Peter, try soaking the coin in a mixture of baking soda [sodium bi-carbonate, 5 parts] and washing soda [sodium carbonate, 8 parts] mix 10 grams of this in 200 ml of distilled water [or de-mineralized] . Let it soak for 24 hours, then brush with a nylon brush [like a stiff tooth brush] and some liquid soap. You can continue the soak and scrub for awhile, it should help.
Bruce