228 CHEMISTRY OF FARM PRACTICE 



170. Blue Pigments. Ultramarine blue is probably a 

 double silicate of sodium and aluminum and sulphide of 

 sodium. The composition is, however, somewhat variable. 

 It is the most important blue pigment. Ultramarine is 

 very sensitive to acids. In addition to its use as a pig- 

 ment in paints it is used in coloring wall-paper, in calico 

 printing and for neutralizing yellow-colored sugar, paper 

 pulp, and cloth. It may also be made by grinding up the 

 mineral lapis lazuli. This is not so easily affected by acids 

 but has not so brilliant a color. 



Prussian blue is ferrocyanide of iron. It is not affected 

 by acids, mixes well with oil, but the color is destroyed by 

 alkalies. It has good coloring powers, but is transparent 

 and lacks body. 



171. Red Pigments. Red lead is lead tetroxide (PbsO^. 

 It is a pigment of great brilliancy when a good compound 

 is secured and it has remarkable covering power. It is 

 made by heating litharge (PbO). 



Iron reds are prepared in large quantities as by-products 

 from other manufactures. They are valuable pigments, 

 being very permanent. These pigments are not very bright, 

 but they have several advantages; they work well in oil, 

 mix with other pigments, have good body and are cheap. 

 They are compounds of ferric oxide (Fe203). 



Vermilion is mercuric sulphide (HgS). It is a heavy, 

 opaque, brilliant pigment which does not work well with 

 oil on account of its weight. It is quite permanent, and 

 readily affected by either acids or alkalies, but it is very 

 expensive. 



172. Yellow Pigments. Chrome yellows are chromate 

 of lead, zinc, or barium, each having a characteristic shade. 



Lead chromate is a brilliant yellow which mixes well with 

 oil and has great covering powers. If treated with a caustic 

 alkali, its color changes to orange or red. 



Yellow ochres are natural mineral products which vary in 

 color, the color being due to hydrated oxide of iron. 



