Object
The lower half of a utensil, most likely a knife. It has a scale tang of iron and two bone scales that are attached to the tang with iron pin rivets. An “X” is incised into one of the scales. It has been stored in a glass olive jar filled most of the way with a clear liquid since its excavation in 1981 from a test pit near the York River in Virginia, USA.
Condition
The condition of the object was re-evaluated several times over the course of its treatment as it became easier to examine. When first removed from the jar, the object was obscured by a layer of a black or dark green substance, most likely a bio-growth such as algae. There was sediment at the bottom of the jar which appeared to be composed of corrosion products.
Conservation
After determining that the liquid inside of the jar was water, it was decanted into a plastic container in which the object could lie flat. Tap water was added until the object was completely covered. A magnet test confirmed that the metal in the object was iron.
A sample of the water from the jar was tested for chlorides with a Chlorotab testing strip, which revealed that it contained about 80 ppm (parts per million). This is a relatively low level of chlorides (seawater can contain as much as 35,000 ppm) but a desalination course was still considered necessary to prevent re-corrosion after drying. To desalinate, the utensil was washed in a series of baths. Osmotic shock, which can occur when an object that has been sitting in saline water is suddenly plunged into tap or deionized water, was considered unlikely, but care was still taken to shift slowly from jar water/tap water, to 100% tap water, to 50/50 tap and deionized (DI) water, and finally to baths of 100% DI water. The water was changed every 5-7 days, testing the chloride levels each time until they were no longer accurately measurable by titration (about 6 ppm).
During the weeks of desalination, the utensil was also X-rayed to better assess its condition. The x-radiographs showed a corroded scale tang and longitudinal cracking in the scales radiating from the bottom two rivets.
Desalination had also removed the coating of bio-growth, making it easier to examine the object. The iron between the scales was deeply pitted and voluminous, and corrosion had stained the bone scale reddish brown, especially near the base where it had been sitting in a silt of its own corrosion products.
It was decided that solvent-drying was the best method for drying the object. Solvent drying is a process whereby the water in waterlogged object is incrementally replaced with a solvent, which puts less stress on the object when it evaporates. Solvent drying would also treat any remaining bio-growth in the object.
The DI water was slowly replaced with an incrementally greater ratio of ethanol to water every few days until no more water was leaving the object and it was sitting in a bath of 100% ethanol. At this point it was removed and allowed to air-dry in a polyethylene bag with an ethanol- rich atmosphere so that it dried slowly.
Next, air abrasion with a fine abrasion pen was employed to reduce the voluminous corrosion on the exposed sides of the tang and at the top of the utensil. The bone scales were wrapped to protect them from the abrasive powder as much as possible. After air-abrasion, a scalpel was used to mechanically remove any remaining corrosion from around the rivets and any areas too close to the bone for the air abrasion pen.
To help prevent re-corrosion, the exposed iron was brushed with tannic acid. Tannic acid is a commonly used cathodic-type corrosion inhibitor for iron. It will stain bone, however, so the surface of the bone was temporarily coated along the edges of the scales for the duration of application and drying.
Although the bone scales appeared to be in good condition from the outside, they seemed slightly less dense as they neared the highly corroded tang, especially at the base of the handle. Consolidating the object with a diluted adhesive would benefit both the iron, bone, and their interface.
A test was conducted to help select the most appropriate consolidant. To replicate the bone scales of the handle, a clean deer bone, was selected and cut into four sections with a Dremel tool. To stress and artificially age the bone, the fragments were boiled for several hours and then frozen for several days. Each fragment was then immersed in a different consolidant under vacuum overnight to achieve thorough penetration into the bone.
5% Paraloid B-72 was selected as the most appropriate consolidant and the utensil was immersed in the consolidant overnight in the vacuum oven.
Finally, a storage container was prepared for the utensil. Space in the collection is highly limited, and the curator expressed preference that the utensil take up the space of the small brown tray. The tray tested pH neutral with a pH marker and deemed safe to use. The polyethylene bag was padded at the bottom and the box padded at both ends. The bag was also pierced to prevent the formation of a microclimate.
Roni.