Bathe the coin in Acetone for a few minutes to arrest the Verdigris (the specs of green which can and will destroy the coin if not dealt with), then rinse with water, then soak in Mineral Oil or Extra Virgin Olive Oil for several weeks or more, checking it periodically with a toothpick or green Rose Thorn by picking at the corrosion and encrustations on the coin. If luck is with you, most if not all of the corrosion and encrustations will come off, revealing more details.