You've probably noticed that holding a copper wire in a candle flame turns the copper black. This is because the high temperature of the flame hastens the natural oxidation of the metal. In the chemical reaction that takes place, two copper atoms combine with an oxygen molecule -- which has two oxygen atoms -- to produce two molecules of copper oxide, which results in the black coating on the wire. It's actually quite easy to remove this coating. All you need is a mild acid. Two readily available ones include lemon juice, which contains citric acid, and white vinegar.
Fill a pot with water, then add a cup of white vinegar and a tablespoon of salt. Mix thoroughly until the salt is dissolved. The mixture of vinegar and salt produces a mild acid that can dissolve copper oxide.
Bring the solution to a boil, drop the burnt wire you want to clean into the pot and let it sit in the boiling water until the blackened coating is gone. This may take one to several hours.
Let the water cool, then remove the wire. Rinse it with clean water and dry it with a rag.
Dissolve a tablespoon of salt in a cup of lemon juice to create a stronger acid to dissolve stubborn tarnish and blackening on copper wire.
Immerse the wire in the solution, and then remove and inspect it. If it's still tarnished, let it remain a little longer in the solution.
Neutralize the wire after removing it from the lemon juice solution by rubbing it with a paste made from baking soda and water. Failure to do this could result in pitting as the acid continues to react with the metal.