I find bright-dip (nitric acid diluted 50:50, with care!) to
work rapidly and wonderfully for removal of cupric oxide.
I made a boo-boo in the above statement – it is the red CUPROUS
oxide that is removed so well by bright dip. Oppi Untracht (Jewelry
Concepts and Technology), p. 416, says, “Copper alloys heated to
high temperatures in oxidizing conditions form a double oxide scale
film The black outer layer is cupric oxide (CuO)… The red inner
layer is cuprous oxide (Cu2O)… Cuprous oxide is tenacious and
more difficult to remove than cupric oxide… When metal is immersed
in a pickling or acid solution, these oxides are converted into a
readily soluble metallic salt…”
The use of a hydrogen peroxide pickle to remove the cuprous oxide has
the advantage of using chemicals which are less tricky to handle than
nitric acid. Its disadvantage is that the hydrogen peroxide doesn’t
last long (Bill Seeley has a nice article on this, available for 50�
from Reactive Metals Studio Inc.). Diluted nitric acid, on the
other hand, will sit for months/years (in a safe place in your work
area) and still be useable. Another advantage is that, assuming your
nitric acid is not worn out, bright dipping only takes a few seconds
to be completed.
Obviously, the use of flux deters but does not completely prevent the
formation of these oxides.
One artist I know uses this tough, red cuprous oxide as part of the
color design of her brass pins and pendants.