5-10 minute soaks in lemon juice followed by nylon brushing. Repeat. When finished, a quick bath in a baking soda/water solution to stop the acidic process and you should be good. This may take a few session. You should also post this in the cleaning section and progress pics if possible. I would move it but my mod powers do not extend to this forum. 
Chris
I would follow this advise. Ammonia does indeed clean tarnished off silver, however, you should want to preserve the
patina on this coin if at all possible. Lemon juice will give you much better control of this and will break up the crud on the coin that you want gone.
It also smells way better than ammonia. That and ammonia can easily damage silver if not diluted to a proper ratio for use on cleaning silver. I would avoid ammonia, even if it meant leaving the coin
as is. Just my opinion.
BTW, the coin is indeed nice as it is IMO.
