What Causes the ‘Rust’ on Coins Over Time?

Examples

1. A copper penny left on a windowsill turns green after months in the sun.
2. Coins in your piggy bank may look dull if they are left there for years.
3. Old coins in your wallet might have a rough texture from oxidation.

More examples

See also

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• What is Temperatures between 60°C and 75°C?

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