Is there some agent or material that corrodes copper quickly?

ammonia. I have heard of people putting items in used kitty litter to “weather it…” (ewww)

use Flux, plumbers use it to sweat pipe joints, use that and let it sit in the sun or try a blow dryer and it should as far as oxidation goes be slightly quicker

If, at some point, you just want to get the oxide off, I suggest Coca-Cola.

If you are in no hurry, copper will discolor all by itself, unless lacquer is applied.

If you’re looking for a certain color, here is a list of chemicals that act on copper to produce specific effects.

If you don’t want to mess with mixing your own, there are prepared solutions available through art supply outlets like this one.