Conservation-restoration of bone, horn, and antler objects involves the processes by which the deterioration of objects either containing or made from bone, horn, and antler is contained and prevented. Their use has been documented throughout history in many societal groups as these materials are durable, plentiful, versatile, and naturally occurring/replenishing.
While all three materials have historically been used in the creation of tools, ceremonial objects, instruments, and decorative objects, their individual compositions differ slightly, thus affecting their care. Bone is porous, as it is a mineralized connective tissue composed of calcium, phosphorus, fluoride, and ossein, a protein. Horn consists of a keratin sheath over a bony outgrowth, as seen with cows and other animals. Antlers are a reoccurring bony growth on the skulls of male members of the deer family (apart from reindeer/caribou, in which both males and females produce antlers.) Unlike horn, which is a permanent feature, antlers are typically shed and regrown each year.
While these materials have a well-documented past as sturdy and reliable choices for tools, decoration, ceremonial objects, and more, they are organic materials that deteriorate if not treated properly. Deterioration may occur if objects made from these materials are subjected to extreme heat, dryness, moisture, or a combination of heat and moisture due to their highly porous nature. Other sources of deterioration include pests, acids, and overexposure to light. It is highly recommended that a conservator be contacted if a museum has bone, horn, or antler objects in need of conservation, as many adhesives, liquid cleaners, and protective coatings may irreversibly damage the object.
Identification and composition Many museums contain objects in their collection that are made of bone, antler or horn. One of the most important steps in the conservation of these objects is determining which material it is.
Bone Bone, which has a very similar chemical make-up to ivory, consists of inorganic materials which provide strength and rigidity and organic components that provide the capacity for growth and repair. Unlike ivory, which has no marrow or blood vessel system, bone has a spongy central portion of marrow from which extend tiny blood vessels; bone is therefore highly porous. Bone is also made of both mineral and carbon-based materials; the mineral-based are calcium, phosphorus, and fluoride; the carbon-based is the protein ossein. Bone also includes the mineral hydroxyapatite, "A calcium phosphate mineral which forms a hard outer covering over the collagen and protein matrix," or organic material.
Bone housed in museum collections come from many different sources; mammals, fish, birds, and in rare cases humans may all be included in a museum's collection. Bones in these collections can come in many shapes and sizes. It can be used in its natural form or polished with sand and other abrasives to create a smooth, glossy surface. It may also undergo a burning process, which gives it a blue-black to whitish-gray color.
Bones of all kinds have been used to create many different objects throughout history, from tools such as hammers and fishhooks to weapons such as spears, arrows, and harpoon points, or other items, such as pendants, hairpins, gaming pieces, musical instruments, and ceremonial objects.
Antler Antler, a modified form of bone, grows out of the skull bones of certain species of animals, such as deer, and is typically shed once a year. It consists of a thick layer of compact bone, an inner section of spongy bone, and internal blood vessels that are fewer in number and more irregular than the ones present in bone. Antler is denser and heavier than skeletal bone and differs from skeletal bone in its external appearance. Skeletal bone is usually smooth except for areas of attachment to muscles, tendons, and ligaments, while antler generally has raised bumps and protrusions across the surface.
Similarly to bone, antler may be used in its natural form, polished with abrasives for a glossy surface, and treated with a burning process for a charred finish and color. Antler has been used for numerous objects throughout history including tools such as hammer batons, knife handles, pressure flakers, and conical arrow points.
Horn Horn is the outer covering of a bony outgrowth on an animal's skull, such as a cow. It consists of a mass of very hard, hair-like filaments called keratin, cemented together around a spongy internal bone core. This layering effect continues to grow over time, resulting in a cone-within-cone structure. Unlike antlers, horns are permanent and not seasonally shed. Another distinguishing factor from bone and antler is the fine parallel lines that are present on the surface of the horn. Horn comes in a great variety of sizes and colors, including white, green, red, brown, and black.
Horn can be used in its natural state, boiled, cut, molded to other shapes, or used in flat sheets. It has been used for a variety of objects including ceremonial decorations, utensils such as spoons and containers, gaming pieces, and combs.
Ivory Many mammals, such as elephants, walrus, narwhals, whales, and hippopotami, produce tusks and teeth of ivory that can be used for carving; these are the most commonly found ornamental ivory objects in collections today. As mentioned above, ivory is very similar to bone in its chemical make-up. Like bone, it is composite, consisting of both organic and inorganic materials that at once allow for its rigid surface as well as its capacity for growth. However, while the chemical compositions are similar, the physical structure of bone and ivory are very different. For example, "ivory is dentine—the part of the tooth that is covered by enamel." Ivory is tooth material, meaning "it is usually whiter, harder, denser, and heavier than bone," which has a spongy central portion that ivory does not. Moreover, "ivory, which has multiple layers, is more dense than bone or antler and is more likely to crack or delaminate while drying."
Identifying the type of ivory you are working with can be done by examining the intersecting patterns that cross the surface of the ivory. Elephant ivory, for example, will exhibit a pattern of intersecting arcs called schreger lines, that hatch across the surface at 115-degree angles.
Examples of ivory objects include: horns, handles and inlays for ceremonial weaponry, jewelry, and decorative arts; statuettes, regalia masks, and even triptychs.
Agents of deterioration
The term agent of deterioration is used to identify the major activities, natural and man-made, that threaten a museum collection. A key preventative measure in conservation is surveying the environment and making a risk assessment of the biggest threats to the collection. The environment ranges from the storage container, storage room, the building, what is outside the building, the surrounding businesses and landscape, the climate and geography, and the year round weather changes.
Physical forces Physical forces that can damage a collection are the vibration, shock, gravity, and abrasion that can be gradual over time or sudden and catastrophic. This can be from an earthquake, being dropped, or stress of compression from the general weight of the object.
The most common physical forces on bone, antler, and horn are mishandling, over handling, being dropped, or being unbalanced while on display or in storage causing stress compression on weak spots. Sweat and oils left behind bare hands assist in promoting mold growth and other damage to the objects. Wearing gloves, handling the object with two hands (rather than one), and avoiding any weak spots on an object will help prevent damage.
Fire Fire can completely destroy the object. Fires are not common, but they are highly destructive. Heat from a nearby fire can cause objects to become brittle or crack, and cause its destruction. Smoke can stain the object. Damage from fire is irreversible. Objects that are made of organic materials are "highly susceptible to combustion, particularly if very dry." The smoke damage can come from a fire "both inside and outside of the museum".
Water Water can come from roofs leaking during rainstorms, floods, fire sprinkler systems, or broken pipes. It can soften and destroy the bone, antler, or horn if it becomes waterlogged. Mold and mildew growth can cause further damage. If the water in the crevices or pores of the bone, antler, or horn were to freeze, it would crack the object.
Crime Theft and vandalism are everywhere, therefore, keeping the objects in locked display cases or locked storage while using surveillance cameras in the gallery and limiting personnel in collections is the most preventative measures.
Pests Pests are a "living organism that are able to disfigure, damage, and destroy material culture. Dermestid beetles, silverfish, and rodents are common pests in museums. There are "beneficial" pests, like spiders and centipedes that feed on harmful pests. By knowing what type of pest, their behavior, and preferred habitat, the pests can be controlled. If there are beneficial pests, then there are harmful pests. Disrupt the habitat of the harmful pest and both will go away.
"Once established, populations can more easily move on to collections storage spaces and other relatively fortified nonpublic spaces." Pests like to snack on organic matter and cause damage to the object. Trap and monitor systems can identify what pest is present while learning about the habits of that pest can tell you how to get rid of it; it may be the temperature that was welcoming.
Light, ultraviolet and infrared "Light, by definition, is the band of radiation to which our eye is sensitive." On the ends of this light spectrum (measured in wavelengths) are ultraviolet radiation and infrared radiation that are not visible to the human eye, but can be very damaging to objects. In museums, filtered lighting with a shorter spectrum that excludes as much ultraviolet and infrared as possible is used to reduce the damage it causes.
Organic materials are highly sensitive to light and the light will cause fading or structural damage if it is not filtered and kept low.
Temperature, relative humidity and pollutants Temperature that is too high, too low, or fluctuates in a large range causes materials to become brittle, fragile, or deteriorate through chemical methods such as acid hydrolysis. Relative humidity (RH) is the measure of "humidity" that we perceive in ranges from dry to damp. It is read as the percentage of moisture in the air. Pollutants are the compounds that can cause chemical reactions to object materials. Pollutants include gases, aerosols, car exhaust, emissions from nearby factories or construction work, even the off-gassing of volatile organic compounds from other museum objects.
A filtered air-conditioned storage area can control the temperature and relative humidity while simultaneously circulating the air which removes pollutants and prevents mold growth while creating an uninviting environment for pests. A relative humidity level that is too low (dry, below 30%) with cause the bone, antler, or horn to crack; too high (damp, over 75%) and they are subject to expansion and mold growth that can disintegrate or discolor. Fluctuations over 3% within a 24-hour period can cause objects to swell and contract, weakening the material and causing damage.
Disassociation and custodial neglect