Calcium Sulfate
| Calcium Sulphate (Gypsum) | Silica (Silex) |
Silex. Mag. 250 Diam.
(The Pigment shows white)
China Clay. By transmitted light
(The Pigment shows black)
Barium Sulphate (Barytes). By grinding the crude ore, treating with acid to remove the iron, and finally washing, floating, and drying, there is produced the commercial form of this valuable pigment. It is used in large quantity as a base upon which to precipitate colors, and also together with other white pigments in the manufacture of ready-mixed paints. It renders the paint coating more resistant to abrasion, and gives to the paint certain very important brushing qualities. It is a very stable pigment, not being materially affected by either acid or alkali, and can be used with the most delicate colors. In oil it is transparent and must be mixed with opaque pigments when used in ready-mixed paints. It is generally used with lighter pigments, such as asbestine, in order to prevent settling. Under the [microscope], both by polarized and transmitted light, the sharp angles of the particles appear distinctly, with no tendency to mass into a compact form. Although transparent in oil, it is valuable in moderate percentage in a ready-mixed paint.