Coins found at beach...how to clean??

[kgillard]

I found some pennys (and silver coins) at the local beach this afternoon. The pennys were encrusted with an almost rock like covering. I tried the hydrogen peroxide and it bubbled for quite a while, but it did not remove the crust around the penny. I took one of the pennys that I could tell was modern and used my grinder's brush wheel to try and remove the crust. All it did was get the penny too hot to hold.

I'm new to all this, so please forgive me if I'm just asking something that has been asked before. Thanks in advance for any help.

Karl

[Farmercal]

rock tumbler and aquarium gravel