4 VINYL-BASED COMMERCIAL FILLERS: GENERAL DISCUSSION

Many vinyl-based fillers are available in the building supply and hardware markets. Three commonly mentioned for use in objects conservation include DAP Vinyl Spackling, Polyfix, and Polyfilla Fine Surface (from England). Another, Perma-fill Ready Mixed Spackle, was unavailable at the time of writing. In January 1997, CPC began to offer BEVA Gesso-P and BEVA Gesso-V as part of its product line. BEVA Gessoes are the first fillers designed specifically for conservation use. These gessoes, which vary from the other vinyl commercial fillers, are based on resins rather than emulsions or dispersions.

The vinyl emulsion fillers contain a polyvinylacetate dispersion or emulsion as the vehicle or binder. The bulking agent is usually calcium carbonate in the form of marble flour or ground limestone. Other ingredients may include biocides and thickeners. Pigments are not added to the vinyl-based fillers as frequently as in acrylic modeling pastes.

The vinyl emulsion fillers can be applied with a spatula as supplied. A drop of water can be added to make the mix softer or creamier. They cannot be easily or successfully thinned with water enough to be brushed on. The addition of adequate water to permit brush application results in a very weak and powdery layer. Tools and surfaces can be cleaned with water. Unlike acrylic-based fillers, vinyl emulsion fillers remain readily water-soluble after drying. The products contain sufficient filler or bulking agents relative to the quantity of vehicle to make sanding or abrasion easy. The resultant films or fills are less flexible and elastic than the modeling pastes. The appearance of the fills is matte but gentle burnishing can slightly increase the gloss and density of the surface.

The BEVA Gessoes are the first commercial vinyl resin fillers designed solely for conservation use. The products have not been available long enough for an adequate evaluation for use on objects, but they appear to have great potential. The setting occurs by evaporation of organic solvents from the bulked resin mixture rather than water from emulsions or dispersions. While the use of resins makes the products toxic to handle when wet, it also makes them indefinitely resoluble and reworkable. The vehicle or binder is based on hydrogenated hydrocarbon resins and ethylene-vinylacetate copolymer (EVA). Adhesive tests have shown that BEVA 371, a comparable vinyl adhesive to those used in BEVA Gesso, is stable (Down et al. 1996). The gesso shares solubility characteristics with BEVA 371; it is soluble in petroleum distillates, ketones, and aromatic hydrocarbons but insoluble in water.

4.1 VINYL COMMERCIAL FILLERS: PRODUCT-SPECIFIC INFORMATION

DAP Vinyl Spackling Compound—DAP Vinyl Spackling Compound has been available for about 20 years. However, during that time the formulation has changed many times, according to a technical representative from DAP, Inc. Volatiles by volume equal about 24%. The dried product is not plastic or flexible, but hard and brittle relative to most of the acrylic modeling pastes.

DAP Fast N' Final—DAP Fast N' Final is a relatively new product. Volatiles by volume equal about 25–30%. The dried fill is lightweight; air bubbles are present beneath the skin. The dried fill is lighter than and floats in water. Finishing the surfaces exposes the voids, making a very smooth surface difficult to achieve. The filler is incompatible with strong oxidizers and caustics.

Polyfix—The MSDS reveals very little information. Practically, the dried fill is much coarser in texture and whiter than Polyfilla. The Polyfix is more comparable to DAP Spackling Compound but is still slightly coarser in texture. It is easily wet or dry sanded to yield a level surface.

Polyfilla Fine Surface, tub—Polyfilla Fine Surface in a tub, manufactured by the Polycell Corporation, has been reported on more than any of the other fillers. The information is still very generic, exemplifying the need for scientific analysis of the commercial products. Caley (1993) lists the basic formulation and the following explanation:

A complete formula was not divulged, but the basis of the material can be summarized:

Fine powder fillers including: ground limestone, ground marble.

Binder: acrylic VeoVa-PVA copolymer, internally plasticized

Additives: cellulose thickeners, boracide preservative, higher alcohols, glycol ethers, amine to raise pH

The “mystery ingredient,” assumed by some to be oil, is VeoVa 10, a product of Shell Resins, described by the company as the vinyl ester of Versatic 10, a synthetic saturated monocarboxylic acid mixture of highly branched C10 isomers, its structure is represented as:

R1, R2, and R3 are alkyl groups (of general formula Cn H2n+1 {univalent}) of which one or more is methyl… . Amongst its notable properties are excellent adhesion and wetting, affording the necessary high CPCV (critical pigment/volume concentration).

A recipe for a similar conservator-prepared fine surface filler and further references for VeoVa are also included in Caley's article, which is not readily available in the United States. Qualitatively, Polyfilla Fine Surface in a tub has the smoothest dried surface and finest particle size of the vinyl fillers. The matte surface is a creamier color than the other vinyl spackles, not including the BEVA Gessoes. Polyfilla Fine Surface is not sold in the United States. There is confusion regarding the name. British manufactured Polyfilla is supplied in two different forms: in a tub and in a tube. A completely different plaster and cellulose fiber filler, which is purchased as a dry powder, is distributed also under the name Polyfilla and is manufactured in Canada and supplied by Conservator's Emporium, Reno, Nevada.

Polyfilla Fine Surface, tube—The only ingredients listed on the MSDS are biocides, which are toxic, corrosive, and harmful to the environment. Other ingredients are not listed. The filler, which is supplied in a tube, is more plastic than the tub version.

BEVA Gesso-P, fine-grained and BEVA Gesso-V, medium-grained—Fine-grained BEVA Gesso-P is grayish when applied but dries white in color. BEVA Gesso-V is coarse, sandy in texture, and beige to brown in color when dry. The gessoes adhere well to most surfaces, better than any of the other products. The bulking agent is identified as a chemically inert compressible mineral powder, a crystalline material having high porosity, large volume, and low density. Both BEVA Gesso-V and BEVA Gesso-P effervesce in dilute mineral acid and seem to contain a carbonate compound, which is not listed as a specific ingredient in the manufacturers' literature. The BEVA Gesso-V contains a reflective particle that appears to be micalike.

Drying occurs more rapidly than with the fillers containing water. The odor of toluene and/or xylenes is strong when working with the wet fillers. Protection from solvent vapors during working is absolutely necessary. Although volatiles are lost and some cracking and distortion of the filler occur during drying, the actual change in volume or shrinkage is small due to the high percentage of bulking agent to vehicle.

The final surfaces are initially matte but can be burnished to a very high gloss. The thermoplastic gesso is easily shaped using heat (65–70�C = 149–158�F). Impasto can be mimicked by shaping with a hot needle microtool (Tomkiewicz 1997). Patterns and textures are readily impressed using heat, pressure, or molds. The impressions are fully retained, and the gesso seems to have little plastic memory. The fills can be finished by sanding, carving, or solvent smoothing.

Fills should be isolated with an alcohol-based coating prior to inpainting, if using a solvent-based system. Inpainting can be executed in acrylic emulsions, synthetic resins including polyvinyl acetates and Paraloids, and natural resins, such as dammar or shellac. Watercolor and gouache are not recommended (Chludzinski 1997; Conservator's Products Company 1997a; Tomkiewicz 1997).

The manufacturer's instructions for use advise kneading plaster of paris into the gesso to create a “claylike” mass for filling. After the addition of just a small amount of plaster, the gesso began to crumble when worked. A drop of BEVA 371 needed to be added to improve cohesiveness.