I've been finding old silver, with chunks of iron adhered to the coin. After some research found that freezing and thawing the coin in a cup of water does the trick. Break up some toothpicks, and put in the bottom of the cup, add coin, water, and freeze. Water slowly works its way between the two metals and creates a wedge between them. Recently cleaned up two barber quarters nicely! You don't have to worry about abrasives or chemical cleaning this way.