How do you assess bone, antler and ivory artifacts and decide on a conservation treatment?

Material Characteristics

Bone, antler and ivory are all classified as bony skeletal material. These materials are related because they are all basically composed of two main components: an inorganic mineral, hydroxyapatite, and an organic protein, collagen.

Bone

Generally, bone is found on archaeological sites in the form of worked artifacts, dietary remains, and/ or human remains. All bones consist of compact and cancellous bone, which is made up of inorganic and organic matter.

As seen in the cross section of a limb bone, the dense, compact bone is found in the shaft, while the spongy, cancellous bone is found at either end (the epiphyses). Generally, compact bone is carved to create tools and decorative artifacts. These worked bone artifacts can be easily distinguished from those carved from ivory if any traces of cancellous bone remain.

Antler

Antlers are outgrowths of bones on the skulls of even-toed ungulates (including deer and antelope.) These outgrowths are essentially fast-growing bones and consist of an outer compact surface that is characteristically rough and channeled from numerous blood vessels, with an inner cavity of spongy cancellous bone. Antlers are shed annually and are initially covered with skin (velvet), and each have a visible feature called a burr, where the antler attaches to the skull. Tougher than bone and usually appearing browner and denser under the microscope, antler is often carved to make tools, cutlery handles and buttons.

Ivory

True ivory is from the upper incisors (tusks) of elephants and mammoths, but the word ivory is often used to describe similar materials including walrus and hippopotamus teeth. Ivory has a marked laminated structure, created by the build-up of layers during growth. In cross-section, elephant ivory has a structure that looks like a checkerboard, or has an “engine-turned” appearance. Ivory artifacts can range in color because ivory bleaches with exposure to light. Ivory appears yellow/brown if frequently handled and gray when burned.

Deterioration

Bone, antler and ivory are all anisotropic and hygroscopic, meaning that moisture changes exert stress that can lead to cracking and splitting of the materials. Teeth and ivory are particularly responsive to changes in moisture and are known to split even further. All of these materials are composed of two components that are preserved at opposing pH, and pH is essentially one of the largest influences on the loss of these materials. The inorganic hydroxyapatite deteriorates in acidic environments, leaving the organic materials rubbery, while the organic collagen deteriorates in alkaline environments, leaving the inorganic components chalky and brittle. These materials are subject to staining from metals, either from the burial soil, nearby artifacts, or attached metal components. For example, bone and iron can form a blue iron-phosphorous compound called vivianite and bone can be stained orange due to contact with iron, or green from contact with copper. These stains are generally harmless to the organic material and stain removal should not be attempted without consultation with a conservator.

Excavation

When bony material is exposed during excavation, if it appears to be in good condition, gently remove as much surrounding dirt as possible. If it is slightly damp, allow it to dry out in situ so that it is not as fragile to lift. Once the artifact is sufficiently dry, it should be carefully undermined and placed in a polyethylene bag, along with any fragments that may have detached or broken off-it is important not to leave any fragments behind as they may be important for identification and research. If possible, it is best to put only one bone and its fragments in each bag as mixing fragmentary pieces can complicate identification and mending. If a bone is particularly weak and fragile, it may be necessary to do a block lift or to consolidate the bone in the field in order to keep the bone from completely falling apart during removal from the field. These procedures should only be undertaken with the advice/supervision/guidance of a conservator.

Post-Excavation/Cleaning

Before attempting to clean any bony material, it is important to ensure that it has been properly identified. Worked bone often is confused with ivory, wood, ceramic, or fired clay pipe fragments. All bony artifacts must also be examined for micro-cracks, fractures and abrasions, and for evidence of paint, gilding, attachments, and other features. These features may be difficult to see without a small amount of cleaning. Ivory is more vulnerable than bone and antler and needs to be handled with greater care and in a different manner. Bone and antler can be treated similarly. If there are no traces of applied surface features, these artifacts can be carefully washed and dried. For dry bone and antler, if the artifact is fairly robust and in good condition, minimal wet brushing/swabbing with a solution of 1:1 ethanol/water may be appropriate. However this technique should not be used for ivory. Do not apply even slightly damp swabs to ivory during cleaning-this moisture will lead to swelling, cracking and delaminating. For more fragile artifacts, dry brushing should be used. If necessary, wooden probes may be used to loosen large clumps of dirt. Do not use metal tools to clean fragile bony materials as they can score the relatively soft surfaces. Remember that when cleaning bones, it is not necessary to remove every bit of dirt. If the bone or antler artifact is damp or waterlogged, it is easier to remove the soil before it dries out. If an artifact is wet and there is even a small suspicion that it is ivory, keep it wet until a conservator can examine it. Treat thin antler and bone artifacts similarly to ivory-if they are wet, they must not be allowed to dry and should be stored in a damp environment or a refrigerator until they can be treated.

In the case of damp bone and antler artifacts (but not ivory), it is important to allow them to dry slowly, and out of direct sunlight. Once a damp artifact is clean, it should be laid out on a table or rack and then turned frequently and monitored. If new cracks develop or other physical changes occur, the artifact can be covered with a polyethylene sheet to reduce the evaporation rate. Waterlogged artifacts will need to be assessed carefully to determine if they can be air-dried successfully.

Conservators sometimes use solvent drying for waterlogged bone and antler if there is any indication that air-drying will cause the object to crack. Typically the artifact is placed in successive baths of water/ethanol mixtures. The ethanol content is increased with each successive bath until 100% ethanol is reached. Bone that has pigment, gilding, staining or other materials on the surface should not be solvent dried.

Consolidation is a preservation technique that can be used on bony material, but that requires consultation with a conservator in order to ensure that it is suitable for the material in question. A common substance used for consolidation of fully dried bony material is Acryloid B-72, which is an acrylic resin valued for its long-term stability and used for a variety of conservation techniques.

Consolidation is often carried out by applying a solution of 5-10% Acryloid B-72 in a solvent such as acetone or ethanol directly to the objects with a brush, or by immersing the objects in the solution. Vacuum impregnation of the B-72 solution into the bones is often employed in order to ensure maximum penetration of the consolidant into the bone structure. Although consolidation may be a desirable treatment for bone that is very friable and/or breaking apart, it is a technique that should not be undertaken without the consideration of future analysis of the bone. While this may not be a consideration with bone artifacts, in the case of human remains, the use of consolidants may affect future analytical work such as DNA analysis and Carbon 14 dating. Additionally, some dietary studies rely on bone weights in order to calculate the biomass equation, which is used in zoo-archaeological studies to estimate dietary contribution. The use of a consolidant may add to the weight of the bones. If the bone weights are altered, the biomass equations will be altered as well, thereby changing the results of the dietary studies. It is important to understand how the bones could be used in future analysis, and why the bones may be breaking apart, before deciding to use consolidation as a preservation technique.

If the surface of the bone is spalling (breaking off) and/or a white substance appears on the surface, the bone may contain soluble salts, which it has absorbed from the ground. If these salts are not removed, they will cause considerable damage to the bone. Desalination of the bone may be necessary. Because of the way in which bone and ivory are structured, some damage may occur during desalination and drying. It is therefore best to consult a conservator before embarking on this process.

Storage

Bone, antler and ivory artifacts should be stored in a stable environment with a relative humidity to between 45-55% RH, and minimal fluctuations in RH. Below 40% RH cracks may appear in the bone, which may destabilize it through time. Above 65% RH mold may form on the bone. All light/UV levels should be low, especially for ivory artifacts, and the temperature should be kept cool and stable. Storing ivory artifacts in a drawer, away from direct heat and light, is most suitable.

What are the conservation issues involved with the recovery of human remains?

The excavation, preservation and analysis of human remains are subjects that provoke strong reactions both within the scholarly community and outside it. Ethical and legal issues play an important role in this debate, especially where the burials or remains of first peoples are concerned. Of additional importance are the roles of the conservator, archaeologist and bio-archaeologist/physical anthropologist in the study and conservation of this material. Each specialty approaches the material from a slightly different viewpoint and has the potential for both aiding and obstructing the other specialties involved. Archaeological constraints and techniques can affect the amount of information that a conservator may be able to retrieve from a burial and its associated materials, conservators may compromise the osteological work through their choice of treatments, particularly if they are not aware of the techniques that may be used for analysis, and osteologists may not always think about the long-term stability of the remains and their careful curation. Opportunities to discuss the excavation, treatment and analysis of human remains in a substantive, interdisciplinary manner have historically been rare and infrequent.

Therefore, when human remains are encountered it is important to initiate a dialogue as soon as possible to ensure that the information recovery is maximized and that it is carried out in the least invasive way possible. There are many ways, other than interventive treatment, that conservators can aid in the recovery and analysis of human remains. These include helping to lift the remains, creating housings that minimize the potential for damage, arranging analysis or in some cases (with all relevant approvals) taking samples for analysis, and the study of any related materials such as clothing and or artifacts. As with most conservation actions, these processes will be most effective if they are supported by as much information as possible.

Additional Resources:

Cassman, V. & Odegaard, N. (2004) “Human Remains and the Conservator’s Role.” Studies in Conservation 49 (4): 271-283. Williams, E (ed) (2001) Human Remains: Conservation, Retrieval and Analysis. BAR International Series 934. Oxford: Archaeopress.

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

 

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