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It is not uncommon to find non-ferrous metals, such as copper, silver, lead, tin, gold, and their alloys, in archaeological sites. These metals have been used in the manufacture of art objects, coins, jewelry, and various utilitarian items such as fasteners, navigational instruments, cooking vessels, and small tools. They are more noble than iron and survive adverse environments in better condition than do iron specimens. Nevertheless, the corrosion problems of each metal varies in different environments. Only those techniques applicable to the conservation of sea-recovered metals are considered here.
Sea-recovered materials are often encapsulated by encrustation. When encrustation is present on non-ferrous metals, it is generally much thinner than encrustation found on iron. Artifacts manufactured from these metals, however, are often found encapsulated in the encrustation that surrounds iron artifacts. Prior to any treatment of the metal artifacts, preliminary conservation steps must be completed. These include (1) initial documentation, (2) storage, (3) encrustation removal, and (4) artifact evaluation. The treatment of each metal group, i.e., cupreous metals; silver and its alloys; tin, lead, and their alloys; and gold and its alloys, is discussed in detail.
STORAGE OF NON-FERROUS METALS
A variety of metal artifacts made of different metals are often found encrusted together in marine sites. In these instances, it is necessary to store the material in such a way that the most susceptible metal is afforded protection, and little to no damage is done to the other metals and non-metals found association with it. Since iron artifacts are the most commonly found metal, the storage conditions discussed under iron are most often used. However, gold, silver, pewter, brass, bronze, copper, and lead artifacts, as well as ceramics, stone, glass, bone, cloth, seeds, and wood, are often all associated in various combinations within a single encrustation. In some cases, the best storage for the encrustation may be simply in fresh water. Once the different objects are removed from the encrustation, they are placed in the most appropriate storage environment for each material.While iron artifacts, as discussed earlier, should at a minimum be stored in an alkaline solution protected from light, this solution is not necessary or even recommended for artifacts made of other metals.
Copper is corroded by oxidizing solutions and strong alkaline solutions. In neutral or slightly alkaline solutions, the copper is passivated and corrosion is halted by the formation of an oxide film on the surface of the artifact. A 5 percent solution of sodium sesquicarbonate or sodium carbonate is recommended for storing copper.
Silver is stable in aqueous solutions of any pH value and in the atmosphere, as long as these environments are free from oxidizing substances. Since chlorides do not affect silver or lead, they do not need to be placed in an aqueous solution and can be stored dry once the encrustation has been removed. Prior to the removal of adhering encrustation, however, it is best to house such objects in an appropriate solution to keep the encrustation from becoming harder and difficult to remove. Silver objects can be stored safely along with iron artifacts in either a 5 percent sodium sesquicarbonate or sodium carbonate solution. When silver that is encrusted to an iron object is stored in a chromate solution, a film of brown Ag2O will form. This film can be removed during the conservation of the object; chromate solutions, however, are not recommended for the storage of singular silver artifacts.
LEAD, TIN, PEWTER
Lead, tin, and pewter are more easily stored. All are often stored dry; when the encrustation on metals is allowed to dry out, however, it becomes much harder to remove. For this reason, lead, tin, and pewter are stored in aqueous solutions. Lead is corroded by aqueous solutions free from passivating substances, especially soft water, de-ionized water, or distilled water, and should never be stored in them. However, since lead is corrosion-resistant in hard, passivating bicarbonate water, and both tin and pewter are passivated in slightly alkaline solutions, all can be stored in tap water, with the pH adjusted to 8-10 by the addition of sodium sesquicarbonate. Lead and pewter can also be stored in sodium carbonate with a pH of 11.5. Tin will resist corrosion in slightly alkaline solutions that are free from oxidizing agents but will react adversely to strongly alkaline solutions. Any alkaline solution with a pH of above 10 is potentially dangerous to tin. Lead, tin, and their alloys, such as pewter, should not be stored in a chromate solution because of the oxidizing effect of chromate, which will form an orange chromate film on artifact surfaces that is difficult to remove. In the absence of passivating substances, an oxidizing agent, such as chromate, can damage lead, tin, and pewter artifacts.