Copper oxides. The red copper (I) oxide (Cu2O) is used in the manufacture of a cheap red glass. The black copper (II) oxide (CuO) decomposes to the red oxide when heated under oxygen at 800 degrees Celsius. Both oxides are used to form cuprate complexes in solid state and solution reactions. The (I) and (II) refer to the oxidation state of the copper in the compound when compared to the zero oxidation state of the metallic element. The oxidation state is a measure of the attraction between the atoms of copper and the electrons in the bonds that it forms in the compound. This attraction is less for the copper in the +2 or (II) oxidation state than in the (I) state.