Cobalt

A metallic element that is a source of blue color in glass making. An impure form of its oxide of cobalt called the zaffer provides an intense color often toned down by fusing with potassium carbonate and a silicate that makes smalt, which is used to color glass blue. Cobalt can be combined with other elements to produce more colors. For example: lead and antimony oxides produces green, manganese and iron produces a fine black.