Hi James--delighted to see you back! Hope to see more of you!
Wouldn't there also be some silver oxide as well (although, granted, much less than copper oxide)?
I haven't done any casting in about thirty years and was really surprised to hear that investment contains sulfur! Why would they put the daddy of all oxidizers in a casting material??
The few times I did cast way-back-when, I had them use 960 silver instead of sterling, and the pieces all came out white instead of black....:-).... The material was only marginally more expensive (quite insignificant at those old silver prices!), and, needless to say, it saved a tremendous amount of work......
Here is an old technique which could conceivably be thought of as 'burning off the oxidation with a torch' using an extreme reducing atmosphere:
I started out doing Yemenite filigree using the traditional benzine can for the fuel (not to be confused with benzene!) and foot bellows for air/oxygen. After soldering, we would often bend the air tube like this ^ with our free hand so that no air was added (by the bellows) to the flame beyond the ambient air in the room. This created an extremely soft, highly reducing flame (yellow/orange) which we would pass over the piece. Any sign of black turned to white (prior to pickling)!
Janet in Jerusalem