Personally I would say that you have a long way to go. I would try a mix of 1
part ordinary vinegar to 20 parts de-ionized water. This is about a 1% vinegar mix, and would
work very slowly. soak the coin for about 15 minutes, then scrub. You have some really hard clay and copper oxide that is
still block int coins details. After the stuff starts breaking up, soak the coin in baking soda and water mix for 24 hours to neutralize the vinegar. You need to watch carefully if you use vinegar around copper based materials
An alternative method, boil the coin in de-ionized water [2 cups] with a teaspoon of dishwasher soap and a teaspoon of baking soda. Boil for about 2 minutes, then let it sit until it is cool. The clay need to be softened. You can scrub it with a stiff nylon brush as soon as you can
handle it.
Bruce
