You could soak it in olive oil a few weeks and see if loosens up ..might sound crazy but I’ve cleaned a few in that condition with hydrochloric acid.. I dip a small acid brush into the acid and brush those few drop of acid back and forth till the crust dissolves. Often the coin will turn grayish white after a few minutes,at this point if there is any detail left you will see it..as long as you don’t take it down to bare copper there is a chance to see detail..after that I rinse it off ,don’t rub it , after that I soak it in olive oil just to slow the corrosion, if I found any detail on it I then let it sit down in my somewhat damp basement a few weeks or months and then tumble it with quarters and water so only the higher portions of the coin will lighten up an give it some contrast.. at this point you gotta check it often to make sure only the high points are bright and the lower areas stay dark... after that a quick rinse followed by a coating of paste wax... there probably are better ways but this works for me and I only due it on coppers that appear worthless anyway... at your own risk...
