Dunk all your copper and bronze coins/artefacts in a pot of petroleum jelly (as long as they are not flecked in verdigris) after first washing them in boiling water to nutrilise the ground salts.After 10-14 days,take 'em out and wipe all the excess off with a cloth.Voila,one piece with a darker,attractive,smooth hue,which can be removed by rinsing it under the hot tap if need be.
Betya wont though!