LOSS COMPENSATION METHODS FOR STONE
JOHN GRISWOLD, & SARI URICHECK
5 PLASTIC REPAIR
While replacement repairs are sometimes required by the size or context of a loss, most conservation treatments now employ plastic repairs because they preserve all original material. The term “plastic repair” defines a moldable fill applied directly to the loss and set into place by its own adhesion to the substrate. It includes mixtures such as mortars and putties. Mortar is a lime-based or cementitious mixture traditionally used to join masonry units. Putty (as described in this article) refers to materials that contain an organic binder and have the working consistency of a dough3. The texture of each type of mix is dictated by the ratio of components and may be fashioned to fit the appropriate scale of a loss.
Many larger-scale plastic repairs, including inorganic mortars and organic systems, often require internal reinforcements, such as wire mesh, epoxy splints, metal pins, or dowels. Often high-grade stainless steel is specified for pinning, but epoxy-coated steel, titanium, and polymeric composites are also used. More complex interlocking or adjustable mounts with the fill materials integrated into the support structure have been designed for reassembly of monumental sculpture (Podany 1987; Garland and Rogers 1995). The use of Plexiglas support rods for resin repairs can provide a transparent support that negates the shadowing effect of an opaque armature within a translucent fill (Strauss 1996).
Plastic repairs are composed of a binder and filler (sometimes called matrix and aggregate), color components, and special additives. Constituents may be either inorganic or organic. The main classes of inorganic binders—plasters and natural and modern cements—as well as the various thermoplastic, reaction-cured, and solvent-cured organic binders will be discussed further. Fillers are inert materials such as glass microballoons, crushed stone, bits of solid resins, and other fine- to coarsely-ground materials. Color may be created by the chosen filler or added to a plastic repair by the inclusion of pigments, colored stone flour or sand, soil, dyes, or crushed enamel glass (Griswold 1990b). Additives modify the working or cured properties of a repair and can include light stabilizers, among other things.
Fillers serve several important functions in plastic repairs. As their name implies, they act to bulk up the binder and in effect temper the qualities of the binder and create specific desirable properties. By the addition of a filler, adhesives may be made weaker (or stronger), dense materials may be rendered less dense, and the autoxidation process of organic resins may be retarded. The addition of inert inorganic fillers can reduce thermal stresses by lowering the coefficient of thermal expansion (CTE) of organic binders (Barov and Lambert 1984). Filler materials, ranging in particle size from fine powders to large aggregates, help create the desired density, texture, porosity, color, degree of translucency, and gloss. Careful selection and mixing of matrix and aggregate can result in a compatible match with a porous stone. In some cases, fillers reduce the necessary quantity of expensive adhesive binders and other components.
Some additives do not just fill up space, but modify the properties of the binder. One of the most common additives is colloidal fumed silica. This material is a thixotropic agent that stiffens a mixture by forming a weakly bonded network of silicon dioxide in suspension. When agitated with a tool, the mixture becomes more fluid (Moll and Schirripa 1980), creating a thick but spreadable putty mixture. Additives of another nature are light stabilizers for polymers, such as UV absorbers. These are commonly added to commercial resins (such as Akemi Marmorkit 1000 polyester resin) and can be included in homemade mixes as well (G�nsicke and Hirx 1997).
When choosing a binder-filler system, the compatibility between properties of filler and binder must be considered. Inappropriate combinations (such as stone chips of a high CTE within a resin of low CTE) may result in differential reactions to changes in moisture and thermal conditions. These reactions can result in internal stress and loss of adhesion. In addition to the proper selection of binder and filler types, the ratio of binder to filler and the relative percentage of aggregate particles with different sizes can be critical to achieving the desired properties of the fill. The shape and size of the aggregate determine the nature of the intergranular porosity, which may or may not be filled with the adhesive binder.