Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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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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