Hardened Dirt On Celtic Silver - Soak In Olive Oil Or Soapy Water?

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with the acetone soak and maybe use a little toothpick and Q-tip going over .

I suspect that the black stuff is a complex of silver / copper oxides and sulfides. Shallow corrosion scarring may be underneath.
Remember, this coin was probably recovered from soil contact burial, and most probably had a primary cleaning already. That would explain it's current appearance.

Try a soaking in sealed small container of acetone first, but I don't think that will do much good.
To remove, it will require physical removal with a Q-tip. Patience needed.