Tarnished copper can be cleaned with a commercial copper cleaner or homemade cleaner. The easiest homemade cleaner is vinegar and salt.
Small articles can be boiled in a pot with one tablespoon of salt mixed in one cup of white vinegar. For large items, mix one tablespoon of salt with one cup of white vinegar and enough flour to make a paste. Rub the paste onto the copper until the tarnish is removed.
The tarnish is removed by a weak acid such as vinegar, so lemon juice can also be used. The salt acts as an abrasive, so rubbing large copper pieces with a cut lemon dipped in salt works as well.