Pigments may be classified as white and colored, or as opaque and extender pigments.
The hiding power of pigments depends on the difference in index of refraction of the pigment and the surrounding medium usually the vehicle of a protective coating. In opaque pigments, these indexes are markedly different from those of the vehicles (oil or other); in extender pigments, they are nearly the same. The comparative hiding efficiencies of various pigments must be evaluated on the basis of hiding power per pound and cost per pound.
Principal white pigments, in descending order of relative hiding power per pound, are approximately as follows: rutile titanium dioxide, anatase titanium dioxide, zinc sulfide, titanium-calcium, titanium-barium, zinc sulfide-barium, titanated lithopone, lithopone, antimony oxide, zinc oxide.
Zinc oxide is widely used by itself or in combination with other pigments. Its color is unaffected by many industrial and chemical atmospheres. It imparts gloss and reduces chalking but tends to crack and alligator instead.
Zinc sulfide is a highly opaque pigment widely used in combination with other pigments.
Titanium dioxide and extended titanium pigments have high opacity and generally excellent properties. Various forms of the pigments have different properties.
For example, anatase titanium dioxide promotes chalking, whereas rutile inhibits it.
Colored pigments for building use are largely inorganic materials, especially for outdoor use, where the brilliant but fugitive organic pigments soon fade. The principal inorganic colored pigments are:
Metallic. Aluminum flake or ground particle, copper bronze, gold leaf, zinc dust Black. Carbon black, lampblack, graphite, vegetable black, and animal blacks Earth colors. Yellow ocher, raw and burnt umber, raw and burnt sienna; reds and maroons
Blue. Ultramarine, iron ferrocyanide (Prussian, Chinese, Milori)
Brown. Mixed ferrous and ferric oxide
Green. Chromium oxide, hydrated chromium oxide, chrome greens
Orange. Molybdated chrome orange
Red. Iron oxide, cadmium red, vermilion
Yellow. Zinc chromate, cadmium yellows, hydrated iron oxide Extender pigments are added to extend the opaque pigments, increase durability, provide better spreading characteristics, and reduce cost. The principal extender pigments are silica, china clay, talc, mica, barium sulfate, calcium sulfate, calcium carbonate, and such materials as magnesium oxide, magnesium carbonate, barium carbonate, and others used for specific purposes.
