Opaque glass or enamel may be made by adding white insoluble substances to the ordinary flint or soda glass while it is in a melted condition. Bone phosphate or bone ash and barytes are most commonly used, but cryolite, white arsenic, and oxide of antimony are also employed. To render the glass dull, add to it as much as possible of either bone ash or barytes consistent with proper working and to keep the temperature high while it is stirred into the glass. The colours used are the same as for transparent glass, but more colouring matter is required to give intensity on the white base. For blues, cobalt oxide, smalt, or black oxide of copper are employed; for violet, oxide of manganese; for ruby, oxide of gold, suboxide of copper; for emerald green, copper oxide and oxide of iron, chromium oxide (chrome green); for yellow, uranium oxide, oxide of antimony, etc.