Tarnish

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Tarnish is a layer of corrosion that forms over copper, brass, silver, aluminum, and other semi-reactive metals as they undergo oxidation. It is analogous to rust, but with a slower rate of occurrence. Tarnish is mainly caused by chemicals in the air, such as sulfur. It often appears as a usually dull, gray or black film or coat over metal.

[edit] Chemistry

Tarnish is a product of a chemical reaction between a metal and a nonmetal or compound, especially oxygen and sulfur dioxide. It is usually an oxide, the product of oxidation.

[edit] Prevention and removal

Using a thin coat of polish can prevent tarnish from forming over semi-reactive metals. Tarnish can be removed by using steel wool, sandpaper, emery paper, or a file to rub or polish the metal's dull surface.