7 ORGANIC BINDERS

Often, a repair system that incorporates an organic binder with transparent or translucent fill materials is found to be the most successful method of emulating a translucent stone like alabaster. A search through the chemical industry's current technical literature on “artificial stone” reveals that most industrial approaches incorporate a multiple organic resin system with organics as binder and filler. Although some treatments using this approach have been successful in the outdoors (Colton 1996), this organic binder-filler system is most widely used in museum contexts where the organic components are not subject to intense UV exposure.

Organic systems can cure either by solvent evaporation (for example, a methacrylate in solution in acetone), phase transition (the cooling of a melted wax), or chemical reaction (the cross-linking of an epoxy). Each type of system has its advantages and disadvantages. Solvent-based systems are often stringy, difficult to work, and exhibit high shrinkage upon evaporation of the solvent. Proper hardness and texture can be difficult to achieve with thermoplastic systems and organic reaction-cured systems. Reaction-cured resins are often excessively strong, toxic, have high shrinkage, are difficult to reverse, and unstable to environmental exposure.

Many combinations of organic resins in organic solvents have been attempted. Shellac diluted in alcohol was popular before the advent of modern materials. Plenderleith and Werner (1988) describe a putty made with nitrocellulose in acetone and amyl acetate, plus added white sand. This recipe was recommended where an anhydrous, nonshrinking fill was needed. “AJK Dough” is a traditional putty made with polyvinyl acetate resin as a binder (Cornwall 1965) used by archaeologists on a variety of artifacts where nonaqueous fills are required.

Today, thermoplastic resin and solvent-based fills often use a more stable acrylic resin such as Paraloid B-72 (an ethyl methacrylate/methylacrylate copolymer) or polymethyl methacrylate (PMMA) as a binder. Variation of the solvent composition can tailor the evaporation and thus the working time of the mix. Unless extensively bulked with an appropriate filler, shrinkage is still a problem. Mixing dry resin powder with the aggregate, then adding an appropriate solvent is one means of achieving minimal shrinkage (Domaslowski and Strzelczyk 1993). Experimentation with other resins continues among conservators. Cyanoacrylate mixed with granulated methacrylates and stone flour was reported as having “the density closest to the stone”(Yakhont 1991) of many adhesives mixtures tested.

Aqueous organic binders have also been used for fill materials on stone. “Gesso” coatings and putties made of glue and added chalk or stone dust have been found on polychrome stone sculpture from a number of periods and cultures. Traditional scagliola recipes are based on a glue binder (Ashurst 1979). Modern acrylic emulsions are used in commercially made artists' modeling pastes such as Liquitex (Pocobene 1994). The slight resiliency afforded such a fill by the acrylic emulsion binder makes it useful for specific applications, but shrinkage and introduction of water to the substrate can be problematic.

An alternative to the aqueous or solvent-based organic mixes is the use of a solvent-free organic binder applied thermoplastically. With no solvent present, shrinkage upon evaporation of a carrier is not a concern. A traditional example of this type of fill is the use of tinted shellac sticks, applied with a torch to preheated, dark-colored stones where the yellow-brown color is not a distraction (Kibby 1996). Hempel (1968) introduced the use of a polyvinyl acetate melted directly onto a stone object for fills, a method which has been modified (Burke 1996; Colton 1996) and published since (G�nsicke and Hirx 1997). The revised method consists of a mix of ethylene acrylic acid copolymers (Allied Signal AC-540 and AC-580) with the PVA AYAC. To impart light-stability to the mix, an antioxidant (Irganox) is added. The mix produces a transparent and colored fill material, which can be manipulated by the addition of other fillers like pigments and marble dust. This mix has been used on marble and alabaster objects in indoor and outdoor contexts over the last 15 years(Colton 1996). The only significant deficiencies appear to be the inability to fill a small-scale hairline loss or shallow, spalled surface and the potential for cold-flow when applied without support in large-scale losses.

Epoxy systems may also be utilized when a strong fill is required for a translucent stone. Epoxies are a class of synthetic resins characterized by a molecular structure with a highly reactive oxirane ring. The oxirane ring acts as the mechanism for cross-linking the polymer chains when catalyzed by an amine hardener. Epoxy resins are generally more expensive than other thermosetting resins. They resist common solvents, oils, and chemicals, are inert, have high mechanical strength, exhibit negligible shrinkage, and can be formulated to have a wide variety of properties, such as resiliency and heat resistance (Brady 1991). Much has been written regarding their use in conservation (Selwitz 1993; Kotlik 1983). The most light-stable epoxies must be used for stone fills, even if the stone is dark. There is a great risk of excess epoxy staining the stone and discoloring, and invisible residue may darken with time. HXTAL NYL-1 has been shown to be the least likely to yellow (Down 1986), but its extremely slow curing time (48 to 72 hours) makes it difficult to work with. In spite of this fact, it may be the most widely used among conservators for strong, translucent fills. Other epoxies have also been used extensively because of their reasonably good resistance to yellowing and their faster cure times. These epoxies include some of the Araldite AY series and Epotek (Down 1986).

The use of epoxy in plastic repairs has also been favored because of the potential range of its optical properties when modified with various fillers. Repairs that incorporate microcrystalline wax along with fumed silica as a filler for epoxy can effectively emulate large-crystal translucent marbles (Craft 1996). The technique of first casting the fill in place with the use of a polyethylene film barrier and then adhering the cast fill with a reversible adhesive improves reversibility and greatly reduces the problem of migration of the hardener or other components into the substrate. Epoxy fills are sometimes colored with organic dyes because their color disperses more readily than pigments. Epoxy solutions in alcohol show superior reticulation capacity (the ability to form a strong network on dilution) compared with solutions in aromatic hydrocarbons (Domaslowski 1990). If a highstrength epoxy binder can be used in dilute form with a carrier solvent evaporating out of the intergranular spaces, a high degree of porosity can be maintained in the fill, and concerns about yellowing are significantly reduced.

One serious problem encountered in “homemade” formulations of fill materials based on epoxy resin is migration of resin or hardener out of the bulked fill and into the surrounding substrate during curing. An isolating coating of a stable resin such as Paraloid B-72 is generally used to mitigate this problem. However, several conservators report they found this barrier layer to be insufficient to prevent staining of the substrate (Burke 1996). This disadvantage of the epoxies leads some to use polyester resins, the most popular being the Akemi Marmorkit 1000, a particularly light-stable polyester resin. It has a faster setting time and greater resistance to penetration into the substrate due to its viscosity (Burke 1996).

Polyester resins have traditionally been used by stonemasons for repairs. They became commonly used for restoration after their adoption by the marble industry after World War II (Brady 1991). Polyesters include a large group of synthetic resins made by the condensation of maleic, phthalic, or other acids with an alcohol or glycol to form an unsaturated polyester. The resin mix is composed of this polymerized polyester, which is copolymerized by an unsaturated hydrocarbon such as styrene monomer. The reaction is catalyzed by other additives such as benzoyl peroxide. The styrene hardener is added in small amounts to catalyze the copolymerization and cross-linking of the resin (Werner 1959; Brady 1991). Thixotropic additives are used to make specific formulations with “knife grade” or “flowing” consistencies. Because of their strength and very quick setting time, polyester resins are often used for adhering large, heavy sections of broken stone. They are still used in many applications because of their lower cost than epoxies, their quick setting time, and their translucency.

Polyesters are subject to deterioration on exposure to weathering, with resulting embrittlement, shrinkage, yellowing, crazing, and failure of adhesion. These problems have been ameliorated by the addition of light stabilizers to compositions such as Akemi's Marmorkit 1000. Their common usage warrants the continual comparative studies of their properties and degradation (Shashoua 1992).

7.1 FILLERS FOR ORGANIC SYSTEMS

There are numerous organic materials commonly used as fillers for organic binders. Broken chunks of precast tinted epoxy or polyester, Plexiglas crumbs, wax, PVAs, and methacrylates can be mixed with adhesive binders and cut to resemble a composite stone, breccia stone, granite, or crystalline marble. Hollow phenolic microballoons can be added to epoxy or polyesters to lower the overall strength of the fill or to induce a degree of porosity (Brady 1991). Epoxy with an organic “blowing agent” additive to induce foaming can be used for lightweight filling of voids (Blackshaw and Cheetham 1982; Sturge 1987). Paper pulp has recently been used successfully by several conservators with a number of different binders including Polyfilla (a cellulose-based plaster), PVA emulsion, or methyl cellulose (Podany et al. 1995). The high strength achievable with variations of this technique make it valuable for some structural applications.

The most common combination of binder and filler types used by sculpture conservators is an organic binder with inorganic fillers. The use of colloidal fumed silica, alumina, or titania preserves translucency while increasing viscosity and can reduce or increase the overall strength. Fumed silica also lowers the weight of fill material per unit volume (Berrett 1996; Byrne 1996; Vine 1996). Fumed titanium oxide has also been used to great advantage by conservators in achieving a white, translucent effect while thickening the mixture (Berrett 1996). Super-loading epoxy with fumed silica (e.g., 10:1 v:v) creates a pliable, marblelike dough (Barenne-Jones 1989).

In addition to fumed silica, numerous inorganic additives are commonly used. Stone flour, sand (e.g., washed silver sand), crushed stone, calcium carbonate, aluminum oxide, and pigments are some of the more common additives. Silica beads have been used in architectural fills, and glass microspheres from 3M Corporation are used in a range of fills in conservation (Hatchfield 1986). Successful results have been achieved with ceramic microspheres (Maltby 1996), powdered fired clay with glass microballoons (Higuchi and Setsuo 1984), and white glass enamel powder (Griswold 1990b) as fillers for epoxies. The glass enamel allows the epoxy to achieve a warm white, highly translucent appearance for outdoors use (Griswold 1990a).

Stone aggregate and sand are commonly added to epoxy or polyester for cast replacements and plastic repair in architectural and monumental contexts, particularly in Europe. This type of polyester cast replacement was used for the restoration of Michelangelo's vandalized Pieta(Wihr 1986), with the original crushed bits of stone as the filler.

Fills on monumental sandstone sculpture in the Czech Republic have been made with epoxies after consolidation with dilute epoxy under vacuum (Selwitz 1993). Recipes for filling losses in marble and alabaster based on polyester resin with marble and alabaster powder have proved effective over time (Larson 1978). In an interesting note, Larson recommends heating water-soaked alabaster to increase whiteness before crushing it into powder.

For a stronger fill, commercial epoxy putties may be used. These include Pliacre, Milliput, and Martin Carbone AB123, which are based on an epoxy resin with alumino-silicate ceramic fillers, titanium dioxide, and other inorganic pigments. Conservators often use these putties “straight” for gap-filling or supportive shells. They are often tinted with artist's pigments or textured with skim coats of other materials and painted to reintegrate the surrounding surface.