One way to clean old pennies is to soak them a solution of salt and vinegar. A mixture of ¼ cup of vinegar and 1 tablespoon of salt cleans 20 to 30 pennies in about five minutes.
Pennies are made of copper plated over another metal. Over time, the bright copper-colored penny takes on a dull brown look as the copper reacts with the oxygen in the air to form copper oxide. The copper oxide turns from brown to a greenish color, called a patina, according to the Copper Development Association. While this green color is valued in architecture and copper sculptures, such as the Statue of Liberty, it causes pennies to lose their new look.
In a solution of salt and vinegar, the acid dissolves the copper oxide on the pennies, leaving the coins clean again. Other acids, such as lemon juice, cause a similar reaction.
If coins are old or valuable, it is better to avoid cleaning them. Cleaning off the natural oxides of coins decreases their value to other collectors. However, About.com recommends cleaning circulated coins using a mild dishwashing soap and water for sanitary purposes. The use of a silver polish, abrasive or the salt-and-vinegar cleaning solution may ruin the coins.