I agree 100% with the Brepohl quote…:-)… Not sure where the problem is.
Doesn’t it follow from the above quote that “silver will oxidize if simply exposed to heated oxygen in the form of a torch”? It in fact says that silver reacts with oxygen even without heat!
When we heat sterling silver or karat golds and they turn black, the vast majority of the black are copper oxides, then (much less) silver sulfides, then (even less) silver oxides.
The most significant point, however, may be that in actual fact I always find that repeated soldering and pickling on a karat gold makes the piece yellow, and if I want to get the original color of the alloy back, I have to polish it, i.e., remove the outer layer of pure-ish gold. And even with white golds, a lot of soldering and pickling make them yellower!
Mind you, green gold is a pretty unusual alloy, so it would help to know what its composition was. Green golds normally have a very high percentage of silver (15%-46% in my own personal alloys list–most alloys with over 25%), and thus it stands to reason that it would be difficult to remove all/most of it. Since the other ingredient often found in green golds is cadmium (4%-12.5%), you might want to look into what would remove that.