Methods of Conserving Archaeological Material from Underwater Sites
by Donny L. Hamilton

Conservation of Bone, ivory, Teeth and Antler

IN THIS FILE:

Removal of Surface Dirt
Removal of Soluble Salts
Removal of Insoluble Salts and Stains
Consolidation
Seeds and Plant Material
Summary


Approximately 70 percent of bone and ivory is made up of an inorganic lattice composed of calcium phosphate and various carbonates and fluorides. The organic tissue of both bone and ivory is ossein and it constitutes at least 30 percent of the total weight of the material. It is often difficult to distinguish between bone and ivory unless the material is examined microscopically. Bone is coarse grained with characteristic lacunae or voids; ivory is a hard, dense tissue with lenticular areas. Both bone and ivory are easily warped by heat and moisture and are decomposed by prolonged exposure to water.

In archaeological sites, ossein is decomposed by hydrolysis, and the inorganic framework is disintegrated by acids. In waterlogged sites, bone and ivory can be reduced to a sponge-like material; in arid sites, they become dry, brittle, and fragmented. In some circumstances, bone and ivory can become fossilized as the ossein is replaced by silica and mineral salts. Archaeological bone and ivory can only be cleaned, strengthened, and stabilized; satisfactory restoration is often impossible.

REMOVAL OF SURFACE DIRT

  1. 1. Wash with soap and water or alcohol (the use of alcohol will facilitate drying). Towel dry.
  2. 2. When washing with water, limit amount time in water.
  3. 3. Brush lightly with brushes and/or lightly scrape with wooden, plastic or metal tools. Dental tools are particularly useful for this purpose.

Note: Structurally weak bone or ivory must be cleaned carefully and the cleaning method used should be dictated by the specimen's condition.

REMOVAL OF SOLUBLE SALTS
Bone or ivory from a salty environment will invariably absorb soluble salts which will crystallize out as the object drys. The action of salt crystallization will cause surface flaking and can, in some cases, destroy the specimen. The soluble salts must be removed in order to make the object stable. For faunal bone, it is usually not necessary to remove all the soluble salts. It is most efficient to rinse faunal bone in tap water until the chloride level in the material being treated equalizes that of the tap water. For more important artifacts made of any of these materials it advisable to remove all the soluble salts by rinsing for an appropriate length of time in tap water and then in deionized water.

1. If the bone/ivory is structurally sound, the salts can be diffused out by rinsing in successive baths of water. While faunal bone can be put directly into fresh water from seawater (there is a slight chance of cracking), the following succession of baths is recommended for important artifacts:

100 percent sea water » 75 percent sea water/25 percent fresh water (local tap water) » 50 percent sea water/50 percent fresh water » 25 percent sea water/75 percent fresh water » straight fresh water.

The object then goes through either running water rinses or numerous changes of the bath water until the soluble salt level reaches that of the tap water or the water supply being used. De-ionized or distilled water is then substituted for the fresh water bath until the soluble salts are removed or reaches an acceptable level.

In order to determine the level of salts in the rinse water, it is necessary to use a conductivity meter. In most cases, one can alternatively use the silver nitrate test to detect the presence of sodium chloride. When sodium chlorides are no longer present, it is reasonable to assume that the bulk of the soluble salts has been removed. The conductivity meter measures the presence of all soluble salts and is thus a much more reliable indicator of the presence and absence of soluble salts in an aqueous solution.

2. If the bone/ivory is structurally unsound, it can be consolidated with a 5 percent solution of Acryloid B-72 and then rinsed. The soluble salts will diffuse through the resin, albeit much more slowly, during the rinsing treatment.

3. Dry bone/ivory in series of alcohol baths (50 percent alcohol/50 percent water, increasing the alcohol content of the baths to 90 percent, 100 percent, and a final bath of 100 percent alcohol). For teeth and ivory, it is sometimes necessary to go through longer dehydration baths in order to ensure that the surface of the material being treated does not delaminate or crack. In such cases, a recommended series of dehydration baths should begin with straight water, followed by 95 percent water/5 percent ethanol (only water-miscible solvents should be used); an additional 5 percent ethanol should be then added to the bath until the artifact is in straight ethanol. To further ensure the integrity of the surface, the artifact should be placed in a second and even a third bath. The object can then be taken through two baths of acetone. In a few exceptionally critical cases, it may be advisable to take the object through at least two baths of diethyl ether. In most cases, after the object has been taken through two baths of acetone, it is reasonable to assume that all the water has been removed. The object should be then consolidated with a proper resin in order to strengthen it and to make it less susceptible to fluctuations in atmospheric humidity (see below).

REMOVAL OF INSOLUBLE SALTS AND STAINS
If it is necessary to remove insoluble salts or stains from bone/ivory, some means of mechanical removal using picks or other tools is always recommended over any chemical treatment. Inevitably, some damage is done to bone and related material when stains and insoluble salts are removed by chemical means. When chemical agents are used, always make sure that the material is thoroughly wetted with water before any chemical is applied. This ensures that the treating chemical remains on the surface of the artifact and is not absorbed.

Calcium carbonate stains:  Structurally sound bone can be immersed in 5 - 10 percent solutions of hydrochloric acid or formic acid. Monitor process closely.

Iron stains:  5-10 percent oxalic acid has been used to remove iron stain stains from bone. For stubborn stains, 5 percent ammonium citrate used alone or 5 percent ammonium citrate followed by 5 percent oxalic acid are effective treatments.

Sulfide stains:  5-10 percent hydrogen peroxide is used to remove sulfide stains. Stained bone may be placed in a hydrosulfite solution followed by dilute hydrogen peroxide to remove any remaining stain.

Unsound bone should be treated with localized applications of the solution with a brush or swab. If unsound bone is submerged, the evolution of carbon dioxide from the decomposition of the CaCO3 will break up the specimen. Very fragile bone may require that the acid be applied locally to stubborn spots, scraped, and blotted; repeat all steps until the area is cleaned.

Following stain removal, it is necessary to rinse the artifact in water to remove all residue of the treating chemical, dry in alcohol baths, and then consolidate with a resin as described below.

CONSOLIDATION
Any resin solution must be diluted to decrease the viscosity and increase its ability to penetrate the material being treated. A 5-10 percent solution of a suitable transparent synthetic resin may be used. When large amounts of faunal bone need to be consolidated, satisfactory results are achieved with water soluble Elmer's Glue All. When bone, ivory, or teeth are treated, slowly dehydrate them in organic solvents as described above then consolide them with PVA (V7) or Acryloid B-72. The use of PVA V7 is encouraged because while the molecules are smaller and are able to penetrate denser material, the resin has enough resilience to mechanically strengthen any object treated with it. For less dense material and large amounts of faunal bone, PVA with a viscosity of V15 is recommended, since it is a stronger, general-purpose resin.

For surface consolidation, apply resin by brush. Good results can be obtained by applying a light coat of resin, allowing it to dry, and then applying a second coat. This procedure should be repeated several times in order to allow for sufficient resin to be absorbed by the material. Complete immersion of the artifact in the consolidating resin gives excellent results, while complete immersion of the artifact in the resin under a vacuum is considered the best method for consolidating most bone or ivory artifacts.

The type of glue used to glue bone, ivory, or teeth together is to some degree dependent upon how the object was treated. If the bone or related material has been consolidated with a resin, a thick viscous mixture of the same resin should be used. PVA with a viscosity of V25, Acryloid B-72, PVA emulsions such as Bull Dog, and in some cases, Elmer's Glue All, are also serviceable glues.

SEEDS AND PLANT MATERIAL
Miscellaneous seeds and plant material are, for all intents and purposes, treated in much the same way, if not exactly the same way, as described above for bone and related material. Once recovered, it necessary to rinse the material to remove any soluble salts that may be present, mechanically clean any material requiring it, chemically treat the material, and rinse out the treating chemical. The material should be dried in a series of water/water-miscible alcohol baths and then consolidated.

SUMMARY
The conservation of waterlogged bone and ivory, as well as most plant material, is a straight-forward process. Complex problems seldom arise, except in cases where bone is so badly deteriorated that it is cannot be treated. In general, intensive rinsing in water to remove the soluble salts followed by complete dehydration through a graded series of water-miscible solvents, and consolidation with an appropriate resin is all that is required. Stains can be removed, but the process may damage bone if care is not taken. The only equipment required is appropriately sized containers, a selection of resins, and a variety of solvents.

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Copyright 2000 by Donny L. Hamilton, Conservation Research Laboratory, Texas A&M University.

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Last updated: Monday, 10-Oct-2011 18:53:03 CDT