Both lead and silver solder have a relatively low melting point, (as compared with the metal of a coin). I'd just use a propane torch to heat the coin and either blow the solder off with a compressor/air gun, or you can wipe it off with a piece of XXXX steel wool. Careful because the coin will be hot,hot,hot, and the steel wool is highly ignitable around a flame. Carefully hold the coin (lightly) at the edge with a small pair of needle nose pliers, or a pr. of long forcipes. After removing the solder and letting the coin cool down use a metal polish like Maas or Flitz to get the coin back to it's original color since the heat from the torch will discolor it. Good Luck!