Iron has been used by man for centuries as a building material. It is one of the most plentiful metals found on the earth. Iron artifacts recovered by divers include portholes, knives, flintlock pistols, running light frames, and a wide array of other interesting items. However, when left in a saltwater environment for long periods of time iron is vulnerable to oxidation, where acids and chlorides create an electrical charge that corrodes the metal compound. In older artifacts, oxidation has caused a level of marine biological growth to encapsulate the object. This growth, called a concretion creates a highly acidic layer on the artifact that greatly increases corrosion.
Iron artifacts removed from sea water should be immediately placed in a fresh water solution. If allowed to dry, the concretions surrounding the object become very hard and cling to the artifact with much more intensity. Regular tap water works for storage, but rain water and distilled water have a higher ph level that allows them to inhibit corrosion much more efficiently.
Removal of the concretion level depends upon the size and general condition of the metal. For large amounts of concretion, a hammer blow can generally remove most of the outer layer. For smaller and more brittle objects, a dental pick, or brush can be used. Remember to always keep the artifact wet when removing concretions.
Laboratories use a technique called glass bead blasting to rid artifacts of the concretion layer. This is similar to sandblasting but the soft glass beads are far less abrasive than sand. The normal intensity of blasting ranges from 25 to 80 pounds while being mindful of keeping the nozzle at least 6 to 10 inches from the item. Each artifact should be carefully examined before this process and with more brittle objects, air pressure should be decreased.
The most effective method for restoring iron artifacts is electrolytic reduction. This process reduces concretions to a more stable form where they can be removed with less stress on the metal. The artifact is completely submerged in tap water with a 2% solution of sodium carbonate (baking soda). It is then subjected to 3 amps of electricity for two to three days.