"Clad" is the term used for the zinc/copper clad coins that are used for most modern coins.
Metal detectorists that find these coins often collect them until they have a big pile and then cash them in, since they have no value for collecting (other than the face value of the coin itself).
Some folks just leave the coins dirty and use a Coinstar machine to cash them in.
If you want to clean them yourself, the best practice is to use a small rock tumbler. Put some aquarium sand or small aquarium gravel in with a few squirts of liquid dish detergent. Do NOT add water, as that will make the soap sudsy and will leak from the tumbler. Put the coins in with the sand/detergent mix and tumble for 6-24 hours, depending on how bad they are.
Do NOT tumble pennies in with nickles, dimes and quarters, or the 'silver' coins will take on the copper color from the pennies. So, tumble your nickles, dimes and quarters together, then do a second batch with just your pennies.