If you've got: tarnished copper
Ideal solution: Copper cleaner containing citric or oxalic acid, or a mild abrasive, such as fine red jeweler's rouge
In-a-pinch fix: Lemon juice mixed with rock salt
How it works: Air causes copper to oxidize and turn color. The acid from the lemon eats away at the oxidation, while the coarse salt provides a hefty dose of scouring power.