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Metal Tarnish Formation
Name: Lisa B.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 4/21/2003
Question:
Why does metals tarnish?
Replies:
Lisa,
There are "precious" few metals that are resistant to tarnish. Such discoloration occurs when the
metal is able to react with or be attacked by something that can make a chemical compound with
the underlying metal. Silver can react with oxygen or sulfur compounds to form a brownish-to-black
tarnish film. Pure gold, on the other hand is resistant to such reactions. Only 24 K gold is pure.
Gold of lesser karat (10K 14K, 18K and 22K) are all alloys of gold and other metals -- usually
copper. As such, all can exhibit
a tarnish capacity that's inversely proportional to their purity. That is, the closer the alloy
is to 24K, the less the tendency to tarnish.
Wherever it occurs, tarnish almost always looks very different from the original polished metal.
Tarnish is removed either by employing another chemical reaction to dissolve the tarnished
surface, or by using a mild abrasive to actually scrape away the discolored compound on the
metal to expose the underlying metal again. Ordinarily, such cleaning processes remove very
little of the original metal.
Regards,
ProfHoff 642
Some, not all, metals tarnish. This is due to the formation of oxides and/or sulfides on the
metal surface. Because of the small particle size, the oxide/sulfide particles on the surface
appear black, so the metal loses its luster.
Vince Calder
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Update: February 2012
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