If you’re like most of us, you probably have a jar or other container of loose change in your home, and it’s probably mostly pennies. One look at them will tell you that pennies become worn and tarnished over time, and you may want to clean them. Various methods exist to clean pennies, but most use the same few simple principles of chemistry. One of the easiest and most common ways to clean pennies is to wash them in a mixture of water, salt, and a mild acid.

The first thing to do in order to clean pennies is to be reasonably sure that none of them is a potential collector’s item. If you got most of your change from buying a morning cup of coffee or from a vending machine, then the chance that you have anything collectible is slim, but it still helps to be sure. If you clean the tarnish off of a collectible coin, you may accidentally destroy its value, so if there’s one you’re unsure about, it may be best to put it aside.

The materials needed to clean pennies are very simple. A shallow plastic or glass bowl should be filled about halfway with water. Add 1/4 cup (60 ml) of white vinegar and 1 teaspoon (5 ml) of table salt to the water and stir until the salt is dissolved. Put 20-30 pennies in the bowl for about five minutes. It’s a good idea to stir them around once or twice during this time. After the five minutes are up, the clean pennies can be rinsed and dried, and they’ll keep their shine for a while.

Pennies get dull and tarnished over time because of the copper that is used in their composition. Copper slowly reacts with oxygen in the air to form copper oxide. This compound has a dull, greenish color to it, and is what we see as tarnish on copper pennies. When pennies are washed in a vinegar and salt mixture, the acetic acid contained in the vinegar is able to dissolve the copper oxide, leaving behind shiny clean pennies. It is possible to use lemon juice in place of vinegar, because of the high acid concentration in lemons.

Ketchup and hot sauce can also be used in place of vinegar, because they both contain acids and salt. The presence of salt is more important, chemically speaking, than the type of acid that is used. If you want to clean pennies that were minted after 1982, keep in mind that around this time, pennies began to be made mostly of zinc, rather than copper, because of the rising price of copper.

Zinc may be more easily degraded by an acidic solution than copper is. If this poses a problem, one final method used to clean pennies is by gentle abrasion. Rubbing a pencil eraser on a penny will remove most of the tarnish, though not as much as salt and vinegar will.