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Thursday, October 4, 2012

Using Electrolytic Reduction to Preserve Historic Artifacts

By Kerri Wilhelm, Historic Sites Curator of Archeology

More often than not, artifacts recovered from an archeological excavation—especially those recovered from marine environments—are heavily corroded and require some form of cleaning for archeologists and researchers to recover information. This is especially true of historic metals and iron-bearing artifacts, in particular.

A proven laboratory method of treating and “cleaning” corroded metals is electrolytic reduction (ER). In this process, a controlled electrical current is applied to a solution called an electrolyte. Correct application of an electrical current to historic metal antiquities will incite the formation of hydrogen bubbles on the artifact, which can effectively lift off much of the corrosion product.

Electrolytic reduction also involves a chemical process wherein reduction occurs and the corrosion products on the artifacts are “reduced” back to their metallic state. This part of the process consolidates the metal corrosion layer. This layer can then be mechanically cleaned by the formation of the hydrogen bubbles on the artifacts that will lift off the consolidated corrosion product.

Anna Lumbroso, a summer intern with the THC’s Historic Sites Division, recently created an electrolytic reduction bath in the Historic Sites Curatorial Repository Processing Lab in Austin. Her process and results are detailed below.

In the photo above, Anna is combining the soda ash and de-ionized water to create the electrolyte for the electrolysis units we will be running in the lab. She must carefully calculate the dry weight of the soda ash (sodium carbonate, Na2CO3), convert the measurement to a liquid equivalent, and very slowly mix the solution to ensure uniform and consistent conductivity in the bath. The measurements must be accurate to ensure the correct pH is achieved: a 5 percent sodium carbonate electrolyte with a pH of 11.5 (extremely alkaline, hence the goggles and gloves).

Jim Jobling and The Texas A&M Conservation Research Lab were kind enough to donate a five-gallon bucket of sodium carbonate for our first few tests.

Next, Anna is “tinning” the leads to the cathode (3/16” brass rods) and anode material (16 gauge mild steel mesh with ½” openings) with steel solder to ensure the conductivity of the lightweight appliance wire we selected for our electrolytic cell. After they are tinned, she attaches them to Mueller clips, which will then be attached to the cathodes and anode material. Learning to solder has been an experiment in patience for both of us!

In the photos below, Anna attaches the positive lead to the anode.

Shown below is the completed electrolytic cell with the sodium carbonate electrolyte.

The ER baths are run inside the fume hood to evacuate the hydrogen gas, which is the byproduct of this process. Once the electrolysis has begun, the electrical current incites the formation of hydrogen bubbles on the leads (clips) and the artifacts themselves as reduction takes place. The purpose of the process is to attempt consolidation of the oxidized layers of corrosion product while also removing chlorides, thus arresting, or at least greatly impeding, the decay of the object.

Anna has created a small ER cell utilizing an old cell phone charger and a simple 6oomL Kimax chemistry beaker that we “liberated” from UT’s surplus. The drawback of using the cell phone charger is being unable to adjust the current density. Fortunately, the electrical draw is low. A positive of the charger use is being able to create a small anode custom fit to smaller, more delicate artifacts.

Anna has enjoyed this laboratory project and has learned a process that she can take forward with her in her academic and professional careers.

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