A coarsely ground blue potassium glass containing small amounts of cobalt oxide, which provides the source of the blue color. It is used as a blue colorant in paints, glazes, glass, bluing paper, laundry blue, starch, textiles, and rubber. Although it was intended as a substitute in painting for natural ultramarine blue, which was very expensive, it was discovered that smalt blue discolors when placed in an oil-based medium. (en)