What's the best way of removing hard red deposits on a silver coin?

The red is probably cuprite, a stable copper mineral. Which means it won’t come off without a fight. Worse, as copper rich soils are relatively rare, the copper in the cuprite probably came from the coin itself. If you get it off you’ll probably be left with a pit. It’s an attractive coin as is. I wouldn’t risk damaging it.

 

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Robin is right, it does look like it came from a British hoard.