If you're trying to come up with a way to get the gunk off an old coin or shine it up for display, stop for a moment and ask yourself why. If you're interested in collecting coins for their resale or investment value, cleaning them is never a good idea. Cleaning can scratch the surface of the surface of the coin and greatly decrease its value. The tarnish on old coins is referred to as "toning" among coin collectors, and it's actually considered to be a good thing. With that said, if you're interested in shining up a coin to make it into a piece of jewelry, do a science experiment with your kids, or use nonvaluable coins to make a decorative display, it can be handy to know how to make them shine. Take a look at some coin-cleaning tips that can help.

Clean Pennies with Ketchup

Parents everywhere know that ketchup is often the source of stains, but very few people think of it as a cleaning agent. However, ketchup can actually clean off the brown tarnish that keeps your pennies from looking new and shiny. All that you need to do is put some ketchup on a plate, put the pennies into the ketchup, and then cover them with more ketchup. Let them sit for a minute or two, then pick up each ketchup-covered penny and rub it with your fingers. Then rinse the pennies (and your fingers) under running water, and you'll see that the pennies are shiny again.

The brown tarnish on the penny is known as copper oxide. It's similar to rust, and like rust, it forms when the penny is exposed to oxygen. The ketchup trick works because the vinegar in the ketchup bonds with the copper oxide to create copper acetate, which dissolves in water. Once you rinse it off, the penny is restored to its former brightness. This is a fun experiment to perform with kids. If you don't like the messiness of ketchup, you can achieve the same results using plain vinegar.

Coca Cola Coin Cleaner

Another option that works for all kinds of coins is soaking them in a cup or bowl of Coke or a similar soda. Rinse the coins that you want to clean in running water first, then drop them into a container of soda. After a few minutes, remove the coins and rinse them. If they aren't yet clean, drop them back into the soda for a while longer, and then scrub the soiled areas of the coins gently with a soft toothbrush. Be careful not to rub too hard or use a brush with hard bristles, as this could scratch your coin. Repeat these steps as necessary until the coins are clean, then pat them gently with a clean towel to dry.

This method works because of the phosphoric acid in the soda. It helps dislodge the coating of grime and oxidation that you will find on old or dirty coins. For a moderately dirty coin, just soaking it might be enough. For a very dirty coin, the acid helps to loosen the grime, but you may need some additional pressure, like brushing with a toothbrush, to effectively dislodge the dirt.

Pencil Eraser

If you're not interested in cleaning coins with food products, there's another simple method that you can use that requires only a pencil eraser. All that you have to do is start rubbing the coin with the eraser. This can be time intensive, and your hand may get sore after a while, but eventually, you'll be rewarded with a clean, shiny coin. If you have access to an electric eraser (you can find them at a drafting supply store) then the process may be a little less strenuous.

While none of these cleaning methods should be used on potentially valuable coins, the eraser one can be especially damaging to coins that might have resale value. The pencil eraser can remove identifying mint marks that let coin dealers know how much the coin might be worth. It can also leave streaks and smudges on the coin. They may not be visible to you, but they'll be obvious to a coin appraiser with a coin magnifier or loupe.

These coin-cleaning methods can be easily used by almost anyone, even kids, and they won't cost you much, if any, money. Just remember that they should only be used on nonvaluable coins that you don't intend to save for their future value. If you're interested in serious coin-collecting, have your coins evaluated and cleaned (if necessary) by a professional coin dealer.