STRUCTURAL FILLS FOR LARGE WOOD OBJECTS: CONTRASTING AND COMPLEMENTARY APPROACHES
MICHAEL S. PODMANICZKY
1 FIRST CONSIDERATIONS
When the perceived need for a structural fill in a wood object arises, a number of questions must be answered before choosing the appropriate material or the particular application technique:
- Is the fill structural, or is merely aesthetic compensation necessary? If the latter, the fill may be selected from a wide range of materials commonly used by objects conservators but not discussed in this article.
- Is the area that needs fill an actual loss of material, or is it a separation or split that will not close? In the latter case, often the result of internal vice, structural integrity may not necessarily be compromised, and a nonstructural, aesthetic fill may be appropriate.
- Is the surrounding material sound or deteriorated? If the latter, a penetrating consolidant that will be structurally compatible with a structural fill is indicated.
- Is the grain of the wood apparent and important to the visual presentation of the object, or is it obscured by paint, dirt, or degraded varnish? Often wood fills are necessary for visual harmony, whereas if a synthetic fill is indicated, it is more easily hidden by an opaque surface coating.
- Is this a surface fill or one that penetrates deep into the object? Surface adhesion is much easier to deal with from the standpoint of reversibility than a deep, mechanically locked fill.
As a nonisotropic or anisotropic material, wood exhibits different properties in different orientations. Practically, wood has greater strength in the direction of the grain than across the grain, and it has dimensional response to environmental moisture variations across the grain, not in the direction of the grain. These properties present both opportunities and dangers for the craftsman and the restorer.
1.2 INTERNAL VICE
Damage from internal vice is by definition the result of the inherent properties of wood, often in combination with the craftsperson's fabrication choices. Nonuniform movement in response to environmental moisture variation can result in splits or breaks along the grain due to internal restraint, as in the case of solid wood, or externally applied restraint, such as cross-grain construction, in a joined wood object. In such cases, the “loss” is due to anatomical deformation under internal stress and is referred to as “compression set shrinkage” (CSS) (Hoadley 1980). Since CSS damage results in merely the separation of grain (a split) rather than breakage across the grain, there is usually minimal loss of strength in the total object.
In these cases, a strong fill is not only unnecessary, it can, under certain circumstances, propagate the split. The traditional method of inserting wood splines or hard fills into splits caused by CSS, no matter how well executed, in a sense “reloads” the system and can result in further CSS and loosening of the fill unless a stable environment can be guaranteed.
Current thinking not only perceives the danger from this type of fill but usually accepts damage from internal vice philosophically, as integral to the character of the object. However, if aesthetic concerns mandate treatment, a soft, nonstructural fill is indicated (Barclay and Grattan 1987; Barclay and Mathias 1989).
As an anisotropic material, wood can also present special compatibility problems with fills. While the traditional wood fill, properly oriented, can move sympathetically with the surrounding material, synthetic fill materials are isotropic. They respond, if at all, in a uniform manner in all directions, an action that in turn can threaten the stability of the repair.
1.3 STRENGTH OF FILL
Engineering requirements for a structural fill and the strength provided by that fill are usually subjective. Restoring adequate load-bearing strength to a large wood or joined wood object does not necessarily mean the complete restoration of original structural qualities, however. Wood as a material often exceeds the strength requirements of objects made from it (Hoadley 1985), so the conservator may choose to vary the potential strength of a fill. For example, if a damaged chair leg started life at 300% of structural engineering requirements, then a fill that restored only 50% of original strength to the damaged element nonetheless returns the object to 150% of engineering requirements. This may give the conservator an opportunity to vary the amount of adhesive contact or the strength of the adhesive itself in favor of greater overall reversibility of treatment.
Since “structural fill” as understood here implies a tenacious adhesive bond somewhere on the object, the issue of reversibility is of more than usual concern.
The AIC Guidelines for Practice recognize that complete reversibility is often an unrealizable ideal. A more realistic concept is “retreatability.” This is a useful double entendre that implies both some degree of reversibility and unhindered future treatment when and if it becomes necessary.
Although strength requirements and “retreatability” are two important characteristics of any fill, for practical purposes strength requirements must supersede retreatability if the two cannot be reconciled. Conscientious attention to actual strength requirements, without overengineering, will ensure maximum retreatability.
In some cases, maximum strength may be a necessity, perhaps sacrificing retreatability in the name of stability. For example, treating the seriously damaged leg of a table with a fill or adhesive that is not realistically reversible may be justified if it ensures against a possible failure and consequent extended damage to the entire piece or to objects that may be displayed upon it.
Other circumstances also highlight the need for flexibility in applying the concept of retreatability. For example, a solid piece of wood that has split or broken has never before been apart, nor was it ever intended to be by the craftsman. Therefore, it could arguably be rejoined with less reversible materials. However, a loss or break in the vicinity of a joint impinges on an area that bears information about fabrication techniques such as tool marks or layout lines. Because of cultural or craft information, a “worked” area or surface is more significant than an “unworked” area and thus requires heightened regard for retreatability. This statement in no way endorses gratuitously irreversible methods, since any object may need to be retreated for a variety of reasons, and previous treatments should have minimal impact on any new work.