We’ve been using the red copper process for some years now, as
accents to jewellery and also in larger copper pieces. It appears
to be a reduced copper oxide (potters recognize the colour from
raku glazes), but like raku is fairly unpredictable.
We heat the piece using propane/oxygen, with a flame that is
just starting to reduce (a hint of yellow, too much yellow and
the flame is not hot enough). Holding the piece can be tricky, as
tweezers will leave an unoxidized part to the surface. If the
piece has a drilled hole, suspend it on stainless steel wire in
the flame. Heat to a bright red heat, even approaching orange
heat, and quench in boiling water (or water that has just been
boiled) with only a moment’s hesitation. The piece sits in the
water in a bubble of water vapour for a few seconds, before it
rumbles furiously and cools down. The hot water can splash, so be
careful. When it works well you get a beautiful crimson colour.
The variables, which we are still learning to control, seem to
be: 1. how hot the metal is (orange heat works great but the
copper can start to melt) 2. how long the piece is exposed to air
before you quench it (one to three seconds)and 3. how hot the
water is. The metal should be polished and clean (grease-free)
before starting. Potential problems? The temperatures needed are
higher than the annealing temperature of copper, so any
work-hardening will be gone, and annealed copper is very soft.
It’s also pretty unpredictable. Some days it just doesn’t seem to
work well as well as others.The oxide is remarkably durable but
can be dissolved with acid (pickle, or skin acids) so it needs a
protective lacquer, spray or wax. In jewellery, you need to
design the copper piece so that it is away from skin: perhaps a
silver hoop around a copper disc, or a silver bezel to hold a
copper shape. Hope this is useful.