Sulfuric acid will remove the oxide without damaging the copper. Tested and known to work: a syrupy-thick 18 molar concentration. So that's a cyanide-free answ

Sulfuric acid will remove the oxide without damaging the copper. Tested and known to work: a syrupy-thick 18 molar concentration. So that's a cyanide-free answer to the problem. Cover the ceiling and walls with copper foil, and regularly mop them down with 18 molar sulfuric acid to keep them pretty. Rinse carefully, especially the ceiling. The cleaning solution is also a great disinfectant all by itself.