Try this as an experiment at home.

Heat a piece of your copper wire until dull red.
Plunge it into an alcohol, like methanol, denatured ethanol, or even
rubbing alcohol.

The hot wire exposed to the reducing environment will reduce the
oxides back to the metalic state.

It has worked pretty good for me when I've needed to use this trick,
but be aware that alcohol is flammable. Don't catch yourself on fire.

Dave