Opaque white glass, or “opal” has been called “milk-white” perhaps to distinguish it from transparent or “clear-white glass.” Resembling fine white porcelain, it was viewed as an inexpensive substitute. Opacity is obtained by adding bone ash or oxide of tin to clear molten glass. By the addition of various coloring agents, the opaque mixture can be turned into blue milk glass, or pink, yellow, green, caramel, even black milk glass. It has been made in numerous forms and shapes in this country and abroad from about the first quarter of the 19th century. It is still being produced and there are many reproductions of earlier pieces.