1 INTRODUCTION

Skin, leather, and fur are materials abundantly found in natural history, ethnographic, historic, and decorative arts collections. Leather is hide or skin that has been chemically treated (tanned) to make it imputrescible. The terms “hide” and “skin,” in the leather industry, are used conventionally: the difference is one of size, weight, and thickness. Hide usually refers to a large, heavy skin such as that obtained from a buffalo, cow, or horse. Skin is simply a small hide or the skin of a small animal such as a bird, rodent, or reptile. Fur is the hair projection of an animal. Skin products are proteinaceous in their original chemical composition but have been treated in a variety of ways to obtain products tailored to different uses as well as different physical characteristics, i.e., tanned, tooled, grained, or waterproofed (Fogle and Raphael 1984).

A variety of conservation issues arise with skin products, dealing with the protein chemistry, shrinkage temperature, tannage interferences, and other matters. Among the variety of conservation issues to be reviewed is the issue of gap-filling when losses occur. The fill depends on such factors as the composition of the proteinaceous materials, their tannages, their finishing treatments, the use of these materials as artifacts, the environments in which they are housed, and their state of deterioration. The aim of this article is to provide information regarding some repair techniques and fill materials for skin and leather that have proven useful for a variety of applications.

One should be cautioned, however, to use the proposed fill materials only on stable skins and leather. Before considering treatment, an accurate assessment of the hydrothermal stability (shrinkage temperature) of the collagen fibers of the artifact should be conducted (Young 1990; Larsen et al. 1993). The following suggestions for fill materials should not be used as a “cookbook” for leather conservation.

Fills in skin products must fit the criteria of appropriate strength, stability, flexibility, adhesion, aging, appearance, and ease in identification. Common drawbacks of fills in skin products are the following:

• imperfect fit resulting from an insert
• ensuing difference in flexibility between the original material and fill
• retraction of the sides of the original skin from the shrinkage of the collagen caused by water contained in an aqueous fill
• change in shrinkage temperature of a severely deteriorated artifact occurring from exposure to water from an aqueous adhesive (Young 1990)
• change in shrinkage temperature occurring from exposure to heat from the use of thermoplastic qualities of an adhesive. (It is generally noted, however, that dry, undeteriorated fibrous collagen is considered stable up to temperatures of 200�C, whereas saturated, deteriorated collagen can exhibit shrinkage at room temperature [Young 1997]).
• lack of reversibility
• leaching of soluble skin materials and subsequent staining caused by solvents used in the repair
• shrinkage of fill materials from their original intended size due to solvent evaporation
• uneven fill surface due to the nature of the fill material, shrinkage or drying stresses as solvent evaporates, and/or separation of the bulking agent from the carrier.

All these negative aspects of fill materials for skin products could contribute to the loss of physical and aesthetic continuity. Some are also deleterious to the skin to be conserved. In addition, it should be noted that degraded skin products exhibit more severe results than those in good condition. Hydrothermal stability testing is of paramount importance in order to assess artifact condition.

Generally, fills in skin products are accomplished using a standard method: a backing is applied to bridge the area of loss when possible (Dignard 1992), and then a fill material is inserted into the area of loss. Most commonly used materials for backings are inert fabrics coupled with either resins, starches, or cellulose ethers with evaporative solvents, or resins or systems requiring the use of a heat-seal process.

Popular suggestions for fill treatments for leather and skin have included a variety of standard and uncommon materials: other skins, gelatin, wax, wax and rosin, white glue emulsions, polyurethane, resins, and pastes made of resins bulked with various materials, such as collagen fibers, cellulose fibers, or glass microspheres.

The following materials have been used by the author to produce versatile and flexible fills in skins and leather products. The first two applications are based on the search for a flexible resin or resin mixture that can be bulked and easily used and manipulated to form a cohesive fill. The third example illustrates the use of a synthetic fabric fill for a natural history specimen with hair. All examples have been made on skins, leather, and fur in good structural condition but are visually disrupted by losses in the skin, leather, or fur. The author has utilized these fill materials on a variety of leathers and skins including 20th-century American commercially chrome-tanned leather and suede materials, 18th-century Italian and French vegetable-tanned and tooled leathers, 20th-century Ethiopian dyed leathers, 19th-century unprocessed calfskin, and 20th-century Native American brain-tanned artifacts. The examples are a representative grouping of the proposed fill applications on stable skins and leathers.