I agree with 4-H a rock tumbler works great, like he said if you clean the pennies and and clad together the clad will be stained a copper color. I use small un colored aquarium gravel a small shot of dish washing liquid and enough water to just cover the coins and gravel. I usually let them run a couple of hours and they are clean, just rinse them off and dry. I bought the Harbor Freight tumbler it works fine but all the belts that came with it broke over the summer. I bought some after market belts on eBay and it hasn't broke yet. I've used the tumbler to clean other finds but I don't think it would work on silver or gold, it would probably ruin silver coins.
Joe