On cleaning coins I found what works the best for me and the cheapest to. I use the rock tumbler with the aquarium gravel, but will seperate my copper from my clads and do each one seperate. I will use Real Lemon juice I get from Sams club as it comes in 2- 1.5 quart bottles, so you get 3 quarts for $5. I add the gravel, the coins, then add about 1/8 of a cup of the lemon juice and fill with water. The copper I will run for a hour to a hour and a half and they will look like new. The clad i find I will run for 2 or 3 hours and dump the tumbler and rinse off the coins. I will pick out the good ones and run the others again with new water and lemon juice for a hours, then unplug the tumbler and leave sit for 5 or 6 hours to let them soak, then run for another 4 or 5 hours and most will look great and some shine even better than new. When I use them people ask why they shine so much and I tell them they are recyled coins from the ground.
Here is a couple of picture of the copper and the clads so you can see how they look.
<img src="http://www.findmall.com/gallery/albums/userpics/10090/cleaned.jpg " alt="" />
<img src="http://www.findmall.com/gallery/albums/userpics/10090/cleaned2.jpg " alt="" />
The quarter had such a shine to them that the ones on the top left had a glare that makes them look yellow. The on on the top right has a slight red tint like the one on the bottom of the picture. The pennies you can tell the zinc ones as the lemon juice started to eat away on the edge and are a little differnt color on the edge.
Rick