, and in a higher molecular weight as Elvacite® 2045 by ICI Chemicals, Wilmington, Delaware. (This was formerly the Elvacite® manufactured by DuPont, which sold its acrylic resin division to ICI in 1993). The solid resins are available through most of the standard suppliers of conservation materials in the United States, such as Conservator's Emporium, Reno, and Talas, New York (See Appendix II, Directory of Vendors).

Chemical and Physical Properties

(1) CHEMICAL CLASSIFICATION

Acrylic resin of the poly(butyl methacrylate) family.

(2) CHEMICAL FORMULA/STRUCTURE

monomer unit: where R = -CH(CH₃)C₂H₅

(Feller 1996)

(3) MOLECULAR WEIGHTS

(a) Weight average molecular weight

Measured with gel permeation chromatography, the molecular weight of Elvacite® 2045 using a polystyrene standard (an older, more common standard, Tom Corle, personal communication 1997) is 193,000 and using a poly(methyl methacrylate) standard is 451,000 (Tom Corle, personal communication August 1995). The weight average molecular weight of Paraloid® B-67 is 44,764 (de la Rie 1987).

(b) Number average molecular weight

Measured with gel permeation chromatography, the molecular weight of Elvacite® 2045 using a polystyrene standard is 120,000 and using a poly(methyl methacrylate) standard is 192,800 (Tom Corle, personal communication August 1995). The number average molecular weight of Paraloid® B-67 is 10,960 (de la Rie 1987).

(4) REFRACTIVE INDEX

1.486ⁿD

(5) SOLUBILITY

The solubility test results for Elvacite® 2045 and for Paraloid® B-67 are shown in the charts at the top of the following page.

Elvacite® 2045
Shell Odorless Mineral Spirits/Shell Sol® 71	IC
Stoddard Solvent/Shell Sol® 340 HT	PIG
Shell Mineral Spirits 145	PIG
Petroleum Benzine	PSG
Turpentine	SV
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	S

Xylenes	S
Toluene	S
Isopropanol	PSG
Ethanol	PIG
Acetone	S*
Arcosolv® PM/1-Methoxy-2-propanol	SV

Paraloid® B-67
Shell Odorless Mineral Spirits/Shell Sol® 71	PIG
Stoddard Solvent/Shell Sol® 340 HT	PSG
Shell Mineral Spirits 145	PSG
Petroleum Benzine	SV
Turpentine	S*
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	S

^* yellow

Xylenes	S
Toluene	S
Isopropanol	SV
Ethanol	PSG
Acetone	S
Arcosolv® PM/1-Methoxy-2-propanol	S

Terms

S	Soluble	clear solution
SC	Soluble Cloudy	completely dissolved, slightly cloudy solution
SV	Soluble Viscous	clear, highly viscous solution
SR	Soluble Residue	residue on the bottom
PSG	Partially Soluble Gel	resin is dissolved in a clear soft gel in a two-phase mixture with solvent on top
PIG	Partially Soluble Immobile Gel	resin is dissolved in a hard gel in a two-phase mixture with solvent on top
IM	Insoluble Milky	insoluble but a minute fraction has swollen and whitened, solvent looks milky
IC	Insoluble Clear	insoluble but a small portion of the resin has swollen to the point that it is stuck to the bottom of the jar, solvent is clear
I	Insoluble	insoluble, undissolved resin sits in clear solvent

Examples: S to PSG = mostly soluble; PIG to I = mostly insoluble

See also Appendix I, Solubility Testing Description and Solvents Used in Testing.

(Note: Information for various individual hydrocarbon solvents was not provided in the brochures from the manufacturers, but one can assume that solubility would be the same as that listed for mineral spirits, in that the resin is soluble in hydrocarbons with a 10% minimum aromatic content)

(7) GLASS TRANSITION TEMPERATURE (Tg)

50°C for B-67 (Horie 1987, 107) and 55°C (131° F) for Elvacite® 2045 (Horie 1987, 107).

(8) BRITTLENESS AND FLEXIBILITY

According to Feller, Stolow, and Jones 1985, the brittleness of the resin film depends on the viscosity grade of the polymer: the higher the viscosity, the less brittle it is.^* According to the literature supplied by ICI, the tensile strength of Elvacite® 2045 is 25 Mpa at 3,600 psi (23°C/73°F, 50% RH) and the elongation at break is 1% (same conditions as listed above).

Preparation/Formulation

(1) TYPICAL BRUSH SOLUTIONS

When used as a varnish, the typical solution is between 8 and 10% weight per volume (w/v) solids in solvent (typically petroleum benzine, xylenes, or a mixture of both), but has also been used at 15 to 20%. B-67 has also been used for facings at much higher concentrations, such as approximately 30%.

(2) TYPICAL SPRAY SOLUTIONS

Typical spray solutions are 10 and 15% w/v, but can be as low as 4%.

(3) ADDITIVES

According to Horie (Horie 1987), Paraloid® B-67 contains an unknown additive which renders it more stable than it would be otherwise (see also Feller 1976, 144). The additive has not been investigated and Horie does not specify if all types of PiBMA have the additive or if it was only found in Paraloid® (i.e., the European version). No other additives are mentioned in the literature supplied by the manufacturers.

(4) STORAGE/SHELF LIFE

Solutions of PiBMA, as well as other acrylics, may turn yellow over time, even if stored in darkened cabinets. This problem may be exacerbated if xylenes or if a less pure grade solvent are used to solubilize the resin. The only precaution put forth by the manufacturers is a warning about the generation of static electricity in handling the resins in the presence of flammable solvents (technical literature supplied by ICI Acrylics, 32).

Working Characteristics and Practical Properties

(1) APPEARANCE

B-67 appears quite glossy immediately upon application, and can have a rather glassy appearance if applied too heavily. Over time, however, the varnish tends to matte down and become somewhat dull. The dullness may be due to the fact that B-67 remains somewhat tacky (given its relatively low Tg) allowing it to retain dust and grime. Conversely, the resin also tends to become brittle with time, and many conservators indicated that this was a main factor in choosing not to use the resin as a varnish, as the surface could become fractured and scuffed quite easily (Bernstein, personal communication August, 1995; Wallace, personal communication December, 1994).

(2) BRUSHING

(a) Open working time

Limited to a few minutes. The working time can be extended by using slower evaporating solvents.

(b) Drying Time

The varnish remains tacky for at least 5 to 10 minutes after application, and should be handled with caution for several hours.

(3) SPRAYING

(a) Open working time

The working time is practically nonexistent. The resin dries quite quickly, expecially if used with solvents such as xylenes.

(b) Drying Time

The spray coatings seem to dry very quickly, although they remain tacky for at least 5 to 10 minutes after application.

(4) MODIFICATIONS FOR SPECIAL APPLICATIONS AND EFFECTS OR TRICKS OF THE TRADE

As few people will admit to using B-67 anymore, only a few tricks of the trade were offered. Among them is the fact that by varying the evaporation rate of the solvents used when applying the varnish by brush, one could work the resin more thoroughly into the surface, thus achieving greater saturation. Conversely, by using a faster evaporating solvent, by adjusting the fineness of the spray, and by varying the distance of the spray gun to the painting surface, it is possible to make the resin appear much more matte than it would otherwise (Bernstein, personal communication August 1995). As mentioned above, fumed silica has also been used to make the resin more matte. B-67 has also been used locally to build up areas of gloss.

(5) OTHER

One conservator discussed using B-67 as a varnish for severely fractured paint surfaces, especially those on checked and flaking panel paintings, because the resin could be applied quite heavily in multiple layers. Once the resin had hardened, it could then be polished down with a very fine grade of steel wool until a smooth, saturating appearance had been achieved. This treatment was developed in the 1970s, and the conservator no longer uses it, as he has doubts about the long-term stability of the resin (B. Rabin, personal communication August 1995).

Aging Characteristics

(1) CHEMICAL PROCESS

PiBMA is known to crosslink on exposure to heat and ultraviolet radiation, with the reactions occurring at the side chains. These reactions can take place even under normal gallery conditions. Once the varnish has begun to age, volatile degradation products are formed which in turn leads to chain breakage (Feller, Stolow, and Jones 1985). However, note that B-67 is much more resistant to crosslinking than Lucite® 2045 (Feller 1996).

(2) RESULTANT CHEMICAL AND/OR PHYSICAL ALTERATIONS

As crosslinking occurs, the resin becomes increasingly insoluble.

(3) IMPACT UPON

(a) Visual Appearance

As the B-67 ages, it is said to become less saturating and can have a very dull appearance. One conservator reported working on a painting which had been varnished approximately 25 years earlier with B-67, and found that the resin was “… crizzled and fractured and was flaking away in places …” (Wallace, personal communication December, 1994). The dust or grime embedded in the surface coating also increased the dullness of the varnish.

(b) Solubility and Removability

The Lucite® 2045 form of PiBMA becomes increasingly difficult to resolubilize, requiring more and more polar solvents, until it reaches a point where at least 50% of the resin is insoluble, at which time the resin can only be swollen. In theory, the resin will ultimately become completely insoluble in organic solvents (Feller, Stolow, and Jones 1985).

(4) ATTRACTION AND RETENTION OF DIRT AND GRIME

Although there has been no formal scientific study quantifying the amount of dirt retained on the surface of a B-67 coating, it will retain more dirt than B-72, for example (see Student Research Project carried out by Patricia O'Regan, Winterthur, 1992). Feller suggests that in the general case, this difference in dirt retention between B-67 and B-72 would not be true (Feller 1996).

(5) THEORETICAL LIFETIME

The studies conducted indicate that the Lucite® 2045 variety of PiBMA will remain soluble for at least 28 years under certain gallery conditions. B-67 is much more stable (see Feller, Stolow, and Jones 1985, 158 and Feller 1976, 142).

Health and Safety

According to the MSDS sheets, the principal routes of exposure to the resin are inhalation, eye contact, skin contact, and dermal absorption, with the problems principally arising from the heated resin product. Inhalation is a problem, causing irritation of the nose, throat, and lungs, as well as nausea, headache, and dizziness. However, the worst effects stem from the solvent used to put the resin in solution. The material is also listed as combustible and “burns vigorously with intense heat.” Under procedures listed in case of an accidental release of the material, caution is recommended to avoid slipping on the loose resin pellets and falling, and all ignition sources should be extinguished. The resin should be stored only in areas equipped with automatic sprinklers, and the minimum and maximum storage temperatures are as follows: 0°C/32°F and 60°C/140°F. Due to the toluene content of the resin pellets (approximately 1.5% maximum), the MSDS sheets recommend that anyone handling the resin wear a respirator equipped with cartridges for organic vapors, chemical resistant gloves, and chemical resistant clothing, and that the area where the material is being handled be adequately ventilated, with an eyewash facility and safety shower located nearby.

Disposal

The MSDS sheets recommend that the resin be incinerated at a facility complying with all local, state, and federal regulations. The United States Department of Transportation considers this material within a class designated as “nonregulated.” No information is given about the material when it has been put into solution in more hazardous organic solvents.

References

Bernstein, J. 1995. Personal communication.

Corle, T. ICI Chemicals. 1995, 1997. Personal communication.

de la Rie, E.R. 1987. The influence of varnishes on the appearance of paintings. Studies in conservation 32(1): 1–13.

Feller, R.L. 1971. Resins and the properties of varnishes. In On picture varnishes and their solvents, R.L. Feller, N. Stolow, and E.H. Jones, eds. Washington, D.C.: National Gallery of Art.

Feller, R.L. 1976. Problems in the investigation of picture varnishes. In Conservation and restoration of pictorial art. Preprints to the 1976 meeting of the International Institute for Conservation of Historic and Artistic Works. N.S. Brommelle and P. Smith, eds. London: Butterworths: 137–44.

Feller, R.L. 1996. Personal communication.

Feller, R.L., N. Stolow, and E.H. Jones. 1985. On picture varnishes and their solvents. Revised and enlarged ed. Washington, D.C.: National Gallery of Art

Gettens, R.J. and G.L. Stout. 1966. Painting materials: A short encyclopedia. New York: Dover Publications.

Horie, C.V. 1994. Materials for conservation: Organic consolidants, adhesives and coatings. London: Butterworth.

Neher, H.T. 1936. Acrylic resins. Industrial and Engineering Chemistry 28:267–71.

O'Regan, P. 1992. Winterthur Student Research Project.

Rabin, B. 1995. Personal communication, August.

Rawlins, F.I.G. and A.E.A. Werner. 1949. The Scientific Department of the National Gallery. Nature 164:601–3.

Wallace, F. 1994. Personal communication, December.

Werner, A.E.A. 1952. Plastics aid in the conservation of old paintings. British plastics 25:363–6.

Paraloid® F-10, Elvacite® 2044, and Plexisol® 550

[poly(n-butyl methacrylate) - PMBA]

a) Historical Background

(1) INDUSTRIAL AND ARTISTIC USE

Poly(n-butyl methacrylate) finds uses in industry as a binder in coatings and inks and as an adhesive according to Tom Corle, Technical Services Representative of ICI Acrylics, Wilmington, DE (Corle 1997). As a varnish for artists' use, the resin has been available since 1938 when Weber introduced its product Synvar® (Feller 1973–74, 12).

(2) CONSERVATION USE

Early n-butyl methacrylate resin, in the form of Lucite® 44, was used experimentally by the Fogg Art Museum in the early 1930s. A common practice in the 1950s was to alternate layers of poly(n-butyl methacrylate) varnish with poly(vinyl acetate) [PVA] inpainting, allowing removal of inpainting without disturbing the varnish (Huston 1996; Keck 1996; Merrill 1996). Conservators treating modern paintings employing media sensitive to the more active solvents have always been interested in varnishes soluble in the hydrocarbons (Potoff 1994, 1996). The usefulness of the poly(n-butyl methacrylate)s as temporary adhesives has long been recognized. Several conservators have spoken of the usefulness of resins such as Elvacite® 2044 or Paraloid® F-10 as temporary facing adhesives because of their softness and ability to conform to impasto, as well as their ease of removal with solvents such as VM&P naphtha (Huston 1996; Potoff 1996). Lascaux® P550–40TB is recommended as a consolidant or impregnation resin (Lascaux 1986, 1989).

Poly(n-butyl methacrylate)s that have been available for conservation use are Paraloid® F-10 and Elvacite® 2044 (formerly Lucite® 44). While Paraloid® F-10 is only available from its manufacturer in solution, Elvacite® 2044 and Plexisol® 550 are available in head form from their respective manufacturers. Proprietary mixtures of poly(n-butyl methacrylate) in solution are also available. These products are Lascaux® P-550–40TB, Gloss, Grumbacher Picture Varnish® (#550–2), Grumbacher New Picture Varnish® Spray, Gloss (#641), Grumbacher Hyplar® Varnish Spray, Gloss (#647), Lefranc & Bourgeois #811, and Lefranc & Bourgeois Vernis á tableaux acrylique (#827). Varnishes made of mixtures of poly(n-butyl methacrylate) and matting agents are Grumbacher New Picture Varnish Spray®, Matte (#642), Lefranc & Bourgeois Vernis mat á tableaux acrylique (#828), Sennelier Vernis acrylique satine mat and Lascaux® P 550–35M, Matte (Williams 1994; Lascaux Restauro 1986; 1989). Krylon® Spray Coating (#41303/41311) has been found to be a mixture of poly(n-butyl methacrylate) and poly(methyl methacrylate). Several varnishes previously thought to consist of poly(n-butyl methacrylate) have been found in recent analyses to contain other resins. These varnishes are Binney & Smith Soluvar® Gloss Picture Varnish and Lefranc & Bourgeois #1186 (Williams 1994).

b) Source

(1) PHYSICAL FORM

The poly(n-butyl methacrylate)s are manufactured as solid, faintly yellow, translucent pellets.

(2) ORIGIN AND MANUFACTURE

Poly(methyl acrylate) was first prepared in 1880. In 1901, Otto Rohm wrote in his doctoral thesis about acrylic acid derivatives and possible polymerization, a process which he later patented in 1915. Paraloid® resins were first marketed by Rohm and Haas in the United States in 1931. (As of summer, 1997, Paraloid® is marketed as Paraloid®, and not Acryloid®, in the United States.)

DuPont began to manufacture Lucite® 44 in 1936 (Weber 1987). According to Lynne Joshi, Reference Librarian at the Hagley Museum, Wilmington, Delaware, the name of the product was changed to Elvacite® 2044 some time during the years 1965 to 1968 (Joshi 1997). The monomer from which poly(n-butyl methacrylate) is derived is the methacrylate ester, CH₂=C(CH₃)COOR, where R= (CH₂)₃CH₃. When Elvacite® 2044 is polymerized in suspension, spheres of resin are produced approximately 232 microns in size.

(3) MANUFACTURERS AND VENDORS

Poly(n-butyl methacrylate) is currently manufactured in the United States as Paraloid® F-10 by Rohm & Haas, Philadelphia, Pennsylvania, as Elvacite® 2044 by ICI Acrylics, Wilmington, Delaware, and in Europe as Plexisol® 550 by Rohm GmbH. Elvacite® 2044 is available as solid pellets. According to Wayne Snape of Rohm & Haas, Paraloid® F-10 is available in a concentration of 40% weight to volume, in a 9:1 mixture, volume to volume, of Shell solvents Varsol® #1 and Amsco® F (evaporation point 150°C) (Snape 1997). Lascaux® Acrylic Resin P550–40TB is in solution of 40% weight to volume in benzine of boiling ranges 100–140°C. P550–35 Matt is supplied in a solution approximately 35% weight to volume in benzine (boiling ranges 100–140°):n-butanol (VM&P naphtha) mixed with pyrogenic silica, which, according to Rolf Burki of Lascaux, is a matting agent of very small particle size made for use with solvent-based varnishes (Lascaux Restauro 1986; Burki 1997). Suppliers of these products in the United States include: a) Talas, New York, NY (Paraloid® F-10, Elvacite® 2044) and b) Conservation Support Systems, Santa Barbara, CA (Paraloid® F-10). [See Appendix II, Vendor Directory.]

c) Chemical and Physical Properties

(1) CHEMICAL CLASSIFICATION

Methacrylate resins are addition polymers of methacrylic acid and their esters (Mills and White 1987, 114–15).

(2) CHEMICAL FORMULA/STRUCTURE

Poly(n-butyl methacrylate)

(3) MOLECULAR WEIGHT

(a) Weight average molecular weight (of Elvacite® 2044)

142,000 by gel permeation chromatography measurement using a polystyrene standard and 337,000 using a poly(methyl methacrylate) standard (Corle 1996).

(b) Number average molecular weight (of Elvacite® 2044)

86,000 by gel permeation chromatography measurement using a polystyrene standard and 95,900 using a poly(methyl methacrylate) standard (Corle 1996). Note that the molecular weight of resin used in Paraloid® F-10 is about 60% of that in Elvacite® 2044 (Feller 1973–74, 13).

(4) REFRACTIVE INDEX

n-butyl methacrylate ⁿD = 1.483 (de la Rie 1987, 3)

Paraloid® F-10 ⁿD = 1.476 (Tennent 1984, 206).

(5) SOLUBILITY

Test results for Elvacite® 2044 and for Paraloid® F-10 are shown in the charts below.

Elvacite® 2044
Shell Odorless Mineral Spirits/Shell Sol® 71	I
Stoddard Solvent/Shell Sol® 340 HT	PIG
Shell Mineral Spirits 145	PIG
Petroleum Benzine	PIG
Turpentine	S
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	S

Xylenes	S
Toluene	S
Isopropanol	PSG
Ethanol	IC
Acetone	S*
Arcosolv® PM/1-Methoxy-2-propanol	SV

^* indicates “yellow.”

Paraloid® F-10
Shell Odorless Mineral Spirits/Shell Sol® 71	I
Stoddard Solvent/Shell Sol® 340 HT	I
Shell Mineral Spirits 145	I
Petroleum Benzine	S
Turpentine	S
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	S

Xylenes	S
Toluene	S
Isopropanol	S
Ethanol	S
Acetone	S
Arcosolv® PM/1-Methoxy-2-propanol	S

Terms

S	Soluble	clear solution
SC	Soluble Cloudy	completely dissolved, slightly cloudy solution
SV	Soluble Viscous	clear, highly viscous solution
SR	Soluble Residue	residue on the bottom
PSG	Partially Soluble Gel	resin is dissolved in a clear soft gel in a two-phase mixture with solvent on top
PIG	Partially Soluble Immobile Gel	resin is dissolved in a hard gel in a two-phase mixture with solvent on top
IM	Insoluble Milky	insoluble but a minute fraction has swollen and whitened, solvent looks milky
IC	Insoluble Clear	insoluble but a small portion of the resin has swollen to the point that it is stuck to the bottom of the jar, solvent is clear
I	Insoluble	insoluble, undissolved resin sits in clear solvent

Examples: S to PSG = mostly soluble; PIG to I = mostly insoluble

See also Appendix I, Solubility Testing Description and Solvents Used in Testing.

(6) GLASS TRANSITION TEMPERATURE (Tg)

Approximate temperature is 22°C (71.6°F) for poly (n-butyl methacrylate). However, Tg varies with molecular weight (Torraca 1968, 313).

(7) BRITTLENESS AND FLEXIBILITY

Polymethacrylates have been shown to be much less brittle than damar or mastic resins (Feller, Stolow, and Jones 1985, 123–4). According to ICI, the tensile strength of Elvacite® 2044 is 3.4 Mpa at 500 psi (23°C/73°F, 50% RH) and the elongation at break is 300% at the same conditions.

d) Preparation/Formulation

(1) TYPICAL BRUSH SOLUTIONS

7–10%, weight to volume, in mineral spirits with an aromatic content of about 18–20% (Carlyle and Bourdeau 1994, Section II, 48).

(2) TYPICAL SPRAY SOLUTIONS

Approximately 15–20% weight to volume in low aromatic solvent, i.e., mineral spirits with an aromatic content of about 18–20% (Carlyle and Bourdeau 1994, Section II, 48). A concentration of 7–10% weight to volume in diluents such as Fisher benzine B-264®, Shell Sol® 332, or Shell Sol® BT-4 has been reported (Miller 1996; Potoff 1996). Evaporation time can be adjusted by small additions of acetone for speedier drying or turpentine for slower drying (Potoff 1996). Slower evaporation rates can also be achieved by adding up to 10% volume to volume of xylene to mineral spirits (Huston 1996).

(3) ADDITIVES

The matting agents found in various commercially available poly(n-butyl methacrylate) formulations have been identified (Williams 1994, Notes on Table 1, note 3). Varnish formulations made by conservators using Elvacite® 2044 or Paraloid® F-10 do not commonly have additives. One unusual instance is that reported by Leni Potoff, who added Irganox® 565, an antioxidant at 1% weight to weight of resin to an Elvacite® 2044 varnish in the late 1970s. The painting varnished with this was recently examined and was found still to be clear and even (Potoff 1996).

(4) SPECIAL CONSIDERATIONS

Conservators interviewed who have used poly(n-butyl methacrylate) in the past as a varnish no longer do so when concern for eventual crosslinking is an issue. Because it softens at 100°F, the tacky varnish surface can attract dust.

(5) STORAGE/SHELF LIFE

Due to the low Tg of resins such as Elvacite® 2044, it is not unusual for the spheres of resin to quickly become block-like masses. Therefore quantities of resin beads should be stored in temperatures lower than room temperature (Corle 1996).

e) Working Characteristics and Practical Properties

(1) APPEARANCE

Varnish coatings are reported to be shiny on drying and are found to become less shiny, though still saturating, in two to three years (Huston 1996; Miller 1996).

(2) BRUSHING

(a) Open working time

Working time is a factor of the aromatic content of the solvent mixture used, i.e., the more aromatic content in the mixture, the longer the working time. To fully saturate a paint film and to prevent lap lines, brush the varnish in several directions (Huston 1996). According to the vendor, if Lascaux® Acrylic Varnish P550–35 Matt is applied by brush, it must be diluted in a mineral spirits or VM&P naphtha of higher boiling point than that used for spray application (Lascaux 1986).

(b) Drying Time

Depends on thickness of varnish applied; the thinnest of layers reportedly dries to the touch in 10 minutes. With good ventilation, a large painting may dry in a day (Miller 1996).

(3) SPRAYING

(a) Open working time

Lascaux recommends applying P550–35 Matt by spray, preferably in a concentration of 10% solids. Varying degrees of matteness can be achieved by mixing with P550–40 TB. Several layers can be applied until an even sheen is achieved. Mineral spirits, benzine, or VM&P naphtha are recommended as solvents (Lascaux 1986). Dilute sprays of Elvacite® 2044 may be applied in multiple layers, allowing each layer to dry for about one hour before spraying again (Potoff 1996). If light coats are applied, they dry too fast to allow any working of the surface (Miller 1996).

(b) Drying Time

Multiple light spray coats dry to the touch almost immediately and allow the conservator to begin inpainting right away (Miller 1996).

(4) MODIFICATIONS FOR SPECIAL APPLICATIONS AND EFFECTS OR TRICKS OF THE TRADE

If shiny areas are visible after drying, they can be dealt with by polishing the fully dried, two-to three-day-old coating with fine grade (0000) stainless steel wool or by applying an additional fine mist of varnish with an airbrush (Huston 1996).

(5) OTHER

According to Lascaux Restauro, P 550–40 TB may be used as a varnish; however, the manufacturer suggests that Lascaux Acrylic Resin® 550/675 is better suited as a varnish due to its slightly higher glass transition temperature and better scratch resistance (Lascaux 1989).

The matting agent in P 550–35M has been known to precipitate out of solution and collect on the surface when sprayed onto absorbent supports (Lascaux 1986).

f) Aging Characteristics

(1) CHEMICAL PROCESS

Poly(n-butyl methacrylate) has been shown to crosslink with accelerated aging and natural light aging. Oxidation and chain-breaking also occur during aging. However, oxidation inhibitors may slow down these effects (Feller, Stolow, and Jones 1985, 157–65).

(2) RESULTANT CHEMICAL AND/OR PHYSICAL ALTERATIONS

Upon artificial aging, methacrylate resins become resistant to removal in solvents such as toluene. The chain-breaking noted above occurs mostly at the top surface of the film and therefore has little effect on the overall solubility of the coating (Lomax and Fisher 1990, 187–8).

(3) IMPACT UPON

(a) Visual Appearance

A 12-year-old surface coating on a Childe Hassam painting was found to look sooty (Huston 1996).

A granular, matte surface has been observed on aged coatings but it is not clear if this is the result of aging or if it is the intended surface (Zuccari 1996).

The varnish has been observed by many to gray over time due to grime absorption.

(b) Solubility and Removability

Artificial aging equivalent to 50 years of daylight exposure of commercial n-butyl methacrylate polymers produced 50–80% insoluble material (Lomax and Fisher 1990, 182). After an estimated 300-year exposure under museum conditions, samples of Paraloid® F-10 were found to still be soluble in toluene (CCI 1988, 1989, ARS 1895). While methacrylate resins continue to be used for conservation purposes, it is important to monitor their reversibility on a regular basis so that removal can be undertaken at an early stage of crosslinking (Feller, Stolow, and Jones 1985, 164). Presumably, crosslinking could continue until these films are completely insoluble (Carlyle and Bourdeau 1994, Section II, 47–8). Conservators who have had occasion to remove naturally aged coatings report no difficulty in doing so after 15 years (Potoff 1996; Heller 1996; Zuccari 1996; Huston 1996) or even 30 years (Keck 1996; Miller 1996), and are able to use solvents such as xylene or toluene alone or in mixtures with VM&P naphtha. The need to add acetone to mixtures or use acetone alone to completely remove naturally aged, 40-year-old coatings has also been reported (Lomax and Fisher 1990, 187). Mark Bockrath reported on being able to remove a 1950 varnish layer with xylene (verbal communication through Barbara Buckley, 1997).

(4) ATTRACTION AND RETENTION OF DIRT AND GRIME

The low glass transition temperature—close to room temperature—of the n-butyl methacrylate resin, accounts for its tendency to attract and retain dirt (Feller, Stolow, and Jones 1985, 146).

(5) THEORETICAL LIFETIME

Under “normal gallery conditions … depending upon the intensity of illumination … films will become about 50% insoluble in toluene in about twenty-five years … and will still be readily removed in solvents that are ‘milder’ than toluene” (Feller, Stolow, and Jones 1985, 164). Most polymethacrylates are resistant to deterioration at normal temperatures except that the poly(n-butyl methacrylate)s and uninhibited poly(isobutyl methacrylate)s tend to crosslink as discussed here and elsewhere (Feller 1996).

g) Health and Safety

The Material Safety Data Sheet for Elvacite® 2044 Acrylic Resin (sold as solid granules) cautions that overheating or processing of this material may evolve into fumes that are irritating on inhalation. Handling the granules themselves may result in dust which can cause eye irritation and respiratory irritation from dust or vapors. The granules are unlikely to cause skin irritation and there is low oral toxicity on ingestion. Protective eyewear and an approved dust mask are recommended if large quantities are to be handled. The material is listed as “combustible but not readily ignited.” However, dust clouds may be explosive. Elvacite® 2044 has a flash point of 300°C (572°F). Appropriate extinguishing media are water spray, foam, dry powder, or carbon dioxide. Recommendations include storage at ambient temperature away from heat sources. If spilled, resin beads can be quite slippery. Spillage should be swept up into waste drums or plastic bags; after sweeping, the spill area should be washed with water.

h) Disposal

The Material Safety Data Sheet calls Elvacite® waste nonhazardous. “Clean scrap may be reprocessed. Incineration may be used to recover energy value. May be disposed of by landfill in accordance with local regulations.”

References

Bockrath, M. 1997. Verbal communication through Barbara Buckley.

Book and Paper Group. 1989. Adhesives: Section 46. In Paper conservation catalog. 6th ed. Washington, D.C.: American Institute for Conservation of Historic and Artistic Works. [Editors: C. Smith, S. Bertalan, A. Dwan, J. English, C. Nicholson, S.R. Albro, K. Schneck, L. Stiber, and S. Wagner).]

Brendley, W.H., Jr. 1973. Fundamentals of Acrylic Polymers. In Paint and Varnish Production. [Rohm & Haas offprint.]

Burki, R. 1997. Facsimile communication. (25 March 1997).

Canadian Conservation Institute. 1988, 1989. Report: Commercial product analytical report, Analytical Research Services. Ottawa: Canadian Conservation Institute.

Carlyle, L. and J. Bourdeau 1994. Varnishes: Authenticity and Permanence. Workshop Handbook. Ottawa: Canadian Conservation Institute, 19–20 September 1994.

Corle, T. 1996. Telephone interview with Jill Whitten. (20 December 1996).

Corle, T. 1997. Telephone interview. (26 March 1997).

de la Rie, E.R. 1987. The Influence of varnishes on the appearance of paintings. Studies in conservation 32 (1):11–13.

Down, J.L. and R.S. Williams. 1988. Unpublished handout report on adhesive testing at the Canadian Conservation Institute. Ottawa: Canadian Conservation Institute (1988 Ottawa Conference).

Feller, R.L. 1957. National Gallery of Art Fellowship Report no. 345–7. (May 31).

Feller, R.L. 1973–74. [From Section 4. Further tests on Soluvar, a brushable varnish]. Quarterly report of the Mellon Institute (November-April):12–13.

Feller, R.L. 1996. Personal communication.

Feller, R.L., N. Stolow, and E.R. Jones. 1985. On Picture varnishes and their solvents. Revised and enlarged ed. Washington, DC: National Gallery of Art.

Heller, B. 1996. Telephone interview. (26 November 1996).

Huston, P. 1996. Telephone interview. (31 October).

Joshi, L. 1997. Telephone and facsimile communications. (26 March 1997).

Keck, C. 1996. Personal correspondence. (5 November).

Lascaux Restauro. 1986. Lascaux® Acrylic Resin P 550–40 TB. R159 (1 August).

Lascaux Restauro. 1989. Lascaux® Acrylic Resin P 550–40 TB R113(1 July).

Lomax, S.Q. and S.L. Fisher. 1990. An Investigation of the removability of naturally aged synthetic picture varnishes. Journal of the American Institute for Conservation 29(2):181–91.

Merrill, R. 1996. Personal communication compiled by Sarah Fisher. (23 October 1996).

Miller, D. 1996. Telephone interview. (1 November 1996).

Mills, J.S. and R. White. 1987. The Organic chemistry of museum objects. London: Butterworths.

Potoff, L. 1994. Telephone interview. (22 September 1996).

Potoff, L. 1996. Telephone interview. (5 November 1996).

Rohm and Haas. 1974. Acryloid® solid grade acrylic resins. Product Literature. Philadelphia: Rohm and Haas.

Rohm and Haas. 1975. Synthetic resins for coatings, Acryloid thermoplastic acrylic ester resins, C335. Philadelphia: Rohm and Haas Company.

Snape, W. 1997. Telephone interview. (1 April 1997).

Tennent, N. and J. Townsend. 1984. The Significance of the refractive index of adhesives for glass repair. In Adhesives and consolidants - IIC Paris Congress Preprints. N.S. Bromelle and P. Smith, eds. London: International Institute for Conservation of Historic and Artistic Works. 205–12.

Torraca, G. 1968. Synthetic materials used in the conservation of cultural properties. The Conservation of cultural properties with special reference to tropical conditions. Paris: UNESCO: 16–20.

Weber, J. 1987. The Development of synthetic polymer resins for artist use. Unpublished student report. New York University, Conservation Center.

Williams, R.S. 1994. Appendix III, Table 1: Composition of commercial artists' varnishes and media. Varnishes: Authenticity and Permanence. Ottawa: Canadian Conservation Institute.

Zuccari, F. 1996. Telephone interview. (5 November 1996).

Proprietary Varnishes Based on Acrylics and Acrylic Blends

Soluvar®

[a proprietary mixture of n-butyl methacrylate and isobutyl methacrylate]

a) Historical Background

(1) INDUSTRIAL AND/OR ARTISTIC USE

Polymers of acrylic resins were developed for use in 1931. The Liquitex® line of acrylics was first developed in 1955 by Henry W. Levison, founder of Permanent Pigments of Cincinnati, Ohio. In 1964 Binney & Smith acquired Permanent Pigments and continued manufacturing the Liquitex® products. Levison continued to develop varnishes for Binney & Smith and in the late 1960s developed Soluvar® as a removable varnish for acrylic paintings, recommended by the manufacturer for artists' use on acrylic polymer paintings after an isolating layer of acrylic gel medium is allowed to dry.

(2) CONSERVATION USE

Early n-butyl and isobutyl methacrylate resins in the form of Lucite® 44 and 45, respectively, were used experimentally by the Fogg Art Museum in the early 1930s. By the early 1950s, reports of their use in conservation treatments were published (Feller, Stolow, and Jones 1985, 135, notes 27, 28, and 29). Since the 1970s, many conservators attracted by the convenience of a premixed varnish have used Soluvar® in treatments.

Conservators who use Soluvar® do so primarily because of the following properties: its solubility in low aromatic hydrocarbons; its ability to be applied thinly, either by brush or spray; and its ability to even out an unevenly saturated paint or varnish surface. Many conservators, however, express concern about its unpredictable content as a proprietary product and/or aesthetic concerns.

b) Source

(1) PHYSICAL FORM

Soluvar® comes prepared as a clear, pale yellow, moderately viscous solution. As distributed, the varnish is 31% solvent, 62% resin, and 7% unspecified material.

(2) MANUFACTURERS AND VENDORS

Liquitex Soluvar® Gloss and Matte Picture Varnishes are manufactured by

Binney & Smith, Inc. 1100 Church Lane Easton, PA 18044.

Soluvar® varnishes can be purchased at retail art material stores. [See Appendix II, Directory of Vendors.]

c) Chemical & Physical Properties

(1) CHEMICAL CLASSIFICATION

Methacrylate resins are addition polymers of methacrylic acid and their esters. According to Binney & Smith, the basic ingredients of Soluvar® are a blend of poly(n-butyl methacrylate) and poly(isobutyl methacrylate) resins in hydrocarbon solvents. The matte varnish contains a synthetic silica flatting agent (Vonderbrink, manufacturer's representative for Binney & Smith, 1984; 1994). It is interesting to note that independent analyses have confirmed the presence of an n-butyl methacrylate polymer, i.e., Paraloid® F-10 some years ago (CCI, ARS 1517.2; Feller 1973–74, 12–13). In a more recent sample, however, the resin was found to be isobutyl methacrylate (Williams 1994 [note 6]).

(2) CHEMICAL FORMULA/STRUCTURE

(3) MOLECULAR WEIGHT

The molecular weight changes depending on the resin content. In an early 1970s comparison, Feller found its molecular weight to be low for its group, about 60% of that of the straight poly (n-butyl methacrylate) used by DuPont in Elvacite® 2044 (Feller 1973–74, 13).

(4) REFRACTIVE INDEX

n-butyl methacrylate ⁿD = 1.483; isobutyl methacrylate ⁿD = 1.477 (de la Rie 1987, 3); Paraloid® F-10 ⁿD = 1.476; Paraloid® B-67 ⁿD = 1.486 (Tennent 1984, 206).

(5) SOLUBILITY

Soluvar Glossy
Shell Odorless Mineral Spirits/Shell Sol® 71	I
Stoddard Solvent/Shell Sol® 340 HT	I
Shell Mineral Spirits 145	I
Petroleum Benzine	S
Turpentine	S
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	S

Xylenes	S
Toluene	S
Isopropanol	S
Ethanol	S
Acetone	S
Arcosolv® PM/1-Methoxy-2-propanol	S

Terms

S	Soluble	clear solution
SC	Soluble Cloudy	completely dissolved, slightly cloudy solution
SV	Soluble Viscous	clear, highly viscous solution
SR	Soluble Residue	residue on the bottom
PSG	Partially Soluble Gel	resin is dissolved in a clear soft gel in a two-phase mixture with solvent on top
PIG	Partially Soluble Immobile Gel	resin is dissolved in a hard gel in a two-phase mixture with solvent on top
IM	Insoluble Milky	insoluble but a minute fraction has swollen and whitened, solvent looks milky
IC	Insoluble Clear	insoluble but a small portion of the resin has swollen to the point that it is stuck to the bottom of the jar, solvent is clear
I	Insoluble	insoluble, undissolved resin sits in clear solvent

Examples: S to PSG = mostly soluble; PIG to I = mostly insoluble

See also Appendix I, Solubility Testing Description and Solvents Used in Testing.

(6) GLASS TRANSITION TEMPERATURE (Tg)

The Tg relates to the component resins. Approximate temperatures are 22°C (71.6°F) for n-butyl methacrylate and 50°C (122°F) for isobutyl methacrylate (Torraca 1968, 313).

(7) BRITTLENESS AND FLEXIBILITY

Polymethacrylates have been shown to be much less brittle than damar or mastic resins (Feller, Stolow, and Jones 1985, 123–4).

d) Preparation/Formulation

(1) TYPICAL BRUSH SOLUTIONS

Commonly Soluvar® is diluted from the jar for brush application, usually 1:1 (in petroleum benzine). For slower evaporation, English distilled turpentine has been recommended in the same proportions (Potoff, 1994). However, that increases its tendency to crosslink and yellow (Feller 1996). Mineral spirits such as Shell MS® 145 can achieve the same results. This concentration is also maintained if Gloss and Matte are mixed. The manufacturer recommends that mixing Gloss and Matte is possible to “any proportion for desired degree of gloss. May be thinned up to 25% with mineral spirits (naphtha) or turpentine.”

(2) TYPICAL SPRAY SOLUTIONS

Solutions of Soluvar® Gloss, diluted 10% to 25% by volume from the bottle are used depending on working properties desired.

(3) ADDITIVES

The silica flatting agent in Soluvar® Matte is suspended in the varnish as a thickened gel (Vonderbrink 1994). Aluminum stearate has also been detected (Williams 1994, note 8).

4) SPECIAL CONSIDERATIONS

Analysis carried out on Soluvar® Gloss in 1978 found it to be composed of poly(n-butyl methacrylate). A sample analyzed in 1988, however, was found to be poly(isobutyl methacrylate).“… [I]f different base resins are used, they may be dissolved in different solvents, and mixing solutions of different resins/solvent composition could create stability or handling problems (e.g., precipitation)” (Williams 1994, note 6).

(5) STORAGE/SHELF LIFE

Anecdotal evidence suggests that Soluvar® Gloss may turn gray after long storage and that Soluvar® Matte may yellow and the matting agent may separate. Conflicting evidence suggests that if carefully stored they retain their original characteristics indefinitely. The matting agent, however, does come out of solution rapidly when the Matte varnish is diluted; only an amount for immediate use should be prepared.

e) Working Characteristics and Practical Properties

(1) APPEARANCE

Soluvar® Gloss dries clear and glossy; Soluvar® Matte dries clear and matte. Acrylic varnishes are less saturating than low molecular weight varnishes. This may be due to the relatively low refractive index for acrylic resins (Mills and White 1987, 114–15) and their polymeric nature (Torraca 1968).

(2) BRUSHING

(a) Open/Working time

Excellent brushability is attributed to the high boiling range of its solvent (178–180°C) as well as to its relatively low molecular weight compared to Lucite® 2044 (Feller 1973–74, 12–13). Working time depends on diluent selected. A mixture 1:1 by volume in petroleum benzine remains workable approximately 30 seconds to two minutes on an absorbent surface, and two to four minutes on a nonabsorbent surface.

(b) Drying Time

Surface is usually dry to the touch within an hour, although complete evaporation of the solvent will take much longer.

(3) SPRAYING

(a) Open working time

The working time depends on the diluent selected. With petroleum benzine the varnish becomes unworkable with a brush almost immediately.

(b) Drying Time

The drying time depends on the diluent selected. With petroleum benzine drying time is very fast.

(4) MODIFICATIONS FOR SPECIAL APPLICATIONS AND EFFECTS/TRICKS OF THE TRADE

On some paintings, a “satin” finish which is crisp but not too glossy yet not plastic looking, can be achieved by brushing a 1:1 matte/gloss mix, itself diluted 1:1 in petroleum benzine (Tomkiewicz 1994). Conservators who use Soluvar® point out that its solubility in petroleum benzine makes it useful on some modern paintings (Lodge 1994). Others use Soluvar® over painting surfaces where an original varnish layer has been partially removed from a solvent-sensitive, resinous paint film. Soluvar® is applied on the assumption that it will remain more soluble in the partially removed, solvent-sensitive original varnish below it for at least 50 years (Tomkiewicz 1994). Light sprays of Soluvar® Gloss can be effective in locally saturating areas of sunken in or blanched medium in otherwise unvarnished oil paintings.

(5) OTHER

Diluted from the jar with petroleum benzine, 1:1, Soluvar® can be applied to large surfaces with some difficulty by rag application, a procedure which must proceed continuously across the surface without going back into the area already coated (Tomkiewicz 1994). Working time may be increased by diluting it with a slow-evaporating mineral spirit such as Shell MS® 145 rather than petroleum benzine.

f) Aging Characteristics

(1) CHEMICAL PROCESS

N-butyl methacrylate, and to a greater degree, isobutyl methacrylate have been shown to crosslink with accelerated aging and natural light aging. Oxidation and chain-breaking also occur during aging. However, oxidation inhibitors may slow down these effects (Feller, Stolow, and Jones 1985, 157–65). [See also Section VII. A. “Phenolic Antioxidants, Stabilizers, and UV Absorbers.”]

(2) RESULTANT CHEMICAL AND/OR PHYSICAL ALTERATIONS

Upon aging, removal of methacrylate resins, particularly isobutyl methacrylate, requires more polar solvents such as toluene (Feller, Stolow, and Jones 1985, 157–65). The chain-breaking noted above occurs mostly at the top surface of the film and therefore has little effect on the overall solubility of the coating (Lomax and Fisher 1990, 187–8). Aging tests on Soluvar® Gloss and Matte varnishes revealed that samples dissolved only partially in a 50/50 mixture of acetone and toluene after 2,275 hours in a fadometer (ca. 100,000 lux) (de la Rie 1988). In 1988 Binney & Smith claimed that instances of insoluble Soluvar® were due to “a bad batch.” Note that as a proprietary product, Soluvar® formulations can change without notice (Lodge 1994).

(3) IMPACT UPON

(a) Visual Appearance

Due to the resins used in this varnish, it will imbibe dirt and become gray.

(b) Solubility and Removability

Aged coatings will require increasingly polar solvents for removal. Their removal may pose a risk to some oil paintings and acrylic emulsion paintings. Artificial aging equivalent to 50 years of daylight exposure of commercial n-butyl and isobutyl polymers produced 50–80% insoluble material (Lomax and Fisher 1990, 182). While methacrylate resins continue to be used for conservation purposes, it is important to monitor their reversibility on a regular basis so that removal can be undertaken, if needed, at an early stage of crosslinking (Feller, Stolow, and Jones 1985, 164). Conservators who have had occasion to remove aged methacrylate resins report solvent mixtures ranging in polarity from petroleum benzine and xylene (4:1) to straight toluene (Lodge 1994; Lomax and Fisher 1990, 187). After only eight years a Soluvar® coating on an acrylic emulsion painting hanging in a public setting was no longer soluble in petroleum benzine and therefore the coating could not be safely removed (Samet 1995).

(4) ATTRACTION AND RETENTION OF DIRT AND GRIME

Soluvar® has been found to accumulate dust (Feller 1973–74, 13). This may be attributed to the low glass transition temperature of the resins.

(5) THEORETICAL LIFETIME

Accelerated aging of n-butyl and isobutyl methacrylates revealed that a period equivalent to 50 years under normal museum conditions resulted in 50–80% insoluble material (Lomax and Fisher 1990, 181).

g) Health and Safety

The Material Safety Data Sheets list no hazardous ingredients for Gloss; however, methyl alcohol is listed for Matte. According to the manufacturer's representative methyl alcohol is used to put the matting agent in suspension; it evaporates from the mixture before bottling takes place (Vonderbrink, 1994). The flash point of these varnishes is 120°F and dry chemical extinguishers (foam or carbon dioxide) are recommended. Inhalation of the varnishes can cause “respiratory irritation, dizziness, headache”; exposure to the skin and eyes can result in irritation; and ingestion is “harmful.” Overexposure can result in eye irritation, skin edema, dizziness, and central nervous system depression. Pre-existing medical conditions such as skin, eye, and respiratory disorders may be aggravated by exposure as well. Relating to carcinogenicity, the following statement is offered: “The ingredients of this product are on the Toxic Substances Control Act (TSCA) inventory.” Protective measures are recommended when using these products. A NIOSH-certified respirator for high exposures; neoprene gloves and safety glasses or goggles when handling large quantities; a window exhaust fan to remove vapors; and adequate cross-ventilation are recommended.

h) Disposal

The Material Safety Data Sheets refer to the latest EPA or state regulations.

References

Book and Paper Group. 1989. Adhesives: Section 46. In Paper Conservation Catalog. 6th ed. Washington, D.C.: American Institute for Conservation of Historic and Artistic Works. [Editors: C. Smith, S. Bertalan, A. Dwan, J. English, C. Nicholson, S.R. Albro, K. Schneck, L. Stiber, and S. Wagner.]

Brendley, W.H. 1973. Fundamentals of acrylic polymers. In Paint and Varnish Production.

CCI. 1988, 1989. Report: Commercial Product Analytical Report, Analytical Research Services. Ottawa: Canadian Conservation Institute.

de la Rie, E.R. 1987. The Influence of varnishes on the appearance of paintings. Studies In conservation 32 (1):1–13.

de la Rie, E.R. 1988. Letter to Brooklyn Museum (October).

Down, J.L. and R.S. Williams. 1988. Unpublished Handout Report on “Adhesive Testing at the Canadian Conservation Institute.” CCI Ottawa Conference.

Feller, R.L. 1973–74. [From Section 4. Further tests on Soluvar, a brushable varnish.] Quarterly report of the Mellon Institute (November-April): 12–13.

Feller, R. L., N. Stolow, and E.H. Jones. 1985. On Picture varnishes and their solvents. Revised and enlarged ed. Washington, D.C.: National Gallery of Art.

Lomax, S.Q. and S.L. Fisher. 1990. An Investigation of the removability of naturally aged synthetic picture varnishes. Journal of the American Institute for Conservation 29:181–91.

Lodge, R. 1994. Telephone interview (22 September).

Mills, J.S. and R. White. 1987. The Organic chemistry of museum objects. Sevenoaks: Butterworths.

Pomerantz, L. Conservators advise artists. Art journal 37(1, Fall):34–5.

Potoff, L. 1994. Telephone interview (22 September).

Rohm and Haas. 1974. Acryloid® Solid Grade Acrylic Resins (Product Literature) Philadelphia: Rohm and Haas.

Rohm and Haas. 1975. Synthetic resins for coatings, Acryloid® Thermoplastic Acrylic Ester Resins, C335 Philadelphia: Rohm and Haas Company.

Rosston J. 1981. Acrylic paints, media and varnishes. Unpublished report (May 7).

Samet, W. 1995. Telephone interview (17 September).

Tennent, N. and J. Townsend. 1984. The Significance of the refractive index of adhesives for glass repair, In Adhesives and Consolidants - Preprints of the IIC Paris Congress, 2–8 September 1984. N.S. Brommelle and P. Smith, eds. London: International Institute for Conservation of Historic and Artistic Works:205–12.

Tomkiewicz, C. 1994. Facsimile communication (24 October).

Torraca, G. 1968. Synthetic materials used in the conservation of cultural properties. In The Conservation of cultural properties with special reference to tropical conditions. Paris: UNESCO.

Vonderbrink, T. 1984. Letter to K. Moser (6 April). The Brooklyn Museum files, Brooklyn, NY.

Vonderbrink, T. 1994. Telephone interview (31 October).

Williams, R. S. 1994. Appendix III, Table 1: Composition of commercial artists' varnishes and media. Varnishes: Authenticity and Permanence. Ottawa: Canadian Conservation Institute.

Golden MSA® Varnish

[proprietary butyl methacrylate polymers]

(Mineral spirits Soluble Acrylic) Varnish With UVLS (ultraviolet light stabilizers), in Gloss, Satin, and Matte formulations. This varnish is primarily n-butyl methacrylate with some amount of isobutyl methacrylate, according to the manufacturers. The exact proportions of n-butyl methacrylate to isobutyl methacrylate, the exact solvent, the exact stabilizers, and their proportions were not released by the manufacturer.

a) Historical Background

According to Golden Artist Colors, the MSA® Varnish product was first introduced in 1982. The stabilizers were added in 1985. It is intended to be used as an artist's varnish.

b) Source

(1) PHYSICAL FORM

Supplied as a solution of 33% resin solids solubilized in mineral spirits (less than 8% aromatics), MSDS lists aliphatic petroleum hydrocarbon [CAS# (Chemical Abstracts Service Registered Number): 8032–32–4], Stoddard Solvent® [CAS#: 8052–41–3], and Aromatic® 150 [CAS#: 64742–94–5]. Available in the following forms:

(a) MSA® Varnish with UVLS (Golden's “standard” product line)

Gloss (Product No. 07730)

Satin (Product No. 07735)

Matte (Product No. 07740)

(b) Custom MSA® Varnish

Golden will produce Custom MSA® Varnishes to specific recipes upon request.

(2) ORIGIN AND MANUFACTURE

Manufactured by:

Golden Artist Colors, Inc. Bell Road New Berlin, NY 13411 Phone: (607) 847–6154 Fax: (607) 847–6767

The company chooses not to reveal the specific sources for the raw materials used in the product.

(3) MANUFACTURERS AND VENDORS

Distributed by Conservation Support Systems, Santa Barbara, California. Also available at local art supply stores, especially those stocking other Golden Artist Colors products. [See Appendix II, Directory of Vendors.]

c) Chemical and Physical Properties

(1) CHEMICAL CLASSIFICATION

Acrylic polymer mixture, primarily n-butyl methacrylate with some isobutyl methacrylate and added stabilizers. Exact composition not released by the manufacturer.

(2) CHEMICAL FORMULA/STRUCTURE

See Section VB.3. Poly(n-butyl methacrylate), where information on the chemical structures of n-butyl methacrylate and isobutyl methacrylate can be found.

(3) MOLECULAR WEIGHT

See Section VB.3. Poly(n-butyl methacrylate).

(4) REFRACTIVE INDEX

See Section VB.3. Poly(n-butyl methacrylate).

(5) SOLUBILITY

Shell Odorless Mineral Spirits/Shell Sol® 71	I
Stoddard Solvent/Shell Sol® 340 HT	I
Shell Mineral Spirits 145	I
Petroleum Benzine	S
Turpentine	S
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	S

Xylenes	S
Toluene	S
Isopropanol	S
Ethanol	S
Acetone	S
Arcosolv® PM/1-Methoxy-2-propanol	S

Terms

S	Soluble	clear solution
SC	Soluble Cloudy	completely dissolved, slightly cloudy solution
SV	Soluble Viscous	clear, highly viscous solution
SR	Soluble Residue	residue on the bottom
PSG	Partially Soluble Gel	resin is dissolved in a clear soft gel in a two-phase mixture with solvent on top
PIG	Partially Soluble Immobile Gel	resin is dissolved in a hard gel in a two-phase mixture with solvent on top
IM	Insoluble Milky	insoluble but a minute fraction has swollen and whitened, solvent looks milky
IC	Insoluble Clear	insoluble but a small portion of the resin has swollen to the point that it is stuck to the bottom of the jar, solvent is clear
I	Insoluble	insoluble, undissolved resin sits in clear solvent

Examples: S to PSG = mostly soluble; PIG to I = mostly insoluble

See also Appendix I, Solubility Testing Description and Solvents Used in Testing.

The MSDS lists the solvent components of the product as: Aliphatic petroleum hydrocarbon [CAS#(Chemical Abstracts Service Registered Number): 8032–32–4], Stoddard Solvent [CAS# 8052–41–3], and Aromatic 150 [CAS# 64742–94–5]. The solvent blend contains less than 8% aromatics according to the manufacturer.

(6) GLASS TRANSITION TEMPERATURE (Tg)

See Section VB.3. Poly(n-butyl methacrylate).

(7) BRITTLENESS AND FLEXIBILITY

From cylindrical mandrel test results reported on Golden Artist Colors'Technical Data Sheet; ASTM D-522, Test Method B (1-second bend duration, 70°F): “3.5 mil thick film passes at two inches diameter mandrel.”

(8) HARDNESS

From Golden Artist Colors'Technical Data Sheet, “ASTM D-3363, Film Hardness By Pencil Test, Scratch Hardness is ‘HB’ (Gloss varnish).”

d) Preparation/Formulation

(1) TYPICAL BRUSH SOLUTIONS

“When brushing, thin with 10–20% mineral spirits or turpentine” (Product label). “Thinning required prior to use. Start with a ratio of 3 parts varnish to 1 part solvent for brushing” (Golden Artist Colors'Technical Data Sheet).

(2) TYPICAL SPRAY SOLUTIONS

“When spraying, thin with up to 100% mineral spirits” (product label).

“Thinning required prior to use. Start with a ratio of between 1 and 2 parts varnish per part solvent for spraying” (Golden Artist Colors'Technical Data Sheet).

(3) ADDITIVES

According to a representative of Golden Artist Colors, UVLS products contain <1% of a hindered amine light stabilizer (HALS) listed by the supplier as “Bis(1,2,2,6,6-Pentamethyl-4-Piperidinyl) Sebacate, mixture.” The MSDS lists “2-(2'-Hydroxy-3',5'-Di-Tert-Amylphenyl) Benzotriazole” (an ultraviolet light absorber sold by Ciba-Geigy Corp. as Tinuvin® 328) as being present. The matting agent is an “Amorphous Silica.”

(4) SPECIAL CONSIDERATIONS

Many “Day-Glo” and “fluorescent” paints (as well as related products containing “optical-brighteners”) depend on the conversion of ultraviolet radiation into visible wavelengths of light for their visual effects. A varnish that contains an ultraviolet light absorber might cause undesirable changes in the appearance of these paints.

(5) STORAGE/SHELF LIFE

Keep away from heat, sparks, and open flame.

Maximum storage temperature: 60°C/140°F

Minimum storage temperature: -18°C/0°F

According to a Golden Artist Colors representative, the shelf life of these products is indefinite. However, research by de la Rie indicates that the effectiveness of UVLS is compromised when stored mixed into solvent-based varnish solutions. [See also Section VII.A. Phenolic Antioxidants, Stabilizers, and UV Absorbers.]

e) Working Characteristics and Practical Properties

(1) APPEARANCE

The appearance is similar to other methacrylate polymer resin varnishes with equivalent viscosities/molecular weights and solvent systems. The Golden Artist Colors' Product Description Sheet states, “The satin MSA® Varnish offers moderate reflection, similar to most matte varnishes. Golden's Matte finish is exceptionally flat.” [Personally, I like the appearance of a very thin spray coat of the Satin MSA® Varnish as a final layer over more saturating hydrocarbon resin layers (such as Laropal® A-81 or Regalrez® 1094), or as a final surface over a layer of Paraloid® B-72 sprayed on top of inpainting.]

(2) BRUSHING

(a) Open/Working time

Similar to mineral spirits but can be varied by the addition of faster or slower evaporating, compatible solvents.

(b) Drying Time

The product label states that the drying time is “3–6 hours. Varies with humidity. Allow 6 hours to dry between coats. Let varnish ‘cure’ for several days before stacking or moving art.”

(c) Solvent system mixtures containing solvents with differing evaporation rates, especially if one or more of the components is a poor solvent for the varnish resins (such as purely aliphatic hydrocarbons), may result in problems in workability during extended brushing out of the varnish.

(3) SPRAYING

(a) The stock varnish solutions are too viscous to be effectively sprayed and therefore must be thinned with appropriate solvents prior to spray application (see “Typical spray solutions” above).

(b) Clumping of the amorphous silica matting agent contained in the Satin and Matte formulations can result in an occasional white speck that is large enough to be noticeable.

(4) MODIFICATIONS FOR SPECIAL APPLICATIONS AND EFFECTS/TRICKS OF THE TRADE

Ultraviolet light absorbers will make paintings appear dark when examined under ultraviolet illumination.

(5) OTHER

(a) The product description sheet states that Matte, Satin, and/or Gloss types “can be intermixed or used sequentially to achieve the desired sheen.”

(b) The product description sheet states “Apply in thin layers. Use as a top coat only. Do not paint over. If applying multiple coats and a Matte finish is desired, thinly apply Gloss coats first, finish with Matte.”

f) Aging Characteristics

See Section V.B.3. Poly(n-butyl methacrylate), where information on the aging characteristics of n-butyl methacrylate and isobutyl methacrylate can be found (See also Feller 1976, 142).

g) Health and Safety

(1) Use precautions appropriate for mineral spirits Stoddard Solvent® and Aromatic 150 (OSHA time weighted average, permissible exposure limit: 100 ppm), and observe warnings listed on the product labels.

(2) The product's hazardous material identification rating for health is 2 (moderate hazard: exposure may result in temporary or minor injury), and for flammability is: 2 (moderate hazard: Flash point is between 100°F and 200°F). The reactivity rating is 0 (minimal hazard: Stable under most conditions), incompatible with strong oxidizers.

h) Disposal

Dispose of as per guidelines and regulations for hazardous waste and combustible liquids. RCRS #D-001 (ignitable); reportable quantity 100 lbs. (SARA/Superfund).

References

Feller, R.L. 1976. Problems in the investigation of picture varnishes. In Conservation and restoration of pictorial art. Preprints to the 1976 meeting of the International Institute for Conservation of Historic and Artistic Works. N.S. Brommelle and P. Smith, eds. London: Butterworths: 137–44.

Feller, R.L. 1996. Telephone Conversation.

Feller, R.L., N. Stolow, and E.H. Jones. 1971. On Picture varnishes and their solvents. Revised and enlarged ed. Cleveland, Ohio: The Press of Case Western Reserve University.

Golden Artists Colors. Product Information Sheet. Manufacturer's Safety Data Sheet, Technical Data Sheet, Product Label.

Rohm and Haas. Acrylic Resins for Aqueous and Solvent Industrial Coatings, Table 1: Typical Properties of Acryloid® Thermoplastic Resins.

Winsor & Newton Conserv-Art® Varnish

[A proprietary mixture of an acrylic resin and a ketone, Conserv-Art® Varnish is a synthetic resin varnish manufactured by Winsor & Newton and is 9 parts isobutyl methacrylate to 1 part polycyclohexanones]

a) Historical Background

An advertising leaflet states, “Developed following thirty years of research carried out by leading conservators” (from Colart Americas, Inc., the American distributor for Winsor & Newton). Conversations with Wendall Upchurch, Winsor & Newton's technical representative in the US, relate that this new varnish is an outgrowth and further refinement of its well known synthetic varnish, Winton, which has been commercially available in North America and the United Kingdom for over 40 years. (See entry IV.D.1 Artists' Gloss Varnish). Augmented by published research and consultations with conservation scientists, Winsor & Newton spent years formulating Conserv-Art® Varnish, which they introduced first in the UK and soon after in the US in 1990. (Upchurch 1994).

(1) INDUSTRIAL AND/OR ARTISTIC USE

Conserv-Art® Varnish was not created for use in industry, and the authors are not aware of any industrial applications for this varnish. Conserv-Art® is advertised by Winsor & Newton as a “superior quality artist varnish for use with oil, alkyd or acrylic colors…” (Winsor & Newton, Leaflet: Conserv-Art Varnishes). As an artist's varnish, Conserv-Art® is considered by the manufacturers to be superior to its predecessor Winton.

(2) CONSERVATION USE

Winsor & Newton states that Conserv-Art® Varnish was created in consultation with conservation scientists (Winsor & Newton Technical Leaflet). However, Alun Foster, Chief Chemist at Winsor & Newton, said that Conserv-Art® was not invented for conservation use (Foster 1996). It was intended to improve on the Winton formula and serve as a replacement for natural resin artists' varnishes.

b) Source

Conserv-Art® Varnish is available in ready-to-use 75 ml and 250 ml bottles. It is manufactured by Winsor & Newton, and distributed by artists' suppliers in the US and the UK.

c) Chemical and Physical Properties

(1) CHEMICAL CLASSIFICATION

Conserv-Art® is a mixture of Rohm and Haas Acryloid® B-67 [now Paraloid® B-67 and referred to thus hereafter] resin (isobutyl methacrylate), ketone resin (polycyclohexanone, Laropal® K80), and at 30% solids in mineral thinners, low aromatic hydrocarbon solvent, and Isopar® H. The proportion of the resins is nine parts isobutyl methacrylate to one part polycyclohexanone. In addition, the following additives are present: Irganox® 1010,a phenolic heat stabilizer; Tinuvin® 328, a UV absorber; and Carstab® DLTDP, a dilauryl thiodipropionate heat stabilizer for the primary antioxidants (Foster 1996).

Since Conserv-Art® is made up of two distinct resins, calculations of molecular weight, etc., are difficult and may not reflect the exact behavior of the varnish based on the characteristics of the resins.

Paraloid® B-67 is a tough, flexible resin. Ketone resin produces a more fragile and brittle film that can be easily abraded. According to the manufacturer, this combination of resins results in a strong and aesthetically pleasing coating that will not bloom or crack (Upchurch 1994; 1995).

(2) PARALOID® B-67 OR ISOBUTYL METHACRYLATE RESIN STRUCTURE AND MOLECULAR WEIGHTS

Please see Section V.B.2. “Paraloid® B-67.”

(3) KETONE OR POLYCYCLOHEXANONE RESIN STRUCTURE AND MOLECULAR WEIGHTS

See Section IV.C. “Ketone Resin Varnishes.”

(4) REFRACTIVE INDEX

Isobutyl methacrylate resin has a refractive index of 1.486 (Horie 1987, 183).

Ketone resin has a higher refractive index, 1.515 (Horie 1987, 183).

The refractive index of Conserv-Art® varnish is not known at this time.

(5) GLASS TRANSITION TEMPERATURE

Isobutyl methacrylate resin: 55°C (Horie 1987, 183).

Ketone resin: 100°C (Horie 1987, 183).

The Tg of Conserv-Art® varnish has not been made available.

(6) SOLUBILITY

The manufacturer claims that the film will remain soluble in mineral spirits for up to 100 years.

Shell Odorless Mineral Spirits/Shell Sol® 71	S
Stoddard Solvent/Shell Sol® 340 HT	S
Shell Mineral Spirits 145	S
Petroleum Benzine	S
Turpentine	S
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	S

Xylenes	S
Toluene	S
Isopropanol	S
Ethanol	S
Acetone	S
Arcosolv® PM/1-Methoxy-2-propanol	S

Terms

S	Soluble	clear solution
SC	Soluble Cloudy	completely dissolved, slightly cloudy solution
SV	Soluble Viscous	clear, highly viscous solution
SR	Soluble Residue	residue on the bottom
PSG	Partially Soluble Gel	resin is dissolved in a clear soft gel in a two-phase mixture with solvent on top
PIG	Partially Soluble Immobile Gel	resin is dissolved in a hard gel in a two-phase mixture with solvent on top
IM	Insoluble Milky	insoluble but a minute fraction has swollen and whitened, solvent looks milky
IC	Insoluble Clear	insoluble but a small portion of the resin has swollen to the point that it is stuck to the bottom of the jar, solvent is clear
I	Insoluble	insoluble, undissolved resin sits in clear solvent

Examples: S to PSG = mostly soluble; PIG to I = mostly insoluble

See also Appendix I, Solubility Testing Description and Solvents Used in Testing.

(7) BRITTLENESS AND FLEXIBILITY

The authors did not conduct testing.

The manufacturers state that the since the formulation of the varnish is part Paraloid® B-67 and part ketone resin, the resultant flexibility is characteristic of a combination of these two resins (Upchurch 1994). Since the authors usually apply a final coat of Paraloid® B-72 over their Conserv-Art® lower layers, they have little practical experience on the comparative brittleness or flexibility of Conserv-Art®.]]

d) Preparation and Formulation

(1) TYPICAL BRUSH DILUTION

Conserv-Art® handles well in brush varnish applications; dilute with a small amount of mineral spirits if desired. Experience has shown that dilution for brushing should not exceed 1:1.

(2) TYPICAL SPRAY DILUTION

For spraying, dilute to desired consistency with mineral spirits. A typical recipe is to thin the varnish with VM & P naphtha at 10% of the initial varnish volume. This proportion would have to be varied depending on humidity and temperature while spraying. The authors rarely use Conserv-Art® as a spray coat.

(3) ADDITIVES

The manufacturer states that Tinuvin® 328, an ultraviolet absorber; and Irganox® 1010, an antioxidant, are present as additives to prevent yellowing and oxidation. Carstab® DLTDP is added to increase the lifespan of the Tinuvin® 328 and Irganox® 1010 (Foster 1996). (Ed. Note: The effectiveness of this additive combination has not yet been validated by conservation scientists.)

(4) STORAGE/SHELF LIFE

Winsor & Newton claims that shelf life is indefinite if the bottle remains tightly sealed to prevent drying. The manufacturer advises keeping the bottle in the cardboard box to reduce light exposure and prolong the effective life of the ultraviolet absorber (Upchurch 1995).

The addition of the Carstab® DLTDP should help to prolong the stability of the antioxidant and ultraviolet absorber (Foster 1996). However, de la Rie states that hindered amine light stabilizers, such as Tinuvin® 292, should be added just prior to application as the shelf life of these products is short (de la Rie 1989).

e) Working Characteristics and Practical Properties

(1) APPEARANCE

Conserv-Art® Gloss provides good saturation of color and contrast. When used undiluted, it forms a glossy and clear film which allows for the viewing of sharp details. Conserv®-Art Gloss and Conserv-Art® Matte varnishes may be mixed to achieve intermediate gloss. Gloss and sheen can also be controlled by amount of dilution. A visual comparison of varnishes on a test painting finds Conserv-Art® varnish to have an intermediate sheen. Samples of Conserv-Art®, Arkon® P-90, Soluvar® Gloss, Soluvar® Matte, Winton®, and damar varnish were used in this visual comparison. All samples were brush-applied in one coat. Conserv-Art® appeared slightly less shiny than damar, Arkon® P-90, and Winton. Soluvar® Gloss and Soluvar® Matte were more matte in appearance than Conserv-Art® Gloss Varnish.

(2) BRUSHING

This varnish handles well in brush applications. It is important to apply brush coats wet-on-wet in a confident technique to avoid hot spots, caused by wet-on-dry overlap strokes. The manufacturer claims that the varnish dries in two hours (Upchurch 1994). Even though the varnish is dry to the touch in that time, it may be presumed that complete solvent evaporation has not occurred.

(3) SPRAYING

Sprays well when thinned with mineral spirits.

(4) MODIFICATIONS FOR SPECIAL APPLICATIONS AND EFFECTS/TRICKS OF THE TRADE

Conserv-Art® Gloss Varnish may be applied thinly with a small, nearly dry, sable brush to locally saturate inpainting which has dried matte. Conserv-Art® Gloss Varnish is a good isolating varnish for achieving saturation for correct color matching of inpainting for paintings to be varnished (Blakney 1995).

f) Aging Characteristics

(1) CHEMICAL PROCESS

The chemical aging characteristics of both Paraloid® B-67 and ketone resin are known (see Sections IV.C. and V.B.2. for further information). Some testing on the combination of these resins has been carried out at the National Gallery of Art (de la Rie 1995) but more comparative testing is needed. It is not known if the presence of the ketone resin will effect crosslinking in Paraloid® B-67. Nor is it known how Paraloid® B-67 affects the rapid oxidation of a ketone resin. In the best case scenario, the two resins work together to improve overall solubility and chemical stability. However, current research by conservation scientists is inconclusive as conservation scientists and manufacturers have opposing views (de la Rie 1995; Upchurch 1995).

(2) IMPACT UPON SOLUBILITY AND REMOVABILITY

Due to the recent introduction of this varnish, its aging characteristics have not yet been tested.

(3) ATTRACTION AND RETENTION OF DIRT AND GRIME

The Tg of the dried, aging film is not known. Because the predominant component is Paraloid B-67®, its surface characteristics may be closest to B-67 (see Section V.B.2.e)).

(4) THEORETICAL LIFETIME

Accelerated aging tests performed by Winsor & Newton indicated that Conserv-Art® Varnish has a theoretical lifetime of 100 years (Winsor & Newton did not disclose their test methods [Winsor & Newton Technical Leaflet]). Some conservation scientists are skeptical of these manufacturer's claims (de la Rie 1995).

g) Health and Safety from MSDS

The MSDS cautions that high concentrations of the vapor may be irritating to the eyes and respiratory system and may cause drowsiness, nausea, and sickness. Contact with skin may cause irritation. The MSDS includes first aid recommendations for eye and skin contact, inhalation, and ingestion. If contact with skin occurs, washing with soap and water is recommended. If eye contact occurs, flush eyes with water for 15 minutes. In the case of inhalation, remove the subject from exposure. Do not induce vomiting after ingestion.

h) Disposal from MSDS

Dispose of as flammable waste in airtight containers, according to local regulations.

References

Blakney, S. 1995. Personal communication.

Carlyle, L and J. Bourdeau. 1994. Varnishes Authenticity and Permanence, Workshop Handbook (Ottawa: CCI): Polycyclohexanones, 52; Butyl Methacrylates, 47;. Suppliers Section II, 57–9.

de la Rie, E. R. 1995. Personal communication.

de la Rie, E.R. and C.W. McGlinchey. 1989. Stabilized dammar picture varnish. Studies in conservation 34(3):137–46.

Feller, R.L., N. Stolow, and E.H. Jones. 1971. On picture varnishes and their solvents. Revised and enlarged ed. Ohio: The Press of Case Western University.

Foster, A., Chief Chemist at Winsor & Newton UK. 1996. Personal Communication.

Horie, C.V. 1987. Materials for conservation: Organic consolidants, adhesives and coatings. London: Butterworths.

Rohm & Haas. undated. Acryloid resins (brochure published by the manufacturer).

Upchurch, Wendall (Winsor & Newton Technical Advisor). 1994. Personal communication.

Upchurch, W. 1995. Personal communication.

Winsor & Newton. Leaflet, Conserv-Art Varnishes (published by the manufacturer).

Winsor & Newton. undated. Materials Safety Data Sheet: Conserv-Art Varnish.

Winsor & Newton. undated. Technical Sheet #17, Solvents and Varnishes.

Poly(Vinyl Acetate) Varnish (PVA)

Poly(vinyl acetate) [PVA] resins are high molecular weight, thermoplastic resins that have been commercially available since the 1930s. Because of their excellent stability, flexibility, and adhesive properties, PVA resins are widely used in commercial and industrial applications, mostly in emulsion form. They are available in a wide range of molecular weights which produce resins with varying physical properties of hardness, flexibility, and viscosity. PVA resins were among the first synthetic materials to be used in conservation, and since their introduction, have been used extensively as varnishes, consolidants, and adhesives.

a) Historical Background

(1) INDUSTRIAL AND/OR ARTISTIC USE

The main industrial and domestic uses for polyvinyl resins are as adhesives for commercial packaging processes, wood adhesives (“white glue”), surface coatings for interior and exterior polymer emulsion “latex” paints, as pigment binders and coatings for paper, and for strengthening and finishing woven textiles.

(2) CONSERVATION USE

The first documented use of PVA in conservation dates to 1932 when it was used as a facing adhesive for the transfer of a fresco (Stout and Gettens 1932, 107–12). Shortly thereafter, PVA was proposed as a varnish for paintings because of its “water white transparency and distensibility” (Gettens 1935, 24). At the Fogg Art Museum, PVA varnish was applied to many of the paintings in the collection from the 1930s to the 1970s. During that time, it was also used in a number of other collections in the United States often in conjunction with other varnishes, probably most commonly with a poly(n-butyl methacrylate) final coat. It does not, however, appear to have found much favor as a varnish in Europe (Mills and White 1994, 132). After the late 1970s its use as a varnish diminished greatly. Currently it is rarely used as the sole varnish on a painting. Some conservators use it between other varnishes to even out paint surfaces prior to the application of a final top coat of a different varnish (Carlyle and Bourdeau 1994, 54) (see also Talens Picture Varnish, Section IV.D.2., page 104, for its use as an interlayer with ketone resin varnishes).

b) Source

(1) PHYSICAL FORM

PVA resins are available in colorless translucent bead form or dissolved in a solvent (e.g., acetone) as a concentrated solution.

(2) ORIGIN AND MANUFACTURE

The vinyl acetate monomer was first prepared by Klatte in 1912 (Gettens and Stout 1966, 74). In 1917 industrial production was begun in Germany and Canada (Mills and White 1987, 130). Commercial production in the United States was well underway by the 1930s. Vinyl acetate is made by the oxidative addition of acetic acid to ethylene. This reaction is carried out in the presence of a catalyst containing palladium and copper salts (Daniels 1985, 1126). Vinyl acetate polymerizes quickly in the presence of radical initiators and ultraviolet light. During its production the polymerization process can be stopped at any stage resulting in resins having different viscosities and molecular weights. Industrially, polymerization of vinyl acetate has been carried out by solution, bulk, suspension, and emulsion techniques. The majority of poly(vinyl acetate) material is produced in the emulsion form. Poly(vinyl acetate) dry resins (homopolymers) make up less than 10% of the current industrial production (Daniels 1985, 1227).

(3) MANUFACTURERS AND VENDORS

PVA is produced by some 40 to 50 companies worldwide. Union Carbide, the largest supplier in the United States, produces the Vinylite® AYA series. Other brand names include Gelva® (Shawinigan Products Corp. Canada); Mowilith® (Farberwerke Hoechst AG, Germany); Vinnapas® (Wacker-Chemie GmbH, Germany); Vinalak® (Vinyl Products, Ltd., England); Rhodopas® (Société des Usines Chimiques Rhone-Poulenc, France); Vinavil® (Società Rhodiatoce, Italy) (Feller, Stolow, and Jones 1985, 227). See also Appendix II, Directory of Vendors.

c) Chemical and Physical Properties

(1) CHEMICAL CLASSIFICATION

Vinyl acetate is an unsaturated ester that contains the vinyl group:

(2) CHEMICAL FORMULA/STRUCTURE

Poly(vinyl acetate) is composed of the repeating units of the vinyl acetate monomer whose structure is:

Polymerization of the monomer produces a long chained polymer that may contain up to 20,000 units and whose structure is:

(3) MOLECULAR WEIGHTS

PVA resins are available in a wide range of molecular weights. Low molecular weight PVAs are highly viscous liquids or soft, low melting solids. As the molecular weight is increased, the tensile strength, ability to elongate, and melting point of the resin are also increased, resulting in a tougher resin. It has been noted, however, that properties such as refractive index and density remain constant, and hardness is changed only slightly within a series (Feller, Stolow, and Jones 1985, 127). The molecular weights of the Union Carbide AYA series are found in the table below and are ranked in order of increasing degree of polymerization (top to bottom).

Molecular Weights of Various PVA Resins (Union Carbide 1989)

AYAC	5,848	12,800
AYAB	not found	8,200
AYAA	31,691	83,000
AYAF	51,370	113,000
AYAT	not found	167,000
= number-average molecular weight
= weight average molecular weight

(4) REFRACTIVE INDEX

ⁿD = 1.4669 @ 20.7°C (Union Carbide 1989)

(5) SOLUBILITY

PVA resins are soluble in a range of solvents including aromatic hydrocarbons, alcohols containing some water (5%), ketones, esters, glycols, and chlorinated hydrocarbons. They are swollen slightly by water, and are insoluble in aliphatic hydrocarbons.

PVA-AYAC

Shell Odorless Mineral Spirits/Shell Sol® 71	I
Stoddard Solvent/Shell Sol® 340 HT	I
Shell Mineral Spirits 145	I
Petroleum Benzine	I
Turpentine	I
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	PSG
Xylenes	S
Toluene	S
Isopropanol	PSG
Ethanol	PSG
Acetone	S
Arcosolv® PM/1-Methoxy-2-propanol	S

PVA-AYAA

Shell Odorless Mineral Spirits/Shell Sol® 71	I
Stoddard Solvent/Shell Sol® 340 HT	I
Shell Mineral Spirits 145	I
Petroleum Benzine	I
Turpentine	I
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	PIG
Xylenes	PSG
Toluene	S
Isopropanol	PIG
Ethanol	PIG
Acetone	S
Arcosolv® PM/1-Methoxy-2-propanol	S

PVA-AYAF

Shell Odorless Mineral Spirits/Shell Sol® 71	I
Stoddard Solvent/Shell Sol® 340 HT	I
Shell Mineral Spirits 145	I
Petroleum Benzine	I
Turpentine	I
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	PIG
Xylenes	PSG
Toluene	S
Isopropanol	PIG
Ethanol	PIG
Acetone	S
Arcosolv® PM/1-Methoxy-2-propanol	S

PVA-AYAT

Shell Odorless Mineral Spirits/Shell Sol® 71	I
Stoddard Solvent/Shell Sol® 340 HT	I
Shell Mineral Spirits 145	I
Petroleum Benzine	I
Turpentine	I
Shell Cyclo Sol® 100/Shell Cyclo Sol® 53	PIG
Xylenes	PSG
Toluene	S
Isopropanol	PIG
Ethanol	PIG
Acetone	S
Arcosolv® PM/1-Methoxy-2-propanol	S

Terms

S	Soluble	clear solution
SC	Soluble Cloudy	completely dissolved, slightly cloudy solution
SV	Soluble Viscous	clear, highly viscous solution
SR	Soluble Residue	residue on the bottom
PSG	Partially Soluble Gel	resin is dissolved in a clear soft gel in a two-phase mixture with solvent on top
PIG	Partially Soluble Immobile Gel	resin is dissolved in a hard gel in a two-phase mixture with solvent on top
IM	Insoluble Milky	insoluble but a minute fraction has swollen and whitened, solvent looks milky
IC	Insoluble Clear	insoluble but a small portion of the resin has swollen to the point that it is stuck to the bottom of the jar, solvent is clear
I	Insoluble	insoluble, undissolved resin sits in clear solvent

Examples: S to PSG = mostly soluble; PIG to I = mostly insoluble

See also Appendix I, Solubility Testing Description and Solvents Used in Testing.

(6) GLASS TRANSITION TEMPERATURE (Tg) (Union Carbide 1989)

AYAC	Tg°: 16°C
AYAB	Tg°: 17°C
AYAA	Tg°: 21°C
AYAF	Tg°: 24°C
AYAT	Tg°: 26°C

(7) BRITTLENESS AND FLEXIBILITY

Poly(vinyl acetate) resins form highly flexible films which exhibit cold flow for long periods (Horie 1987, 93) and thus retain their flexibility for many years (Lewis 1995).

elongation at break (at 20°C and 0% RH),%	10–20
modulus of elasticity, GPad	1.275–2.256
tensile strength, MPd	29.4–49.0
(Values from Daniels 1985, 1227)

(8) CRACKING OF PVA RESIN FILMS (Measured at 21°C, 50% RH)

Resin	Diameter of Mandrel, in Inches, Necessary for Cracking
Poly(vinyl acetate) AYAB	0.6
Poly(vinyl acetate) AYAA	<0.1
(from Feller, Stolow, and Jones 1985, 124)

d) Preparation/Formulation

It is recommended that for single varnish coat applications, PVA resins of viscosity grades between 4 and 15 be used (between AYAA and AYAF, Union Carbide grades) (UNESCO 1968, 311). Those with molecular weights/viscosity grades above and below that range should be avoided because they are either too soft or too viscous in solution. Historically, the first type of PVA varnish used at the Fogg Art Museum was Vinylite® A (Bakelite Division of Union Carbide). This resin is similar to the present day AYAF grade.

A review of treatment records at the Fogg indicates that the standard PVA resin grade was AYAF. This was applied in a number of ways to the surfaces of paintings in the collection. It was often applied as the only varnish, either brushed or sprayed. Sometimes the PVA varnish was coated with a layer of hard wax or with a poly(n-butyl methacrylate). In fewer instances PVA-AYAC in toluene was brush applied as an isolating layer followed by an AYAF top coat, or the AYAC coating, which has a lower molecular weight and somewhat glossier appearance, was applied over an AYAF layer to saturate the surface. In one instance, a PVA varnish was applied over a methacrylate resin varnish layer. (Ed. Note: Other conservators cite AYAB as the most commonly used resin for varnishing, replaced later by a mixture of 50% AYAF/50% AYAC when AYAB was no longer available.)

(1) TYPICAL BRUSH APPLICATION

For brushing, the standard PVA formula used at the Fogg Art Museum from the late 1930s to the early 1970s was:

(a) PVA dissolved 10g–20g/100ml in MI2 solvent. The composition of the MI2 solvent, which has a slower evaporation rate than ethyl alcohol, is:

toluene	70%
ethyl alcohol	10%
ethylene dichloride	10%
Cellosolve®	5%
Cellosolve® Acetate	5%
(from Gettens, 1935, 19)

This solvent formula was also used extensively for spray applications.

(2) TYPICAL SPRAY APPLICATIONS

For spraying, the standard PVA recipes used at the Fogg Art Museum from the late 1930s to the early 1970s were as follows: (Ed. note: The reader must exercise caution with the use of ethyl alcohol on certain types of paintings.)

(a) PVA in ethyl alcohol:

10 g PVA (AYAF) dissolved in 100 ml ethyl alcohol

(b) Vinylite® A - spray solution:

PVA (10g/100ml) in ethyl alcohol	500 ml
Cellosolve® Acetate	100 ml
diacetone alcohol	35 ml
(from Bradley 1950, 120.06)

(3) ADDITIVES

Because of its stability and natural flexibility, additives were not commonly included in typical PVA varnish formulas to alter its physical characteristics. Additives such as wax were not used because PVA varnishes are more matte than natural resin varnishes. However, wax was often applied over PVA coatings in the belief that the wax would protect the paint layers against humidity (Ruhemann 1968, 270).

(4) STORAGE/SHELF LIFE

PVA resins exhibit cold flow over long periods at room temperature. The resin beads, especially the lower viscosity grades, have a tendency to cake into a solid mass. The dry resin should therefore be stored in small quantities in a cool environment. While the resin is very stable, it should not remain dissolved in a solvent for a long period of time. It is recommended that PVA dispersions be used within six to nine months (Baer, Indictor, and Phelan 1971, 37) so perhaps PVA in a solvent should not be kept longer than that period of time.

e) Working Characteristics and Practical Properties

(1) APPEARANCE

As soon as they were introduced in conservation, it was noted that PVA resin varnishes produced surface coatings that had optical properties that were different from those of the natural resin varnishes. PVA varnishes have a lower refractive index than do the natural resin varnishes. This property, along with significantly higher molecular weights results in a surface coating that does not saturate, level out, and darken paint layers to the same degree as damar or mastic. Furthermore, PVA varnishes are also less glossy. When compared with a number of other synthetic resin and natural resin varnishes, PVA varnishes produce intermediate gloss and distinctness of image (D/I) values over substrates with low gloss and sometimes can even exhibit the lowest gloss and D/I values over substrates of high gloss (de la Rie 1988, 12). Although used rather frequently in America from the 1950s through the 1970s, their matte appearance and low saturation have made them, in the minds of most painting conservators, unsuitable for old master paintings. PVA varnishes have therefore been removed from paintings for these aesthetic issues. On the other hand, these properties might make them suitable for some 20th-century paintings where darkening and saturation are undesirable.

(2) BRUSHING

For brushing, PVA resins should be dissolved in slower evaporating solvents to increase the working time (see recipes above). Therefore, a PVA in ethanol solution should not be used for brush varnishing because the working time is greatly reduced. (Ed. note: and because it could damage certain types of paint layers.)

(3) SPRAYING

When spraying, the PVA resin can be dissolved in mixtures of various slow and fast evaporating solvents. Of most concern in this operation are the drying rates of the solvents and the distance at which the spray gun is held from the surface of the painting. If held too far away, the resin can be deposited as distinct granules because the solvent has more time to evaporate before it is deposited on the paint layer. This results in an appearance that is very matte and rough. Substitution of fast evaporating solvents with slow evaporating solvents can greatly improve the saturating effects (Feller, Stolow, and Jones 1985, 141).

Spraying can provide greater versatility in the application of various layers of different varnishes than brushing techniques. PVA varnishes can be used as isolating layers for an original surface and areas of retouching. The PVA varnish can then be used to produce a consistent foundation over an otherwise porous and uneven paint onto which a more aesthetically pleasing varnish could be applied. As an intermediate layer between retouchings and a top layer of a hydrocarbon-soluble polymer such as Paraloid® B-67, the top layer can then be safely removed in the future without disrupting the PVA varnish or the underlying retouchings (Hulmer 1972) (See also Section IV.D.2., Talens [Rembrandt] Picture Varnish, p. 99).

Gustav Berger presently markets a PVA spray varnish to be used as an isolating layer to fix retouches and to protect them prior to the application of a suitable finishing varnish. The varnish is ready to use, that is, should be sprayed without dilution at 40 psi pressure and is available from Conservator's Products Company, Chatham NJ (See Appendix II: Directory of Vendors).

(4) MODIFICATIONS FOR SPECIAL APPLICATIONS AND EFFECTS/TRICKS OF THE TRADE

PVA varnishes can be applied as an interlayer locally over a matte or absorbent area.

f) Aging Characteristics

(1) CHEMICAL PROCESS

PVA resins have excellent aging properties and are among the most stable polymers used in conservation. Thompson (1963) indicated that there was no evidence to suggest that these resins crosslink. Subsequent experiments indicated that, “polyvinyl acetate has an extremely low tendency to cross-link in the fadeometer and in sunlight” (Feller 1963, 174). Although minor chemical alterations can occur (Horie 1987, 92), PVA resins are resistant to oxidation and are inert to near ultraviolet and visible light radiation (Kirk-Othmer 1985, 1227).

(2) RESULTANT CHEMICAL AND/OR PHYSICAL ALTERATIONS

No resultant chemical or physical alterations have been noted.

(3) IMPACT UPON VISUAL APPEARANCE/SOLUBILITY AND REMOVABILITY

The stability of PVA resins result in varnish films that do not yellow. They can however, take on a grayish appearance caused by dirt becoming embedded in the soft varnish film (see following paragraph). They remain fully soluble, as evidenced by light aging tests (Feller 1963) and by solubility tests on naturally aged samples which indicated that Vinylite® A (similar to AYAF) was easily dissolved after 30 years of natural aging by the solvents in which it was originally soluble (Feller, Stolow, and Jones 1985, 171–5). PVA resin varnishes that were applied to paintings at the Fogg Art Museum more than 30 years ago have recently been removed with toluene and acetone.

(4) ATTRACTION AND RETENTION OF DIRT AND GRIME

The glass transition temperatures of PVA resins range from slightly below to slightly above room temperature (see Tg° table above). Because of their softness, dust and dirt are easily embedded in the varnish film and cannot be removed with aqueous cleaning solutions. This is especially so with low viscosity grades such as the AYAC, AYAB, and AYAA series (Union Carbide). PVA-AYAF, which was the PVA resin most frequently used for varnishing at the Fogg, has a Tg° just above room temperature (24°C) but it too develops a grayish appearance as it picks up dust over time.

(5) THEORETICAL LIFETIME

PVA was classified by Feller as a class A resin with a theoretical lifetime of > 100 years (Feller 1978).

(6) OTHER

It has been noted by some that, over time, some PVA varnishes have developed adhesion problems with the paint layers to which they were applied. Thomson (1957) reported that the adhesive strength of PVA varnishes was poor and at the Metropolitan Museum it was noted that “the varnishes show little adhesive strength and can often be peeled off easily” (de la Rie 1988, 16). (Perhaps this is caused by the high molecular weight and the long lasting flexibility of PVA resin over a brittle and aged paint film, thereby resulting in shearing?). It was noted by both authors that this did not necessarily mean that the adhesion was incomplete at the time the varnish was applied.

g) Health and Safety

Poly(vinyl acetate) is a nontoxic material that is approved by the FDA for the packaging of food (Kirk-Othmer 1985, 1226). Many of the solvents that are used to dissolve PVA are toxic. MSDS should be referred to when dissolving the resin.

h) Disposal

PVA resins are to be disposed by incineration in a furnace or otherwise disposed of in accordance with appropriate federal, state, and local regulations.

References

Baer, N.S., N. Indictor, and W.H. Phelan. 1971. An Evaluation of adhesives for use in paper conservation. Guild of Book Worker's journal 10(1):17–35.

Billmeyer, F.W. 1984. Textbook of polymer science. 3d ed. New York: John Wiley and Sons.

Bradley, M.C.,Jr. 1950. The Treatment of pictures. Cambridge, Mass.: Art Technology.

Carlyle, L and J. Bourdeau. 1994. Varnishes: Authenticity and permanence. Workshop Handbook. Ottawa, Canada: Canadian Conservation Institute.

Daniels, W. 1985. Poly(vinyl acetate). In Kirk-Othmer Concise encyclopedia of chemical technology. 3d ed. M. Grayson et al., ed. New York: Wiley-Interscience Publications: 817–47.

Doolittle, A.K. 1941. Vinyl resins. In Protective and decorative coatings. J.J. Mattiello, ed., Volume 1. Washington, D.C.: U.S. Government Printing Office: 433–6.

de la Rie, E.R. 1988. Stable varnishes for Old Master Paintings. PhD Oral Dissertation, University of Amsterdam. The Netherlands. Meppel: Krips Repro Meppel, B.V.

Feller, R.L. 1957. Factors affecting the appearance of picture varnish. Science. 125(3258): 1143–4.

Feller, R.L. 1963. 1963. New solvent type varnishes. In Recent advances in conservation. Contributions to the Rome Conference, 1961. G. Thomson, ed. London: Butterworths: 171–5.

Feller, R.L. 1972. Problems in the investigation of picture varnishes. In Conservation of paintings and the graphic arts, Lisbon Congress, International Institute for Conservation of Historic and Artistic Works, 1972. London: Butterworths: 201–9.

Feller, R.L. 1978. Standards in the evaluation of thermoplastic resins. In ICOM Committee for Conservation. 5th triennial meeting, Zagreb, 1–8 October 1978. Preprints. Paris: ICOM: 78/16/4.

Feller, R.L., N. Stolow, and E.H. Jones. 1985. On picture varnishes and their solvents. Revised and enlarged ed. Washington, D.C.: National Gallery of Art.

Gettens, R.J. 1935. Polymerized vinyl acetate and related compounds in the restoration of objects of art. Technical studies in the field of fine arts 4(1):15–27.

Gettens, R.J. and G.L. Stout. 1966. Painting materials: A short encyclopedia. Unabridged and corrected 1942 ed. New York: Dover Publications.

Horie, C.V. 1987. Materials for conservation: Organic consolidants, adhesives and coatings. London: Butterworths.

Hulmer, E.C. 1972. Notes on the formulation and application of acrylic coatings. In Conservation of paintings and the graphic arts, Lisbon Congress, International Institute for Conservation of Historic and Artistic Works, 1972. London: Butterworths: 211–13.

Lewis, J.S. (Customer Services Area Specialist, Union Carbide Corporation). 1995. Telephone Communication.

Mills, J.S. and R. White. 1994. The Organic chemistry of museum objects. Sevenoaks: Butterworths.

Ruhemann, H. 1968. The Cleaning of paintings: Problems and potentialities. London: Faber and Faber; New York: Frederick A. Praeger.

Skeist, I. 1977. Handbook of adhesives. New York: Van Nostrand Reinhold Company.

Stout, G.L. 1966. Repair in retrospect. Fenway Court 1(2):9–15.

Stout, G.L. and R.J. Gettens. 1932. Transport des fresques orientales sur des nouveaux supports. Mouseion 13/18:107–12.

Thomson, G. 1957. Some picture varnishes. Studies in conservation 3:64–78.

Thomson, G. 1963. New picture varnishes. In Recent advances in conservation, G. Thomson, ed. London: Butterworths: 176–84.

UNESCO. 1968. The Conservation of cultural property with special reference to tropical conditions. Paris: UNESCO.

Union Carbide. 1989. Polyvinyl acetate resins for coatings and adhesives. Danbury, CT: Union Carbide Chemicals and Plastics Company Inc.

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^*Viscosity in centipoises at 70°C 20% solids in toluene: B-67, 18; 2045, 55 (Feller Stolow, and Jones 1985, 122).

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