In the early 80s I built a number of Columbus SL frames that were either brass, nickel or copper-plated.
The nickel and copper plating are in any case the basis of any top-notch chrome-plating process..or should I say they were, some years ago..
As has already been identified, the trouble with copper is that it develops a greenish coating of verdigris..an acetate of copper, if not protected from the atmosphere. Once plated, the copper should be given at least one coating of hard clear lacquer to seal the surface to prevent the verdigris forming.
The other problem is, of course getting a lacquer or varnish that will key on to the shiney copper plate. In 1982, my shop manager Alan Gornall went to France to join the famous ACBB-Peugeot racing squad, along with Sean Yates, Graham Jones, Alan Peiper. Because his "Team Peugeot" bike wasn't ready he set off with a new "Bespoke" resplendent in its copper-plate finish, complete with what we thought was an adequate lacquer coat.
The team's training HQ was situated on the Riviera, so Alan had to drive south from Calais to Paris..and onwards down to the Riviera, with the bike mounted on a boot-top rack. En route the weather deteriorated, and it began to sleet and snow. According to Alan, when he arrived at Nice, his immaculate bright copper-plated frame was covered in numerous small patches and dots of verdigris, where the lacquer had not been applied well enough. A couple of days later an aerosol spray had covered the damage..and the frame became an immaculate white, and transferred up in Peugeot livery.
For whatever reason, and I can't think of one, I had far more luck with nickel-plated finishes, protected with lacquer coats. The lacquer seemed to stick more firmly to that metal than to either copper or brass.
Norris Lockley