I found my 1st and only silver dollar 5 years ago.? Unfortunately, it has some evil black junk stuck on the front.? It's raised like a hard tar or something, and isn't the lovely toning I see on some old silver.? The back is also very dull and dingy unlike any other silver I have found.? I'd appreciate any ideas on how to help it out.? The usual baking soda rub did nothing to help.? I'll try something mild but nothing drastic because it's a nice conversation piece the way it is.