226 CHEMISTRY OF FARM PRACTICE 



it will dry and set in a hard film, due to the absorption of 

 oxygen from the atmosphere. Corn oil and soybean oil, 

 to a limited extent, behave in a similar manner. The dry- 

 ing of these oils is aided by sunlight and hindered by mois- 

 ture; it does not occur in the absence of oxygen. Boiled 

 oil dries more quickly than raw oil. 



167. Driers. The drying of paints may be hastened by 

 certain compounds of lead and manganese which are known 

 as driers. When the drier is applied in the form of a liquid 

 it is known as a Japan drier. The use of a small amount 

 of a drier seems more effective than when a large amount is 

 applied. With an excessive amount of drier the film pro- 

 duced is not so durable. Some pigments, red lead for 

 example, act as driers as well as pigments. It is generally 

 believed that the greater the proportion of the pigment the 

 more resistance the film will show, provided that all of the 

 particles of the pigment are covered with oil. 



For thinning paint to make it work easier, certain vola- 

 tile materials are used, such as turpentine or benzine. 



168. White Pigments. White lead is the most important 

 of all pigments. This material is a basic lead carbonate in 

 which there are two molecules of lead carbonate to one mole- 

 cule of lead hydroxide (2PbCO3-Pb(OH)2). White lead 

 is very heavy, being 6.47 times heavier than the same volume 

 of water. Its great value as a pigment is due to its covering 

 power, its permanency, and to the readiness with which it 

 mixes with other pigments. Like all lead compounds it is 

 poisonous. 



Sublimed White Lead. This has recently come into ex- 

 tensive use. It has good covering power and color, mixes 

 well with pigments containing sulphur, and is more durable 

 in sea air than is white lead. With linseed oil it dries rapidly, 

 forming a tough, impervious coating. Sulphur fumes will 

 not affect it quickly. For these reasons it is frequently 

 used as a substitute for white lead. It is composed of 75 

 per cent lead sulphate, 20 per cent lead oxide and 5 per cent 



