Oxides or salts of some metals are added to the collected glass to make coloured glass. The substance to be added:Cupric oxide, CuO or cobalt oxide: Blue ColorCuprous oxide, Cu2O or selenium oxide: Red ColorChromic oxide, Cr2O3 or ferrous salts: Green ColorSodium uranate or ferric salt: Yellow ColorCadmium sulfide, CdS: Lime-like yellow ColorManganese dioxide, MnO2: Violet or Pink ColorFe2O3 or excess quantities of CuO: Black Color