(1) Cobalt oxide Explanation: The addition of oxides, sulfides, salts, and other compounds of different metals to molten glass at the time of manufacture adds colour to glass. The introduction of electrically charged ions which are evenly distributed in the glass during manufacture also gives glass a characteristic colour. Cobalt oxide: deep blue colour; copper oxide: turquoise; chromium: dark green to black; cadmium sulfide: deep yellow, etc.