A few years back I was hunting with a team of archaeologists in Spain with my kids and they showed me a trick to clean crud off of coins - and we were using 2,000 year old Roman bronze coins.
When we got back to the condo at night, they would soak the coins in DI water (the stuff you get at the shopping center) for an hour or two and then pop them in the freezer overnight. What happened was that the water penetrated the crud - not the coin - and when the water froze, it expanded and cracked the crud off the coin. It took several times to get most of it off but the neat thing was that the water did not get into the coin and hence could not damage the coin.
I have tried it on coins here and it works pretty darn well.
You can always soak them in olive oil at the end to give it some patina again
Just another trick to try.