Conservation and restoration of bone, horn, and antler objects
The conservation and restoration of bone, horn, and antler objects is the preservation of objects made from or containing these organic materials by preventative methods and treatments. Because these materials are both versatile and durable, they have been used throughout history for various tools and/or ornamentation including clothing, jewelry, and decorative art. These objects can be found in personal collections as well as cultural institutions and their collections.
- 1 Identification and composition
- 2 Processing
- 3 Types of deterioration
- 4 Preventative conservation
- 5 Display issues
- 6 Interventive conservation
- 7 References
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, 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 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 comes from many different animals, including mammals, birds, and fish, and can come in many shapes and sizes. It can be used in its natural form, can be polished with sand and other abrasives to create a smooth and glossy surface, or can be treated with a burning process which leaves it a blue-black to whitish gray color.
Bones have been used to create numerous objects throughout history, from tools such as hammers and fishhooks to weapons such spear, arrow, and harpoon points to pendants, hairpins, gaming pieces, musical instruments and ceremonial objects.
Antler, a modified form of bone, grows out of the skull bones of certain species of animals and is 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 less and more irregular than the ones present in bone. It is also denser and heavier. Antler also differs from bone in its external appearance. Skeletal bone is usually smooth except for areas of attachment, while the surface of larger antlers generally has raised bumps and protrusions.
Similar to bone, antler can be used in its natural form, can be polished with abrasives for a glossy surface, and can be treated with a burned process for a charred finish and color. Antler has been used for numerous objects throughout history including tools such as hammer batons, pressure flakers, and conical arrow points.
Horn is the outer covering of a bony outgrowth on an animal's skull. It consists of a mass of very hard, hair-like filaments cemented together around a spongy internal 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 in 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.
In general, the same preparation process can be applied to bone, horn, and antler for conversion into usable materials for tools or ornamentation.
- Detaching, or separating the bone, antler, or horn from the muscles and connective tissue of the animal, is the first step.
- Cleaning, the second step, involves removing the blood, marrow and cells from the interior.
- Drying in a slow process to avoid cracking and splitting is the third.
- Working the bone, antler, or horn into the desired shape and finish is the final step. These methods include cutting with stone or metal tools, polishing with abrasives, burning, and/or decorating with pigments such as ochre or charcoal.
Types of deterioration
There are numerous causes for the deterioration of bone, antler, and horn.
These causes include:
- Extreme dryness
- Excess heat, which leads to the destruction of the protein portion and loss of moisture, resulting in shrinkage and surface cracking
- Excess moisture, which leads to the swelling of the protein portion and promotes mold growth
- A combination of both excess heat and moisture can lead to the destruction of ossein, resulting in warping and cracking as the materials dry out.
- Pests such as rodents, which may chew and attack these materials, especially bone
- Exposure to strong visible and ultraviolet light, which may bleach the natural color of these materials
Like any museum objects, the handling of bone, antler, and horn should be conducted in a manner conducive to maintaining the health of the object. These items can be handled with bare hands that are clean and dry. However, body oils can stain the surface of these materials, especially because of their matte, porous surface texture, or those that are light in color. Wearing cotton or latex gloves is one way to prevent the transition of these harmful oils.
Items that contain bone, antler, or horn should always be lifted and moved in a manner that is fully supportive and does not place unnecessary stress on weak areas or attachment points; using an acid-free tray is highly recommended. These objects can be protected further by being wrapped in unbuffered, acid-free tissue paper and/or placed in a sealed polyethylene bag when being transported.
When objects are in good condition, normal surface dirt and grime is removed safely in a few different ways. One way is by using a soft brush to lightly dust the object, dislodging dust and debris. To remove surface dust, a variable speed vacuum, soft lint-free cloths, vinyl eraser crumbs, volcanized rubber sponges, and micro-attachments may be used.
If dusting alone is not enough, smooth materials are then cleaned with water and mild soap. A cotton swab is used to apply a soapy solution sparingly. The surface can not remain damp longer than a few seconds and will immediately be dried with a soft tissue or another cotton swab. The same area will be cleaned again in the same manner to remove any soap film. If the surface is cracked or porous, water will not be applied. Bone should never be soaked.
Bone, antler, and horn objects are stored in tightly closed display cases or drawers to buffer them from sudden changes in temperature and relative humidity while shielding them from dust and dirt. By storing them in the dark, these light-sensitive materials that are dyed or painted are protected. To prevent bumping and chipping, the storage drawers and shelves are lined with a chemically stable cushioning material such as polyethylene or polypropylene sheeting, as opposed to a rubber-based material that can produce unnatural yellowing. Items with holes, straps, appendages, etc. are never hung or supported by said attachments. Instead, they are stored with a support at the base of the item, and a support for the natural position of the handle or strap.
Proper storage also aids in the regulation of temperature, relative humidity, and safe illumination levels, which can have disastrous effects on these organic materials if they fluctuate. Bone, antler, and horn objects are never stored near radiators, heat pipes, outside windows, or incandescent lights, because these can cause excessive drying and temperature fluctuations.
- Temperature is kept as constant as possible, no greater than 68 degrees Fahrenheit with no fluctuations of more than +/- 3 degrees a day.
- Relative humidity is kept at a level between 30 percent (in the winter) and 55 percent (in the summer) with fluctuations of no more than 15 percent during each season.
- Illumination is kept below 150 lux, with the ultraviolet (UV) component restricted to 75. Dyed objects are extremely light-sensitive and being exposed to lux levels more than 50 is damaging. Maintaining low light levels and using lights that emit less radiant heat is an effective way to prevent damage. Shining direct bright light on tightly sealed storage units can reproduce high temperatures and high RH internally, putting the stored objects at risk.
Integrated pest management
Although bone, antler, and horn are not particularly susceptible to insect damage, they can be damaged by rodent attacks and mold growth. Rodents and other small mammals can gnaw on the surface of these items, causing structural damage. Maintaining good housekeeping, following an IPM program, and holding regular pest control checks will help prevent a rodent infestation.
Mold can infest these organic objects when the relative humidity in storage and display areas exceeds 60 percent for extended periods of time. White or greenish fuzzy growth on the surface of these objects is an indicator of a mold infestation. Good ventilation and air circulation prevents mold as well as properly regulating relative humidity levels.
Preventive care can protect bone, antler, and horn objects from damaging elements, but objects on display are put at risk and therefore need to be carefully monitored. Maintaining proper temperature and relative humidity is important to protect the health of objects on display. Rotating bone, antler, and horn items on display on a set timeline prevents them from being exposed for extended periods of time.
- Light damage: extended exposure to harmful light results in surface color being bleached and excessive drying. Consulting a conservator or exhibition light specialist for proper exhibition lighting can help prevent this.
- External supports: supports and mounts are fabricated from safe materials. Attaching items to mounts is done with padded wires or flat acyclic plastic clips. Metals in direct contact with these organic objects cause damage; fats that may remain in these organic items react with the metals, forming corrosion. The use of adhesive mounts is avoided.
When issues arise in objects made of bone, antler, or horn that need extensive treatment, a conservator should be consulted. Some treatments include the following.
- Removal of surface dirt: when minimal dusting and maintenance are not successful in removing surface dust, a conservator may perform more intensive cleaning, using water and a safe cleaner.
- Removal of soluble salts: organic materials from a salty environment will invariably absorb soluble salts that will crystallize as the object dries. Salt crystallization will cause surface flaking and may result in irreversible damage. Conservators will remove the soluble salt to make the object stable by using water with certain levels of chloride and ionization, only if the object is structurally sound. If the object is not structurally sound, an Acryloid B-72/water solution will be used.
- Removal of insoluble salts and stains: to remove insoluble salts and stains, conservators either use a mechanical method with picks and other tools, or a chemical treatment.
Harmful conservation treatments
When addressing minor repairs and minimal cleaning of objects containing bone, antler, or horn, there are some methods/products that are avoided.
- Liquid-base cleaners or detergents used to clean surface dirt and dust can damage the objects.
- Over-the-counter adhesives used to repair cracks and breaks can stain and become brittle over time. It is important to note that cracks and breaks in these organic materials can be indicators and evidence of an object's use and history, and therefore should not be addressed unless the health of the object is in danger.
- Wax or other protective coating used to make repairs can obscure surface details, cause discoloring of the surface and are often impossible to remove without causing further damage.
Always consult a conservator before progressing with a treatment.
- "Care of Ivory, Bone, Horn, and Antler". Canadian Conservation Institute. Retrieved 24 April 2016.
- "Bone, Antler, Ivory, and Teeth" (PDF). Minnesota Historical Society. Retrieved 24 April 2016.
- "Anthropology: Bone, Antler, and Tooth Artifacts". Carnegie Museum of Natural History. Retrieved 24 April 2016.
- "Bone Tools". The Office of the State Archaeologist. The University of Iowa. Retrieved 24 April 2016.
- "Horn". AIC Wiki. Retrieved 24 April 2016.
- Hamilton, Donny. "Methods of Conserving Archaeological Material from Underwater Sites" (PDF). Center for Maritime Archaeology and Conservation. Retrieved 24 April 2016.