Assuming you're talking about the copper firescale (pink color), use
a 50/50 mixture of nice strong and fresh pickle with H202 - Hydrogen
Peroxide. Drugstore strength. Works like a charm.
You don't need to use it hot (I never do).
I keep a tupperware container of it by the pickle pot and drop in
any pieces that show sign of copper firescale when I take them out of
the pickle. Usually only takes a minute or so to dissolve the copper
oxides. You may need to still wash with a brass brush to get the
residue off, by the way.
Hope this helps!
Handcrafted and Unique Artisan Jewelry