With the harsh nature of maritime sites artifacts such as timbers, cloth, and organic materials can quickly deteriorate, leaving more durable artifacts such as metals, ceramics, and glass. Though metals may be less prone to deterioration under certainty circumstances, they are by no mean free from corrosion. Metal artifacts recovered from marine and submerged sites are very unstable due to the corrosive nature of the minerals dissolved in water. Metal artifacts recovered from marine sites must be treated in order to stop artifacts from deteriorating beyond repair. Removal of the artifacts from the water may actually do more harm than good and quicken the deterioration of the artifacts. When metals are recovered from the sea they are commonly encrusted in a conglomerate of different materials (Rodgers, 2004). Common encrustation of metals can consist of calcium carbonate, magnesium hydroxide, metal corrosion, clay, sand, and marine life (Hamilton, 2010).These problems may often be solved with a conservation method known as electrolytic reduction or electrolysis. Electrolytic reduction is the process of submerging metal in a chemical solution, then running electrical current through the solution and metal artifact, resulting in the movement of ions from the artifact to the cathodes creating a low amp current (Kipfer, 2000). Electrolytic reduction is one of the most effective methods to conserve metals (Hamilton, 2010). Besides being an effective method, electrolysis is also very cost effective and relatively easy to produce.
Mechanical cleaning of an artifact is recommended to quicken the process of electrolytic reduction, but care should be taken to not damage the artifact in the process. Pneumatic tools are very efficient and less destructive than some chemicals used to remove encrustation. A weld-flex chisel and the smaller Chicago Pneumatic Airscribe are particularly serviceable for removal of encrustation (Hamilton, 2011). When these tools are not available a standard hammer and chisel can be used with some degree of success.
Electrolytic Reduction Setups
There are three different electrolytic reductions setups that can be used in the conservation of metal artifacts. The first setup would be described as the most ideal in that the artifact is submerged in its own vat, surrounded in a form fitting anode that is equal distance around the artifact (Hamilton, 2011). The artifact is then connected to a regulated direct current (DC) power supply. This setup allows the conservator to monitor just a single artifact and allows for electrical current adjustments specific to that artifact. Chloride levels can be tested to determine if the artifact is chloride free with this setup while with the other two this is not possible. This setup is ideal for artifacts of great significance or that need careful conservation.
The second setup has multiple artifacts in a single container, but each artifact is surrounded by its own form fitted anode and is connected to its own DC power supply. Artifacts in this setup must have ample space in order to prevent cross currents. Chloride levels with this setup cannot be tested for each individual artifact, but can be tested to determine when the electrolyte solution should be changed. This setup is sufficient for artifacts that do not need constant monitoring but are still of some importance.
The third setup is the most commonly electrolytic reduction configuration that is used in the conservation of metal artifacts. The setup consists of multiple artifacts all in the same vat and all artifacts connected to a single DC power supply. This configuration is the least desirable of the three setups, though it does provided advantages in processing a larger quantity of artifacts. The previous setups needed a DC power source for each artifact, which takes up space that can be used for conserving more artifacts. Multiple artifacts connected to a single DC power source is cost-effective and can provide more lab space for artifact conservation and analyses.
Post Electrolytic Reduction Procedures
Electrolytic reduction is used to stabilize an artifact, but without proper conservation efforts after electrolysis, the artifact will start the corrosion process over again. One method to prevent corrosion from occurring is the application of tannic acid. Tannic acid creates a corrosion resistant film that seals the metal from moisture in the atmosphere. To apply the tannic acid the artifact has to be heated, most commonly by boiling in distilled water, to open the pores in the metal and then tannic acid is applied. The artifact is then left to dry and the process repeated a minimum of three times to ensure that the artifacts are completely sealed. The tannic acid will stain the metal black which can often be seen on metal artifacts such as cannons. The application of tannic acid is commonly the last step in the preservation of a metal artifacts, though it is recommended that an additional sealant, such as microcrystalline wax be used (Hamilton, 2011).
With most metal artifacts recovered from maritime site being conserved with the use of electrolytic reduction it is important for maritime archaeologists to understand this essential process. Electrolytic reduction produces stable artifacts with little effort and does not consume large amounts of a project’s budget. Ample laboratory space is needed for larger artifacts while smaller artifacts could be stored and conserved in a relatively small amount of space. The processes of electrolytic reduction can also provide curators with more time to focus on other aspects of a project, due to the fact that constant monitoring of the electrolytic reduction process is not needed. Though stabilization of artifacts may be crucial to a project it is important to note artifacts that need to have their surfaces maintained, such as coins or artifacts with inscriptions and designs, may not be suitable for the processes of electrolytic reduction.
Hamilton, Donny L., 2011, Archaeological Metal Artifact Reduction/Cleaning by Electrolysis. Texas A&M University. http://electrochem.cwru.edu/encycl/art-a04-archaeology.htm
Hamilton, Donny L., 2010 Methods of Conserving Archaeological Material from Underwater Sites. ANTH605 Conservation of Cultural Resources I. Nautical Archaeology Program, Texas A&M University http://nautarch.tamu.edu/CRL/conservationmanual/index.htm
Kipfer, Barbara A.,2000 Encyclopedic Dictionary of Archaeology. Kluwer Academic, Plenum Publishers, New York.
Rodgers, Bradley A.,2004 The Archaeologist’s Manual for Conservation: A Guide to Non-Toxic Minimal Intervention Artifact Stabilization. Kluwer Academic, Plenum Publishers, New York.