smalt
A potassium cobalt silicate glass of a deep blue
color. It is pulverized under water for use as a
coloring material in papermaking. It consists of
50 to 70% silica, 12 to 22% potassium oxide, and 6
to 16% cobalt oxide, and is prepared by fuzing
crude cobalt oxide (called "zaffre") with silica
and potassium carbonate. It must be pure;
otherwise its color is adversely affected. It is
usually used as a loading agent, and because of
its resistance to acids, alkalis, heat, and
moisture, its fastness is excellent; however,
because of its relatively low coloring power it is
expensive to use, and is, therefore, used mainly
for the more costly handmade and better
machine-made azures used for writing papers. Its
high specific gravity usually causes it to sink
through the pulp and color one side (the wire
side) more than the other. At one time the term
"smalt" was used to describe the vitrified pigment
used for painting on glass and was not necessarily
limited to blue. (156 , 197 )
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