I was thinking about the reaction of silver and sulphur. I've read that when copper is oxidised by oxygen, after the formation of the first film of Cu2O, it is actually the copper atoms diffusing out of the bulk copper through the Cu2O layer to the air, where they are oxidised. Does anyone know what happens when silver is in contact with pressed sulphur tablets? I guess Ag2S forms instantaneously (we did this in a lab session and saw indeed black circles, but we didn't manage to interpret the X-ray diffractograms in a way that made sense), but what happens then? Can we exspect silver to diffuse through the Ag2S layer, or will sulphur diffuse through Ag2S towards silver and oxidise it? Of course, I can put a tablet on a silver plate and leave it for a year and see if the reaction has stopped because of complete formation of an Ag2S film, or if the silver plate seems porous or hollow (or vice versa the sulphur tablet). Could I deduce from ionic radiuses and silver sulphide's structure which species should diffuse through which medium?