Tarnish
From Academic Kids
Tarnish is a layer of corrosion that develops over copper, brass, silver, or aluminum as they undergo oxidation.
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How to treat
When Tarnish occurs, it is treated chemically, usually with strong acids. Many commercial products are dedicated specifically to this.
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How to prevent
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.
