How to clean Nasty green from ocean coin?

[Dirty Digger Doug]

So we found a coin on the beach today. It's slightly larger than a penny and slightly smaller than a nickel. It's thick like a fatty Indian. Problem is the green grunge hardened on the coin won't come off. We got the sand, shells and barnicals off but this green crust won't budge. How do I get this off without damaging the coin?

 

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[alloy_II]

So we found a coin on the beach today. It's slightly larger than a penny and slightly smaller than a nickel. It's thick like a fatty Indian. Problem is the green grunge hardened on the coin won't come off. We got the sand, shells and barnicals off but this green crust won't budge. How do I get this off without damaging the coin?

That's strange, copper based paints are used as anti fouling agents to keep marine life from attaching itself to the bottom of a ship or boat.

Green indicates your find is copper but you claim your find had barnicals.

I'm not aware of any nondestructive means to removed the green copper oxide.

 

[Dirty Digger Doug]

Well not barnicals per say but the eighth inch of crust ball that harden onto coins found on the beach. Casting a light from the side it kind of looks like Lincoln's siloete.