Sometimes we WANT to copper flash metal. When soldering copper, even with copper solder, the solder join is usually visible. After cleaning all the extra solder from the join, try this: put some iron binding wire on your metal piece and put it into a pickle solution that has been used long enough so that the solution is a blue/green colour (that’s the copper in the pickle solution). Leave your piece in the pickle for about 10 minutes, and the copper flashing may be thick enough so that it can still be polished.
Here is the explanation of what happens:
Sparex pickle dissolves copper oxides from the jewelry metals being pickled. If you then put a metal like silver, copper, or gold, which has a low degree of chemical reactivity, into the liquid while it is also in contact with a metal, like iron, with a higher degree of reactivity (actually, the property is called ‘electronegativity”, but thinking of it as reactivity may help), then you set up what amounts to a battery, or galvanic cell. the iron dissolves, and in doing this it becomes ionized. when this happens, other ions in the solution become UNionized, balancing the
electrical charges. The result is that copper, which is less reactive than iron, comes out of solution when the iron goes in, and it does it by plating out on the silver, copper, or gold. The iron does not make the solution a copper
plating solution. What the iron does is supply the electical voltage needed to bring the copper back out of solution.
If you do NOT want to copper flash your jewellery piece, make sure it does NOT touch iron while in the pickle pot!!
(This explanation of the copper flashing PROCESS was found online, but I do not remember the source – sorry)