60 PAINT TECHNOLOGY AND TESTS 



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 ten- 

 dency to mass into a compact form. Although transparent 

 in oil, it is valuable in moderate percentage in a ready-mixed 

 paint. 



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, con- 

 sisting of calcium sulphate and about 21% of water of combi- 

 nation, 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. 



