How do you assess objects made of horn, tortoiseshell or baleen and decide on a conservation treatment?

Material Characteristics

Horn, tortoiseshell and baleen are all composed of keratin, a proteinaceous complex secreted by the epidermis of the skin of vertebrates. Wool is also composed of keratinaceous fibers but will be covered in the section on textiles. Horn is derived from an outgrowth of the skin of cattle, sheep, goats and antelope. Horn forms over a vascularized core of bone that supports a germinal layer that produces keratin throughout the animal’s life. This gives horn the appearance of growth even though it is dead tissue. It also produces a characteristic cone within a cone structure that makes horn susceptible to delamination. Commercially, horn was manipulated by soaking and heating to soften, shape and reform it and was used for a variety of small artifacts such as powder horns, combs and decorative components on small boxes.

Tortoiseshell is derived from the shells of sea turtles. Only three (the Hawksbill, Green and Loggerhead turtles) of the seven species of sea turtles produce a shell that is thick enough to be commercially viable. The Hawksbill’s shell has traditionally been the most highly valued source. A turtle’s shell consists of three key elements: the flat ventral plate or plastron, the domed carapace that covers the back of the turtle, and the bridge, which forms a rigid connection between the two. Each of these elements consists of spongy bone plates, which are covered with an outer shield of keratinaceous scales. The scales are laid down annually, with increasingly broader layers of keratin-containing cells added to the bottom of the scales, producing characteristic protruding scales with sloping laminated sides. These growth rings produce an optical ripple effect, which can be used as a diagnostic tool for identifying tortoiseshell. The color and patterning of tortoiseshell varies depending on the species, the age of the individual and the location on the shell. The Hawksbill’s plastron produces a lightly colored shell, referred to as “Blonde shell,” which was particularly prized in the 17th and 18th centuries. One of the first written references to tortoiseshell, by the Roman author Pliny, records its use as a veneer. It was used in many of the same ways that horn was and in the 18th and 19th centuries elaborate combs were made from it. Due to the rarity of tortoiseshell, other materials such as horn and bone were sometimes dyed and stained to imitate it. One of the first uses of modern plastics was to emulate tortoiseshell.

Baleen is derived from whales of the suborder Mysticeti, such as humpback, finback, minke, sperm, sei, blue and right whales. These whales employ a filtration system for feeding. The system is composed of baleen plates that grow from the upper jaws of these whales. The baleen plates grow continuously from the epidermal layer of the gum and can reach lengths of 13-14 feet. They have a triangular shape and are placed approximately 1 cm apart. Baleen was most commonly used for “boning” in clothing.

Identification

Correct identification of horn, tortoiseshell and baleen is critical to ensuring that it receives proper treatment. Often these materials can be confused with bone and ivory or with a variety of plastics that sought to imitate the appearance of the materials (particularly horn and tortoiseshell). Pigment variations in tortoiseshell and horn are often used to tell the two materials apart. The pigment in tortoiseshell is evenly distributed throughout the thickness of the piece and appears slightly granular, whereas the patterning in stained horn is a surface phenomenon. Additionally, horn generally appears to have striations or corrugations in its surface that tortoiseshell lacks. These variations can be seen with a low powered microscope. Sonia O’Connor’s article “The Identification of Osseous and Keratinaceous Materials at York” is the definitive article on the identification of these materials.

Deterioration

Keratin is very sensitive to changes in humidity. It responds by taking up or losing water, which affects its mechanical properties and can cause dimensional changes. As the RH rises the material can become weaker but more pliable. Some bacteria and fungi specialize in digesting keratin. Alkaline burial conditions can disrupt the disulphide links in the keratin, speeding up hydrolysis. Visible changes occur in the translucency of the piece-it will become increasingly opaque as it deteriorates. Additionally, the material will begin to delaminate. Heavy overburden (resulting in a high degree of pressure) in the burial environment can cause deformation of the material.

Treatment

Unlike other materials where it has been possible to discuss some of the pros and cons of treatments that are in wide usage, there are very few standardized treatments for horn, tortoiseshell and baleen. Due to the relative rarity of these materials, the trade restrictions on tortoiseshell, and the fact that they are usually found individually or in very small groups there has not been a substantial amount of research on their treatment. There are no standard treatments that are used and each object must be treated on a very individual basis. The susceptibility of horn and tortoiseshell to damage from changes in humidity, makes it important to call a conservator as soon as possible after the artifact has been excavated.

When horn, tortoiseshell or baleen artifacts are found in waterlogged conditions, they should not be allowed to dry out, as this will cause them to warp and crack. Either place the artifact in a 4mil polyethylene ziplock bag with water and place that bag within another bag to ensure that if it leaks the water will be captured and remain in contact with the artifact, or place the artifact in a Rubbermaid® or Tupperware® container filled with water (this will provide some rigidity and handling should be minimized as fragile objects can float into the walls and be bent or damaged. If possible, refrigerate the bags or containers to reduce the potential for biological growth.

Storage

The storage requirements of treated pieces of horn, tortoiseshell and baleen are similar to those for other organic materials. It is particularly important to keep these materials away from heat, a factor that should be carefully considered should these materials be put on display, as lighting in display cases can warm them too much.

Additional Resources:

O’Connor, S. (1987) The Identification of Osseous and Keratinaceous Materials at York. In: Archaeological Bone, Antler and Ivory. Occasional Papers Number 5. London: The United Kingdom Institute for Conservation of Historic and Artistic Works of Art. Available at http://www.ukic.org.uk/pubtable.html.

Wardlaw, L. & Grant, T. (1994) “Treatment of Archaeological Baleen Artifacts at the Canadian Conservation Institute”. Journal of the IIC-Canadian Group 19: 31-37.

Sourche from; http://www.sha.org/research_resources/conservation_faqs/treatment.cfm

 

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