There is one unavoidable constant for any person who owns silver, tarnish. Whether its the cutlery you only pull out for Christmas dinner or a pair of earrings. Here in the museum we also have to deal with tarnished silver, most recently a silver tea set from the McCook House.
With as many objects as we have in the collections at the Ohio Historical Society and our affiliated sites, keeping every piece of silver properly polished is almost an impossible feat. Over time, unpolished pieces build up a dark residue. Silver is effected by the environment, particularly hydrogen sulfide, which causes a chemical reaction on its surface known as tarnish. This discoloration begins with a milky white color, progressing through yellow and brown. In sever cases, tarnish can appear nearly black. This was the case with the McCook silver.
So how does a museum clean heavily tarnished silver? That is something varies depending on the material (solid silver v. plated silver) and curator/conservator preference. It should be said that there are many products available for silver cleaning. For museum silver, I personally dislike using commercial treatments that can contain unwanted ingredients. They can leave a residue that, over time, can built up and cause harm to the collections and are often far too abrasive, leading to damaged or lost silver.
The first step in any cleaning or conservation process is observation. You have to know what is going on with the piece to know how to treat it, and its important to look beyond the obvious. Yes, the set was heavily tarnished. What else? Looking closely, there was observable silver loss on raised decorative elements throughout the set. This meant that we had a damaged silver plate, which is more susceptible to further loss if the cleaning process was too abrasive. So what did we do?
Chalk! Well, close. Calcium Carbonate is a soft abrasive, meaning it will affect the tarnish without risking unnecessary damage to the already compromised silver plate. This compound can be purchased though numerous vendors online at very reasonable costs. To make the cleaning paste, you need to combine your calcium carbonate (a white powder) with deionized water until it is the consistency of marshmallow fluff. You dont want it too runny or too firm.
Using soft cloths and cotton swabs, we proceeded piece by piece. Apply, rub, wipe away. Repeat. Repeat. Repeat. Because the cleaning mixture is such a mild abrasive, and the tarnish so heavy, the entire process took two staff members one full work day. Long, time consuming, but oh so satisfying. Once the tarnish had been removed, detail work could begin. Bamboo skewers (found in the grocery store) and soft bristled brushes helped us remove the excess mixture from the various nooks and crannies.
The last step is a good solid polish, using a silver polishing cloth (or rouge cloth) and elbow grease. You never want to submerge silver plate completely in water because the base metal can corrode. Buffing with the polishing cloth also helps to dry the small amount of water used in the calcium carbonate mixture.
Once the silver shines again, regular polishing with the cloth will help prevent similar build ups. I recommend once every 2-4 weeks.