Barium Sulphate (Blanc Fixe). Blanc fixe is the precipitated form of barium sulphate, resulting from the action of soluble barium salts on soluble sulphates. The specific gravity (4.2) of this compound is lower than that of barytes. Possessing greater opacity in oil, it is of more value as a paint pigment for some purposes. It comes in for its greatest use as a base on which to precipitate lake colors. The [very fine particles] show a slight tendency to agglomerate.
Calcium Carbonate (Whiting). The natural form of calcium carbonate, prepared from chalk, has a much higher specific gravity (2.74) than that of the artificial form (2.5) prepared by the precipitation of calcium carbonate. The latter, however, possesses greater hiding properties. Both grades find a wide use in distemper work and in the manufacture of putty. It is often used in small percentage in many ready-mixed paints. The [photomicrograph] of the pigment shows the presence of many large particles.
Calcium Sulphate (Gypsum). The mineral gypsum, consisting of calcium sulphate and about 21% of water of combination, is sometimes used as a paint pigment after grinding and dehydration. Being slightly soluble in water it has a tendency to pass into solution when exposed to atmospheric agencies. It lacks hiding power in oil. Its specific gravity is 2.3. As in the case of all pigments prepared directly from mineral substances, the many-sized and shaped particles appear clearly when [enlarged]. Partially and wholly dehydrated forms of gypsum are also used in paint.
Silica (Silex.) This white pigment possesses great tooth and spreading properties. It is of use as a wood filler and as a constituent in combination paints. It wears especially well when used in combination with zinc oxide and white lead. Its purity often approaches 97%. The particles when [enlarged] are seen to have sharp angles and are not uniform in size, which accounts for its marked tooth and properties.
Aluminum Silicate (China Clay). China clay, or aluminum silicate, is a permanent and valuable white pigment showing very little hiding power in oil. It is found widely distributed in granitic formations. It is very stable, with a gravity of 2.6. [Particles] are found in many shapes and sizes, showing sharp and definite angles.
Ochre. Ochre is a hydrated ferric oxide permeating a clay base, largely used as a tinting material. It has a specific gravity of about 3.5, and a decidedly golden yellow color. A good quality should contain 20% or over of iron oxide. The [particles] of this pigment are flocculent and very uniform in appearance.
Sienna. Sienna, like umber, is essentially a silicate of iron and alumina, containing manganic oxide. It contains, however, a lower percentage of the latter than in the case of umbers. The [photomicrograph] of the burnt variety shows clearly the fine condition of the pigment, while large particles are shown in the raw variety.
Umber. Umber, another naturally occurring pigment, consists of iron and aluminum silicates, containing varying proportions of manganic oxide, its color and tone varying according to the percentage of the latter. The raw variety is drab in color, which in burning changes to reddish brown. A marked percentage of large-sized [particles] exist in this pigment.