Some of the old "Cash" coins are valuable but...go slowly. Try olive oil first (neutral pH) as it is a solvent and will remove dirt and grime. Next you might try acids like lemon juice ( pH 2) or white vinegar (pH 2.4 @ 5%) or coffee (pH 5) and these will remove any shells and incrustation. [I just removed shell and sand encrustation from the top of a 2 1/4" silver topped alligator tooth by soaking the top only in white vinegar overnight and it came out great.] Finally, if all else fails, try hydrochloric acid (pH 0), also known as Muriatic acid, for a few seconds at a time, wash-off with water and check results, repeat. FYI Handle HCL with extreme care, goggles, rubber gloves, etc.