Here is another interesting cleaning method I have not tried yet....
How To Clean Coins by Frozen Water Expansion
This method effectively works on coins that have hard deposits of dirt on their surfaces. This cleaning technique is simple and consists of only two steps:
1) Place a coin in distilled water and soak it for a while so that the water molecules would have enough time to penetrate the dirt and, if possible, other deposits on the coin's surface. Some dirt may break off during soaking, so you just toothbrush it off the coin.
2) Place the coin in a plastic container and put the container in a freezer. As the water gets frozen, its ice crystals will expand and break the dirt deposits apart. To completely rid the coin's surface of dirt, you need to repeat the soaking/freezing procedure several times. This cleaning technique will not damage the coin.