I’ve used simple Johnson’s paste wax quite successfully - that
doesn’t mean that eventually the piece doesn’t darken somewhat, but
I’ve two bowls that I raised and then heat treated in my oven - one
is wonderful shades of brown, fuscia, blue and gold and the other
one mostly gold with a bit of blue. Although it’s been longer than 10
years since I waxed the bowls, the colors are still visible (not as
bright as they once were). I simply rubbed a thin coat of wax on, let
it almost dry, then buffed it, and then applied a second coat and
buffed. It looked nice and though over the years the copper is
darker, it still looks pretty good.
I’ve also used liquid turtle wax with the same effect.