STRUCTURAL FILLS FOR LARGE WOOD OBJECTS: CONTRASTING AND COMPLEMENTARY APPROACHES
MICHAEL S. PODMANICZKY
2 TWO TYPES OF FILLS
Generally, fills are two-part systems: an adhesive or mechanical interface with the object and a sympathetic mass of cohesive material that replaces the loss. These characteristics can literally be separate, as in the case of a well-fitting wood fill affixed with an appropriate adhesive. Or the fill and its adhesive can be combined into a single component, such as bulked epoxy paste. The appropriate fill balances characteristics of both parts of this system to attain the best combination of strength and retreatability.
2.1 WOOD FILLS
Although wood fills are structurally and aesthetically desirable, they do pose some obstacles. Matching grain pattern and color demands a skilled and experienced eye, and fitting a solid fill to an irregular or deep area of loss requires a high but by no means unattainable level of skill. In the past, their high skill level notwithstanding, traditional restorers did not apply the concept of integrity to the original material of an object. In the interest of saving time and effort, a wood fill would often be treated much like inlay. It would be cut to a convenient shape, i.e., slightly larger than the loss, straight-edged, sharp-cornered and slightly taper-sided for a tight fit. It would then be laid over the area of loss and traced out with a sharp knife or pointed scribe; the area so outlined would be relieved with a knife and chisel. Glued into place and leveled with the surrounding area, this method produces a tidy appearance but results in the loss of original material.
With heightened regard for original material, the contemporary conservator carves the fill to fit the loss instead. This approach does indeed require skill, and many conservators are able to execute the task by eye. However, it can be facilitated with carbon paper. The paper is laid in the interface, ink toward fill, so that when the fill-in-progress is temporarily pressed into place, high spots are registered on the surface. These spots in turn are progressively carved away until a good fit is achieved and carbon registers overall.
It is important not only to choose but also to align the grain of wood fills to closely approximate the surrounding wood. When the fill must match visually as well, grain orientation must be exact. Grain orientation includes the degree of tangential vs. radial exposure as well as the angle and direction of the pores relative to the surface. Otherwise, light reflected from the surface will belie even the best-fitting fill. One of the benefits of a wood fill fitted to a odd-shaped loss is that the irregular interface at the surface is less eye-catching than the straight lines of the traditional fill.
It is accepted, particularly in architectural conservation, that for ethical identification of compensation, wood of a different species from the original should be used. However, conservators of decorative objects produced with a heavy reliance on the figure and color of the wood accept that wood fills are virtually never completely undetectable, so they generally use the same species whatever the circumstances.
Although thermoplastic wood “glues” such as PVA and aliphatic resin emulsions produce excellent bonds, reversibility is problematic for wood fills. For strength, versatility, and retreatability, traditional hot animal-hide glue, produced in a wide range of gelling characteristics and tensile strengths, remains the best adhesive for today's conservator. (Fish glue, which has the same general characteristics as hot animal glue, is traditionally preferred when working in the mixed medium of Boulle work, i.e., tortoise, brass, and pewter.) So-called liquid hide glue is adulterated with a gel suppressant such as urea. The experience of many conservators is that while such glue is sometimes desirable for nonstructural repairs, it may fail at elevated temperature or humidity. Liquid hide glue also has a shelf life, and manufacturers do not always provide an expiration date on the package.
2.2 SYNTHETIC FILLS
If a wood fill is not practical or desirable, a gap-filling adhesive paste or bulked liquid can be used. As a general rule, thermoplastic fills such as bulked acrylic, PVA or PVOH resins, or even the traditional sawdust and glue fill, are less structural than thermosetting fills. These thermoplastic polymers have adequate theoretical cohesive strength, but they are generally unsuitable for gap-filling due to shrinkage during solvent release curing. Bulking with glass microspheres, glass microballoons, phenolic microballoons, or microfibers can increase the viscosity of these adhesives, but enough bulking to create a thick paste can result in reduced cohesive strength (Grattan and Barclay 1988).
Thermosetting resins are irreversible with standard solvents, and although gap-filling properties of catalyzed epoxy and polyester resins are excellent, the bond with a wood substrate that is mostly mechanical is also virtually impossible to reverse without damage to the object.
It is most practical to utilize the desirable characteristics of each of the resin types by using a thermosetting material for the gap-filling, structural fill but introducing a thermoplastic adhesive “bridge” at the interface for retreatability. There are a number of techniques for this system. Some conservators cast the fill in place with a nonadhesive barrier at the interface. The fill is then removed, trimmed, perhaps even colored, and then re-adhered. Complex or deeply undercutting fills may have to be built up in separate pieces so that they can be individually removed, worked on, and readhered with a skim coat of filler to blend the surface. This method is desirable if the mechanical undercutting of the loss could make later removal difficult.
Others prefer to accomplish the fill in a single step. First, the surface of the loss is coated with the appropriate thermoplastic resin or adhesive. The thermosetting gap-filler is then poured, spread, or otherwise molded in place, adhering to the bridge. These fills must be trimmed and colored in place (Anderson and Podmaniczky 1990). In both cases, practical reversal usually requires destructive removal of the mass of the fill in order to deliver the solvent to the barrier.
For maximum structural qualities, epoxies remain the favorite over polyesters for the majority of conservators polled for this article. Ciba-Geigy epoxies such as Araldite 1253 (paste) and Renweld 306 (thick liquid) are reported as widely used. Although technical representatives of epoxy manufacturers are hesitant to comment on the strength of bonds that are, in a very real sense, disrupted from the start, initial testing by conservators has provided useful results and the technique has been widely accepted. Some barrier adhesives that have been used include Paraloid B-67, Paraloid B-72, “Butvar” B-98, and hot hide glue.
Hide glue is quite popular as a bridge or barrier. If it is allowed to dry completely to a hard gloss, the surface of the glue must be roughened for good bonding of the epoxy fill. It has been found that the epoxy bond is enhanced if applied before the hide glue is completely dry. This technique avoids the need for roughening the surface. Although there has been little scientific testing of this specific system, conversations with technical representatives suggest that there may be some degree of primary bonding between epoxy catalyzed in direct contact with collagen.
In many cases, even fills that are required to provide structural support do not have to fill losses completely. This is particularly important when considering retreatability factors. For example, an epoxy fill on a barrier resin may not need to adhere to the entire surface of a deep void to provide adequate strength (Podmaniczky 1988). One way to reduce the adhesive contact area is to apply paste wax to inner surfaces so that there is only structural bonding at the perimeter despite complete filling of the loss. (Dental crowns exhibit this principle, adhering only to the outer layer of a tooth stub.) The author has also incorporated a void in the interior of a fill to provide an access reservoir into which solvent could be injected to saturate the barrier if removal was ever necessary. On one occasion this procedure was accomplished by packing cotton balls into the center of the loss. A mass of microcrystalline wax has also been used for the same purpose.
To reduce penetration of the epoxy paste on the surface of a loss that may have many voids or undercuts, a two-dimensional barrier, such as Japanese tissue, fine-wove polyester, or another sheer fabric can be incorporated into the bridge adhesive in order to restrict penetration of the fill material (Anderson and Podmaniczky 1990). In the absence of specific engineering data on the strength of restricted bonding, consulting colleagues or one's own past work experience usually provides guidance in these cases.
2.3 WOOD/EPOXY FILLS
Often it is desirable to combine wood with synthetic resin fills. In these cases, the wood fill is fitted well, but only to the perimeter of the loss, saving time by not going through the laborious work of fitting to the entire interface. A bridge adhesive is applied, and the fill is then adhered with epoxy paste.