Daily Wire Tip Oct. 3: The Sterling Silver Tarnish Culprit
Daily Wire Jewelry Making Tip for
October 3, 2010
Question:
In one recent Tip of the Day, you said “without the presence of an alloy such as copper, fine silver will not tarnish.” So it’s the alloy that causes silver to tarnish?
-Julia in Honesdale, Pennsylvania
Answer:
Yes, sterling silver is composed of 92.5% silver and the most common alloy added is 7.5% copper. Copper tarnish is composed of CuO, otherwise known as oxidation when moisture is present in the air.
When there are also acids present in the air, along with carbon dioxide (such as pollution) a greenish tarnish/patina occurs, which can be seen on landmarks like the Statue of Liberty.
Answer contributed by Dale “Cougar” Armstrong
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copper, fine silver, nontarnish, silver wire, sterling silver, tarnish
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Technically, silver and gold are unable to interact chemically with oxygen to form tarnish. Tarnish is a type of corrosion. Copper and oxygen form cuprous oxide (Cu2O). Silver will interact with sulfur to form silver sulfide (Ag2S) which is called antiqued silver by jewelers; this is actually a tarnish or corrosion layer. I believe the silver sulfide is self-limiting in that it will only cover the surface and not penetrate deeply into the silver. Gold can develop a thin black film that can be wiped off, but I can’t remember or find what chemical interacts to cause that; I know it’s not oxygen. I used to work with a corrosion expert and I learned some corrosion trivia from him.
Thanks for the additional info Diane.
You forgot to mention that the alloy Argentium (patented, I think) is also by law Sterling, and though I think it has copper, it also has germanium, which keeps it from tarnishing.
All this brings up the question of wire storage. I put my wire into plastic zip lock bags and then a put them into a binder all together. Can tarnish be advanced through the plastic from copper wire to silver? Also I store my scraps together in one bag…maybe I should not be doing that if I intend to use them later??
Karen – copper will not permeate the plastic between it and silver but you really should separate your scraps if you intend on using them at a later date and do not want to have to clean them first.