3 VULNERABLE MATERIALS IN ARTIFACTS

Several categories of artifacts were systematically observed in this study because they exhibited characteristics or flaws making them potentially vulnerable to the damage mechanisms described above. The following sections will discuss the potential for damage in artifacts with multiple material compositions, cracks, lamellar structures, waxy or oily elements, or cultural sensitivity.

3.1 MATERIALS IN COMPOSITE OBJECTS

The vulnerability of composite objects involves two areas of concern. The first category includes objects featuring different materials in close contact inhibiting independent movement of each material according to change in temperature or humidity. Tension may be built into an object during natural formation of the material (such as a tooth) or human creation of an object (such as a drum). Cracking, splitting, or warping may occur when one material is restricted by another. The second category involves materials that are not generally frozen (particularly inorganics such as metals, glass, ceramics, or stone) attached to materials that are good candidates for freezing. Garments with metal, glass bead, or tooth adornments are common. If the packing material or the more thermally robust part of the composite cannot mitigate the effect of the freezing environment, there is the potential for the vulnerable material to suffer from cracking or surface condensation. Condensation from poor packaging may result in staining, corrosion, or other changes in surface characteristics. Examples of vulnerable materials in composite objects include rawhide or sinew wrappings (arrows, hammers, snowshoes, etc.), metal (buttons, cone tinklers, brass tacks, inlay, etc.), tooth (garments, masks, jewelry), ceramic (beads, pipes), stone (beads, pipes), glass (beads), wood objects with inlay or tight joinery, and wax on wood or other organic substrates.

3.2 CRACKED OBJECTS

Shrinkage, swelling, embrittlement, and other phenomena in the freezer may lead to propagation of existing cracks or formation of new ones, especially in materials made up primarily of inorganic compounds. Ivory, tooth, and bone are of special concern since they have been shown to crack under changes in humidity, apparently from the stresses of shrinking and swelling. Tooth, bone, ivory, and baleen contain both inorganic and organic components that behave differently under temperature and humidity changes. Bone, for example, is anisotropic (it has a higher percentage of change in the long axis) and responds to environmental changes differently in different directions (Williams et al. 1993). Cracking is most easily caused in thick objects (such as stone, glass, and ceramic) having poor thermal conductivity, a high CTE, and high elastic stiffness when they are subjected to large, sudden changes in temperature. Beads with the tiny cracks associated with glass disease may be at additional risk because of their sensitivity to changes in humidity. Improper packing for low-temperature treatment could theoretically expose the glass to the elevated RH that occurs with decreased temperature (see sec. 2.1.3). Glass disease involves deterioration of the glass structure from leaching out of water-soluble components with flawed compositions created during manufacture (Lougheed 1988; Erhardt and Mecklenburg 1994). Michalski mentions that craquelure on painted or coated wood may crack further at –50�C (Michalski 1996). Candles are thought to crack if placed in the freezer, suggesting thick wax layers may be vulnerable. Some botanicals such as seeds or gourds might have the potential to crack, although the National Museum of African Art recently treated 60 gourds, including some with crack and ethnographic repairs, and noted no visible damage (Hornbeck 2001). The cracking danger in the freezer does not seem to be from low temperature alone as much as from mishandling while cold objects are embrittled. Examples of materials vulnerable to cracks include bone, tooth, ivory, diseased glass, painted or coated wood, plant materials, wax, and inorganic materials such as metal, ceramic, and stone.

3.3 LAMELLAR OBJECTS

Delamination is the peeling apart of materials with a layered structure. Examples of particular concern include painted objects such as masks and furniture, particularly if the pigment is well bound in media that will behave differently from the substrate. Amorphous and semi-amorphous polymer media (oil, varnish, glue, gum) can suffer shrinkage on the scale of 0.7% per 70�C decrease in temperature. Leaner paints with less binder and a larger percentage of pigment, however, fare better with shrinkage in the range of 0.4% per 70�C decrease in temperature (Michalski 1996). It is interesting to note that certain woods, such as cedar, cypress, and redwood, hold paint better than others, due in part to their dimensional stability but also because they are relatively porous and the wood-paint bond is thought to be largely mechanical (Mecklenburg et al. 1997). Another area of concern involves adhered or glued objects such as feather tipping on war bonnets, inlaid objects, furniture joinery, and past treatments. Most adhesives are stronger, more brittle, and more reactive to increases in RH at low temperatures (Erhardt and Mecklenburg 1994; Erlebacher et al. 1992). Joins that are under stress are at additional risk for failure as the adhesive becomes brittle at low temperature. The adhesive and adherend may also shrink or swell at different rates, causing failure. Objects with accretions from burial or use may be vulnerable for similar reasons. There have been reports of successful low-temperature treatment of leather with adhesive repairs, including BEVA 371 film, silicone adhesive SF2, and wheat starch paste mixtures (Kite 1992), as well as the successful low-temperature treatment of Japanese lacquer wares (Tanimura and Yamaguchi 1995). Shell (turtle shell, marine shell, snail shell) is thought to be potentially vulnerable to damage at low temperature due to its lamellar structure. This natural lamellar structure provides areas of weakness for stresses to be released if shrinking or swelling occurs. This damage could manifest as an opening up of these layers, with associated peeling and loss. Byne's disease is another source of concern: the appearance of powdery deterioration on the surface of a shell could indicate that the calcium carbonate has reacted adversely with acid vapors off-gassing from wooden shelves or cabinets, forming hygroscopic salts that could swell in elevated humidity (Tennent and Baird 1985). Horn, as a keratinaceous material similar to hair, has the ability to absorb limited amounts of moisture. However, the lamellar structure of its growth makes it prone to crack with age, and it is these cracks and microcracks that may be propagated if stressed. Baleen is similar to horn, but further calcified. Changes in RH between 25% and 85% do not seem to affect the dimensional stability of skull bones in mammals (Williams et al. 1993). Teeth, which are hygroscopic, anisotropic lamellar structures, suffer more from low RH than from temperature changes. If damage occurs, canines are more prone to crack than molars, in part because the hollowness of molars constrains movement less than the more solid interiors of canines (Williams 1991). It is interesting to note that industrial cleaning techniques recently developed for large wooden surfaces such as floors employ low temperature expressly to force failure between layers, including wood, dirt, wax, varnish, and overpaint (Piening and Schwarz 1999). It is worth repeating that proper packaging should eliminate the elevated humidity that occurs with low-temperature treatment (see sec. 2.1.3). Examples of materials vulnerable to delamination include adhesive joins and repairs, painted or gilded objects (masks, furniture, beads), turtle shell, marine shell, snail shell, horn, baleen, bark, resins, and accretions.

3.4 WAXY OR OILY OBJECTS

The possibility of waxy, powdery, or crystalline formations developing on the surface of some materials during treatment is another area of interest (see sec. 2.2.4). Waxes, oils, and fats found in some objects (oiled ropes, food bowls with residues, dressed leather) may undergo a polymorphic (solid-to-solid) phase change during cycled changes in temperature, resulting in an opaque, powdery wax formation on the surface (Pearlstein 1986). Another explanation suggests that bloom may be the result of having materials with different coefficients of thermal expansion in contact with each other. For example, if a high CTE wax is sandwiched between two low CTE fibers (or vice versa), the wax will be squeezed (extruded) from between the fibers as temperature is increased (Elzey 2001). Spew from dressed leather exposed to low temperature for pest control has been reported (Baughman 1999). Cold temperatures may also cause waxes to become brittle (Victoria and Albert Museum 1970). Examples of materials vulnerable to bloom include bark, botanicals, wooden food dishes, dressed leather, wood with waxed surfaces, and oiled ropes.

3.5 OBJECTS WITH NONTANGIBLE SENSITIVITIES

Low-temperature treatment may be inappropriate for objects with cultural sensitivity determined by traditional care. Some objects are considered to be sacred or living members of certain cultural groups. The bagging required for a low-temperature treatment may constitute mistreatment from a traditional care perspective. Some Native American medicine bundles at NMAI are sometimes allowed to deteriorate naturally and be consumed in isolation from other objects within the museum environment. Examples of materials with potential cultural sensitivity include bundles, masks, pipes, sacred or ceremonial objects, medicine objects, fragments of human remains, and associated funerary objects.