Olive oil will work a bit better than water for getting the green stuff off copper coins.
Let it soak in there for a day or so and then take a tooth pick and prick what green will come off, then back into the olive oil bath for a while and repeat. I've dug old coins that were undiscernable as to what they were (other than shape and diameter) that I got to where I could tell what they were from doing the above.