Conservation

Metal Artifacts

Metal artifacts found on the Monitor include those that are made from iron, copper, lead, silver and tin. As a result of their malleable nature when heated, they can be made into many different forms. In contrast to non-metal artifacts, the most important type of deterioration in metals is a result of chemical deterioration rather than physical deterioration. Most metals react with chemicals in the environment such as oxygen and salts to form corrosion products in the forms of oxides, carbonates, and sulphates.

Monitor’s rotating gun turret. Photo Credit: Monitor Collection/NOAA

 Iron artifacts from the sea are often the most problematic to clean and stabilize of all submerged materials. Since chlorides destabilize iron and accelerate corrosion, and chlorides are common in sea water, these artifacts tend to be significantly corroded and are rarely found in great states of preservation. During conservation processes, it is essential to stabilize the artifact immediately after removal from the ocean in a wet storage container to begin reducing the chloride levels. This is commonly done by soaking and sometimes through a process known as electrolytic reduction which helps remove chloride compounds from the interior of the artifact, and encrustations from the exterior. The object is placed in a solution of sodium carbonate or sodium hydroxide where an electrical current is passed through it to remove the chlorides and stabilize it through the process of reduction.

Lantern recovered from the Monitor. Photo Credit: Courtesy of The Mariners’ Museum

In contrast, copper artifacts may appear bright and virtually uncorroded even if they have been under water for many years as long as they have been in an environment where oxygen is scarce. In addition, copper in solution is toxic to most marine organisms so the presence of encrusting marine life is a good indication that the artifact surface is free from active corrosion. Patinas and corrosion crusts that do form on copper surfaces are typically more fragile than they appear and can be seriously damaged even by casual cleaning. Artifacts that are made from lead usually do not require extensive work because surface encrustations are rarely obscuring. Tin and lead do not need extended desalination, because they usually do not react with any residual chloride in the metal after treatment. Any cleaning that is necessary is done with wooden tools to minimize the damage to the artifact. Tin artifacts also require little cleaning other than dirt removal. If they were found in areas with corrosion stimulators such as chlorides and sulphates, localized roughening of the surface may turn into small pits. These pits of tin oxide cannot be removed without exposing the surface to additional corrosion and therefore remain even after stabilization and cleaning.