Tarnish

From Freepedia

Tarnish is a layer of corrosion that develops over copper, brass, silver, or aluminum as they undergo oxidation.

Treatment

When tarnish occurs, it is treated chemically, usually with strong acids. Many commercial products are dedicated specifically to this.

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.