Thank you for your responses otlichnik and Robert_Brenchley.
After watching the coin carefully with a magnifying
glass 15x, it looks to me as the corrosion is all on top of the silver...So it can be extracted.
I think, and in books it says, the coin is Silver but it has less silver than usual...Its better than a rich
billon but worse than a denario.
otlichnik what chemical treatment do you advice? Amonia? Baking Soda? citric acid?
Cheers