CCI Notes 6/1

Care of Ivory, Bone, Horn and Antler

Introduction

Many museums have objects which are made from ivory, bone, horn or antler. Proper identification of these materials is important, as it may determine the type of care required for the object's continued well-being.

Bone and Ivory

Bone and ivory are very similar in chemical make-up, but their physical structure is quite different. Both consist primarily of inorganic materials (which provide strength and rigidity), and they also have a soft organic component to provide the capacity for growth and repair. The main structural difference between the two is that bone has a spongy central portion of marrow from which a series of tiny blood vessels extends into the more solid areas of bone. These vessels appear as tiny dark spots, pits or lines on the surface of bone objects. In the bones of sea mammals (extensively used by coastal peoples) there is generally a larger proportion of spongy material than in the bones of land mammals. Ivory, on the other hand, has no marrow or blood vessel system. It is essentially tooth material. It is usually whiter, harder, denser and heavier than bone.

Walrus and Elephant Ivory

Walrus and elephant ivory are the two most common types found in Canadian collections. Almost all Inuit ivory carving is walrus ivory. In cross section, it is characterized by a central core which has a rather marbled appearance, surrounded by a smooth creamy white dentine layer (Figure 1a). Elephant ivory, on the other hand, is characterized in cross-section by a series of intersecting lines (Figure 1b) which are whiter and more opaque than the surrounding material. These lines are frequently noticeable on the underside of carvings of elephant ivory, where it has been cut across the tusk.

Figure 1a. Cross-section of a walrus tusk. A - Primary dentine B - Secondary dentine

Figure 1b. Cross-section of an African elephant tusk. C- Characteristic intersecting lines

Antler

Antler is a modified form of bone, as it is an outgrowth of the skull bones of certain species of animals. It has a thick outer layer of compact bone and an inner section of spongy bone. The internal blood vessels are less numerous and more irregular than in bone; antler is thus denser and heavier. The outer surface of antler often has a dark, rough texture.

Horn

Horn is made up of the same material as hair, hooves and fingernails. If examined closely, it resembles a mass of very hard, hair-like filaments cemented together. Horn grows around a spongy core. If the core is removed from the horn, a hollow object, such as a powder horn, can be made. Horn can also be boiled, cut and moulded to other shapes, or used in flat sheets.

French Ivory

"French Ivory" is a synthetic material usually composed of pigmented cellulose nitrate and/or casein (CCI Notes 15B, Display and Storage of Museum Objects Containing Cellulose Nitrate). It was commonly manufactured in the late nineteenth and early twentieth century. It looks very similar to elephant ivory. It can even exhibit the intersecting line pattern, although this is due to its manufacture, and the pattern is more regular than that of the natural ivory. Occasionally it is marked as synthetic. "French Ivory" and "India Ivory" are two of the more common markings. It is not as heavy as real ivory and its overall colouration is more even. Cellulose nitrate objects can be identified using a chemical spot test with diphenylamine. (CCI Notes 15C, Diphenylamine Spot Test for Cellulose Nitrate in Museum Objects). Please contact the Analytical Research Services Laboratory at the Canadian Conservation Institute, for more information.

Relative Humidity and Temperature

Of these materials, ivory is the most sensitive to changes in relative humidity (RH), while horn is the least sensitive. Ivory is hygroscopic material like wood — it absorbs or releases moisture with changing humidity, swelling or shrinking. Cyclical changes in RH cause very severe cracking and warping. This problem is particularly acute with thin ivory objects, such as miniatures.

Ideally, ivory, bone, horn and antler should be displayed and stored at a temperature not greater than 25^oC and an RH in the range 45% to 55%. However, in practice, these conditions are often difficult to achieve in many Canadian museums, especially in winter. It is desirable to try and maintain as steady an RH and temperature as possible and, most important, to prevent exposure to extreme conditions or rapid fluctuations. It is important to keep the objects away from direct sunlight, hot light sources, ventilation or heating ducts, fireplaces, the top of appliances (such as radios or television sets), exterior walls or cold windows.

A special caution applies to French Ivory: as this material is usually cellulose nitrate, it is potentially flammable. Care should be taken to keep it away from heat or other sources of ignition.

Illumination

As mentioned above, these materials should not be placed in bright areas, such as direct sunlight or near spotlights. In fact, the illumination should be kept below 150 lux with the ultraviolet component restricted to 75 µW/lm. Objects which are coloured with dyes are extremely sensitive and should only be exposed to 50 lux at most; limiting exhibition time will help reduce the damage which is cumulative and irreversible. CCI Notes 2/4, CCI Environmental Monitoring Kit and 2/5, Using a Camera to Measure Light Levels describe some of the measurement methods for light levels.

One problem to be aware of is that in tightly sealed display cases, radiant heat from light sources can produce very high internal temperatures which can damage sensitive materials by causing rapid fluctuations of relative humidity. In addition, a bright light beam directed at an artifact can raise its surface temperature. Maintaining low light levels and using lights that emit less radiant heat will reduce the risk of damage.

Storage

Keeping the artifact in a tightly closed display case or storage drawer provides a considerable degree of buffering against sudden changes in temperature and relative humidity and is protection against dust and dirt. Dark storage also eliminates damage due to illumination.

Bone, ivory and antler are rarely attacked by insects, but horn is often seriously damaged by the larvae of carpet beetles and clothes moths. Objects should be inspected at least twice a year for insects and mould growth (CCI Notes 3/1, Examining for Insect Infestation).

Storage drawers and shelves should be lined with a chemically stable cushioning material such as polyethylene or polypropylene sheeting (e.g., Ethafoam or Microfoam).

Avoid using rubber-based materials for storage or packing as these can produce unnatural yellowing of ivory.

Handling

If removal of artifacts of bone, ivory, horn or antler from the museum is unavoidable, make sure they are well wrapped and insulated from sudden changes in temperature and humidity. This can be done by wrapping them in unbuffered, acid-free (neutral pH) tissue paper and placing in a sealed polyethylene bag for transportation.

Caution should be exercised during handling or cleaning, particularly of porous bone such as Inuit whalebone sculpture. Its porosity makes it very fragile and easily broken.

Basic Cleaning

Ivory, bone, horn and antler are all porous materials susceptible to staining if left in contact with corroded metals (such as copper or iron) or coloured materials. Stains of this nature should be referred to an experienced conservator for treatment. However, if the objects are in good physical condition, normal surface dirt and grime can be removed safely. A light dusting with a soft brush is often all that is necessary to keep objects clean.

Dusting alone is sometimes not enough to clean bone and ivory. It is possible to clean ivory and smooth non-porous bone with water and mild soap such as Ivory Snow or WA Paste. Using cotton swab (Q-tip), apply the soapy solution sparingly, just dampening the surface. Clean only a few square centimetres at any one time. The surface should remain damp for no more than 15-20 seconds, and then be dried immediately with another cotton swab or soft tissue. Clean the area a second time in the same manner using only water to remove any soap film. Avoid applying water to cracked or porous surfaces and, above all, never soak a piece of bone or ivory.

After many years, ivory and bone may develop an attractive brownish yellow "patina". This is the result of a natural ageing process and, as there is rarely any aesthetic reason for trying to whiten the artifact, no attempt should be made to remove this patina.

Many types of Oriental ivories on which a patina was deliberately produced by special surface staining are also vulnerable to careless handling or cleaning. Processes such as sanding or bleaching will almost certainly cause serious damage to artifacts.

It should be noted that cellulose nitrate also develops a similar "patina", which in this case indicates the first stage of potentially hazardous degradation of the object. In thin films, degraded cellulose nitrate has been known to undergo spontaneous combustion. Objects showing signs of advanced deterioration should be isolated from the rest of the collection.

Horn is somewhat more durable than bone and ivory, and the above cleaning methods can be applied to it as well, providing the surface is intact and not delaminating. However, horn in the form of thin films (as in lanterns) can be softened very quickly by contact with water and may well be deformed; keep such artifacts away from water.

Broken, friable or extremely dirty objects requiring repair, consolidation or extensive cleaning should be referred to an experienced conservator. Please contact the Ethnology Laboratory at the Canadian Conservation Institute for advice.

Suppliers

Cotton swabs (Q-tips), mild detergent (Ivory Snow):

• drug or grocery stores

Unbuffered, acid-free (neutral pH) tissue paper:

• The Hollinger Corporation
  Box 6185
  Arlington, Virginia 22206
  (703) 671-6600

Microfoam:

manufactured by

• E.I. DuPont de Nemours & Co. Ltd.
  1007 Market Street
  Wilmington, Delaware 19898
  (302) 774-1000

and distributed in Canada through
Canadian Paper & Packaging Co. Ltd.
800 Cochrane Drive, Unit 2
Unionville, Ontario
L3R 8C9
(416) 499-7200

WA Paste:

• Contact Canada Packers sales office in Vancouver, Edmonton, Winnipeg, Toronto or Montreal (which serves Quebec and the Atlantic provinces).

Bibliography

1. Canadian Conservation Institute. Anionic Detergent. CCI Notes 13/9. Ottawa: Canadian Conservation Institute, September 1983.
   
   
2. Canadian Conservation Institute. CCI Environmental Monitoring Kit. CCI Notes 2/4. Ottawa: Canadian Conservation Institute, June 1983.
   
   
3. Canadian Conservation Institute. Examining for Insect Infestation. CCI Notes 3/1. Ottawa: Canadian Conservation Institute, September 1983.
   
   
4. Canadian Conservation Institute. Ultraviolet Filters for Fluorescent Lamps. CCI Notes 2/1. Ottawa: Canadian Conservation Institute, June 1983.
   
   
5. Canadian Conservation Institute. Using a Camera to Measure Light Levels. CCI Notes 2/5. Ottawa: Canadian Conservation Institute, September 1983.
   
   
6. Lafontaine, R.H., and P.A. Wood. The Stabilization of Ivory Against Relative Humidity Fluctuations. Studies in Conservation 27, no. 3 (1982): 109-117.
   
   
7. Penniman, T.K. Pictures of Ivory and Other Animal Teeth, Bone and Antler. With a brief commentary on their use in identification. University of Oxford Pitts River Museum Occasional Paper on Technology no. 5. Oxford: Oxford University Press, 1952.
   
   
8. Victoria and Albert Museum. The Care of Ivory. Technical Notes on the Care of Art Objects, no. 6. London: Victoria and Albert Museum, 1971.
   
   
9. Williams, R. Scott. The Diphenylamine Spot Test for Cellulose Nitrate in Museum Objects. CCI Notes 15/4. Ottawa: Canadian Conservation Institute, October 1987.
   
   
10. Williams, R. Scott. Display and Storage of Museum Objects Containing Cellulose Nitrate. CCI Notes 15/3. Ottawa: Canadian Conservation Institute, October 1987.
    
    

Copies are also available in French.

Texte également publié en version française.

© Communications Canada, 1988
Cat. No. NM 95-5/76-1-1988E
ISSN 0714-6221

September 1983
Reprint March 1988