Is a layer silver chlorid (horn silver), find
patina?
His other coin has silver chlorid, too.
It can be removed with either sodium thiosulfate or ammonia but under silver chlorid is very often corrosion or heavy corrosion and this substances can remove oxidation layer and a little
bit of silver, too.
I do not like silver chlorid and I always remove it but I only buy coins where I think that there is almost no corrosion under silver chlorid.