I think you can do this by placing the copper in water with some salt
added.

Attach a jumper cable from the positive side of a car battery to the
copper and the negative side to something made of galvanized metal
that is also in the water.

The flow of electrons from the copper to the zinc should put a patina
on the copper rather quickly.

Note: this should be done with DC current AC will not have the same
effect.

And the container for the water needs to be non conductive.

I've never tried this but I've always wanted to.

If you do please let me know how well it works.