Oxidising/Copper and Brass Query

[Dolly]

Good afternoon, all. I've been experimenting with silver with copper and brass, which I've done before. However, for the first time, I've got patches of what I think is some kind of oxidisation (hopefully photo attached). May I pick your brains as to the best way to remove this? I've got salt, vinegar, biocarb and a stiff clean toothbrush at the ready (not all at once perhaps). Am I on the right lines?

Secondly, I have several batches of domes and spheres - none of which are affected. I'd like to find out why this one has 'gone' (and one other) but not the others, so I can refine what I'm doing and avoid it happening again.

Many thanks - I appreciate any help.