So your cleaning out grandpa's basement and you come across some old pennies. They look green and blackish. Are they worth anything? You can't see the dates. What should you do?
First, the best thing is to step back and make sure you want to clean the pennies at all. Some of those old coins can be fairly valuable, and cleaning them will devalue them as much as 90%. So take the time to look at them enough to identify them, and then use a coin guide or take them to a coin dealer to make sure that you don't have the family heirlooms in the bag.
If you've determined the coins are simply old, and not valuable, then your can move on to actually cleaning the coins. If you look around the web, you can find a whole host of ideas on how to clean a copper penny. But make sure you are cleaning a copper penny. Since 1982, pennies have been minted with 97.5% zinc and only 2.5% copper. This was done as the price of copper kept climbing, and it was not going to be long until the copper content of a penny was worth more than the face value of the penny.
But zinc reacts differently than copper to some of the popular cleaning approaches to cleaning pennies, and in this case differently means that it will degrade more quickly, so any solution using acids or other reactive agents would best not be used on the newer pennies.
There are a whole host of ideas on cleaning pennies. Some ideas include:
Rubbing them clean with pencil erasers
Mixing a solution of vinegar and salt and soaking them
Using taco sauce as a cleaning agent
Using a brass cleaner such as Brasso
Using a wire brush with a small tool like a Dremel tool
Now some of these ideas will work much better than others. But others are actually destructive to the coins, reducing the coin's value...