Oxidising/Copper and Brass Query

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.

The colour changes in these metals when heated, are related to oxidation. So they become more obvious if you concentrate a fierce flame with a sharp blue cone on any place for long.
You should work in a darkened place so as to see hot spots more easily, reduce the air vent on your torch a little, and keep the torch moving about.

Sterling when heated can have cuprous oxide on the surface, but this gets dissolved in your pickle.
It can also have cupric oxide, which is more deeply embedded, and is the dreaded firescale.

Brass is an alloy of copper and zinc. Prolonged heating followed by pickling will remove the surface zinc, so allowing patches of red copper to show

Copper where heated with an oxidising flame, will change to the dark colours of copper oxides.

Any discolouration that will not dissolve in safety pickle, will need aggressive removal with abrasives, or strong acids. Dennis.

Thank you Dennis - I really appreciate your reply, and the information. I need to work on my flame control and make sure that I pay full attention to the process.