I use aluminum foil, vinegar, baking soda, and a pinch of salt. You get a small pan or anything that will hold liquid that you can heat up on the stove. Line it with the aluminum foil shiny side up. Put the vinegar, salt, and baking soda in and boil it. Drop the coin in for a couple minutes and the stuff will come right off when you use a cloth on it. I'm sure you know cleaning it devalues it. Also you only need a little vinegar just enough to completely submerge the coin. You can use this method if you want all that off quick.