Tarnish

Tarnish is a layer of corrosion that develops over copper, brass, silver, aluminum as well as a degree of semi-reactive metals as they undergo oxidation. It is analogous to rust, but with a slower rate of occurrence. It is mainly caused by chemicals in the air such as sulphur. It appears as a gray or black film over the metal.

Prevention

Most tarnishing is prevented by tinning, a process by which the reactive substance is coated in a non-reactive substance, such as tin or wax, and thus protected from oxygen.

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