FILLS FOR BRIDGING STRUCTURAL GAPS IN WOODEN OBJECTS
PAUL S. STORCH
Cracks and surface gaps in wooden objects have been filled with various materials to restore surface integrity. Waxes are the traditional choice, since they are inert, easily reversed, and can be color matched to the surrounding surface. The main drawback with wax fills is that they do not expand or contract with the hygroscopic movements of the wood and can become dislodged. Epoxies have also been utilized as fillers, as well as being used as consolidants for rotted structural members in buildings (Phillips and Selwin 1978) and more recently as a fill material in weak wood with a low modulus of compressibility (Barclay and Mathias 1989). Since they cure by chemical reaction and soak into the wood fibers themselves, epoxies should be considered irreversible. It is granted that, in some cases, their strength is required and the principle of irreversibility is modified by the necessities of the treatment, as was thoroughly discussed by Barclay and Mathias (1989). The use of soda lime borosilicate glass microspheres mixed with Acryloid B-72 as an easily reversible void-filling compound in moisture-sensitive wooden objects has been presented by Hatchfield (1986).
For architectural members that may be exposed to moderate stresses during exhibition or use, conservators have utilized the type of silicon-based polymers referred to as room temperature vulcanizing (RTV) silicones as a fill material when infilling cracks in surfaces (Fogle 1987; Barclay and Grattan 1987; Grattan and Barclay 1988).
The RTVs, which are available in proprietary formulations, are chemically known as dimethyl siloxanes and cure by the emission of either methyl alcohol (alkoxy curing, noncorrosive) or acetic acid (acetoxy curing, corrosive). RTV silicones are available at most hardware stores and stores specializing in lacquer and adhesive products. The RTVs chosen for this treatment project were Dow Corning single-component RTVs 734 and 738 due to their availability, ease of application, and the author's previous experience with them as mold-making materials for large metal objects. RTV 738 may be obtained in a clear, white, or black coloration. The RTV used here was the clear type.
RTVs have a compression modulus less than that of most wood, are stable for at least 10 years when used outdoors, do not penetrate into the wood fibers, and are easily reversed by mechanically pulling them out of the gap. They are available in both gel and self-leveling (flowing) viscosities, and the viscosity may be adjusted by adding glass microballoons or powdered fiberglass. While the addition of the fillers to a self-leveling RTV, such as Dow Corning 734, will increase its viscosity, filler added to Dow Corning 738 will cause leveling due to the mixing action required to add the pigments. In other words, the particles added to the gel increase its flow properties. The fillers also provide a rough surface that may be colored with acrylic paints or dry pigments.
The acetoxy curing 734 is used as a surface coating only. In this application, 734 does not present a problem since there is no contact with metal. During work, the ventilation in the lab was adequate to remove vapors as they were produced.
The 738 is applied from a tube using a caulking gun. The 734 comes in a can and can be applied with a spatula. This material is thinner and fills in the irregularities in the surface of the thicker, more puttylike 738.
Similar products such as General Electric 1200, the Silastic series of RTV rubbers made by Dow Corning, and the RTV-M series made by Wacker-Chemie of Germany were not examined in this project. The GE product, which is acetoxy curing, was not appropriate in this application. The latter two silicones are two-component systems developed for mold making (West 1985). They are used in applications thinner than that needed in this situation. A systematic comparison of different caulking silicone RTV products with various fillers was beyond the scope of this project. Such a study was undertaken by Grattan and Barclay (1988), who described the details of the different products. Their conclusions support the choice of fillers described above.
The filling methods described above have been used for gaps and cracks in a surface where the fill did not have to support its own weight. The problem to be solved here was how to adapt a reversible, lightweight fill material such as RTV silicone to bridge a structural gap.