Try soaking it a few days in a Rx bottle with few drops of Dawn Liquid soap, hot tap water, shake every now & then over a period of 2-3 days, rinse with hot tap water, pat dry with soft paper towel.
See Sleepy Hallows "most wanted Roosevelt dimes" post #491 in CRH forum about his dirty merc dime, and used Dawn on it, see his before & after pics of using Dawn.