It does, indeed sound like copper plating. Perhaps next time, use
stainless steel binding wire instead of copper wire for this tumbling
process? And, maybe don't mix a mystery metal with something
If it is copper-plating, I'd try a hydrogen peroxide pickle
solution. The exact proportions are flexible. It works best hot. I
put the copper-plated metal into a glass or ceramic container, cover
with hydrogen peroxide, and add pickle until it starts to bubble.
Rinse and check frequently. I've had students and friends forget
about it, and find that the acid had gone beyond cleaning and etched
a lovely texture.
Another thought would be to use a mildly abrasive media, using
Best of luck, and please tell us what happens!