you wanted to know about ivory,
but were afraid to ask
WHERE DOES IVORY
In strict terms, true ivory comes from the tusks of elephants. This includes both African and Asian species and their prehistoric
ancestor, the mammoth.
Tusks and teeth from walrus, hippopotamus, boar, and whales (sperm, killer and narwhal) are
commonly referred to as ivory, but some do not consider these true ivories. Generally speaking,
however, ivory refers to the tusks or large teeth of these mammals.
There is an imitation ivory manufactured from cellulose nitrate
or casein (a plastic made from milk). Faux ivory was commonly used in
the late nineteenth century, and had many names such as "Genuine French
Ivory," "Ivorine" or "Indian Ivory." There is even a vegetable ivory made from the inner seeds of the South American Tagua palm tree.
MFMHS Conservator, Monica
Brook cleans coral concretions
off an ivory tusk found at the Henrietta Marie.
WHAT IS IVORY?
Ivory tusks and teeth consist of an inner pulp cavity surrounded by
dentine. Dentine is a combination of mineralized connective tissues and collagen proteins. An external bony layer called cementum, commonly called bark or rind, covers the dentine. Most tusks and teeth have an
additional external enamel layer covering the cementum, but young elephants and walruses only have enamel on the very tips of their tusks. Their enamel is quickly worn away and it is not replaced. Dentine is the main component in ivory and it is
this layer that is utilized for carvings, inlays, etc.
Ivory forms in layers upon layers like placing a cone on top of a cone.
Over time these layers form a pattern of concentric growth rings similar to growth rings in trees. If you make a cross-section in ivory you will see these lines, called "Lines of Owen."
ARE THERE DIFFERENCES IN THE TYPES OF IVORY?
After ivory has been worked it is difficult to determine which species the ivory came from. Understanding the structure of natural ivory will help to recognize the differences. Dentine contains microscopic nerve canals that radiate to the cementum layer. After ivory is worked and polished, the
canals are visible and appear as irregular lines. The arrangements of these lines differ among species. It is this difference that is used to help identify the origin of the ivory.
There are other characteristics that differentiate the varieties of ivory.
For example, walrus ivory has a primary and secondary dentine
layer. The primary dentine layer has a typical ivory appearance, but the
secondary layer has a marbled or oatmeal-like appearance.
Lines of Schreger are characteristic lines found only in elephant and
mammoth dentine. The lines make a unique crosshatch pattern and
are commonly referred to as cross-hatchings or engine turnings. The two species have variations in
the angles in the pattern of the Schreger Lines.
It is easier to tell the difference between natural and synthetic ivories by the lack of irregular canal lines in synthetic ivory. High quality imitations can have simulated lines, but often these lines will have a repeating pattern. The fail-safe way to detect a fake is with an ultra-violet light (UV). Natural ivory will fluoresce bright blue under UV light, and synthetics will fluoresce a dull blue.
IS THERE A DIFFERENCE BETWEEN AFRICAN AND ASIAN IVORY?
There are subtle differences. African ivory is harder and has a translucent, yellow appearance.
Asian ivory is softer and has an opaque, white appearance.
IS IVORY ORGANIC OR INORGANIC?
Ivory is unique because it is classified as both organic and inorganic. The inorganic components (mineralized
tissues) give ivory its strength and
rigidity, while the organic components (collagen proteins) function in
processes for growth and repair. Unlike bone or antler, ivory does
not have blood vessels.
HOW DO YOU CONSERVE IVORY?
The ivory we conserve in our lab has been recovered from the ocean. Its deterioration has been accelerated by its lengthy exposure to seawater. Consequently, it will require special treatments to become stable.
Ivory we treat will typically have a thin layer of concretion on its surface. This layer is not harmful to ivory, but it is not esthetically pleasing, and it can obstruct important diagnostic and
decorative features. Concretion can
be chemically removed with acid, but ivory is very sensitive, and it is
easily damaged. The preferred method of removing concretion is
mechanical removal that involves carefully picking off the concretion with a sharp tool.
Ivory is porous and it readily absorbs soluble salts. The high
salt concentration in seawater is a concern. If salts are not removed
they will crystallize and expand when the ivory dries. The expanding salt crystals will create pressure within the structure of the ivory causing it to crack or delaminate. To remove these harmful salts, ivory is rinsed in a series of water baths that slowly progress from fresh water to distilled water. This process may take
several weeks or months depending on the size and
thickness of the ivory. When all the salts are removed the ivory can be safely removed from
Ivory is hygroscopic, meaning it absorbs and releases moisture in response to the changing conditions in its surrounding environment. This will cause ivory to swell and shrink, and in extreme conditions, the ivory will crack or warp. For this reason it is important to let ivory slowly adjust to its surrounding climate after it is removed from the final water bath.
Ivory is also susceptible to staining. Iron stains are common in ivory recovered from underwater archaeological sites because of the abundance of iron
artifacts associated with sunken ships. Special bleaching solutions can be
prepared and applied locally to remove stains.
Severely deteriorated ivory will delaminate and its cone within a cone structure will separate,
causing irreversible damage. To prevent this from happening ivory is immersed in a special adhesive that will strengthen and consolidate the separating layers.
RECOMMENDED STORAGE CONDITIONS FOR IVORY
Temperature: no higher than 25°C or 72° F
Relative humidity (RH): 45-55% avoids fluctuations
Light levels: 150 lux or 50 lux for dyed ivory
Ultraviolet light: 75 micro-watts/lumen, the lower the better.
In general, avoid direct sunlight, hot light sources, ventilation or heating ducts, exterior walls and windows.
Canadian Conservation Institute, CCI Notes 6/1, "Care of Ivory, Horn and Antler"
George Bernstein the Ivory Hound,
www.ivoryhound.com/article_glossary.htm (June 29, 2002)
www.uniclectica.com/conserva/ivory1.html (June 28, 2002)
National Fish and Wildlife Forensics Laboratory, www.lab.fws.gov/IVORY/text/elephant.html (June 28, 2002)