body ..but zinc oxide and other sub- 

 'fy&p4f*^ t ^^ 

 of white pam?, a pigment is added. 



y^ 504 SMITH'S INTERMEDIATE CHEMISTRY 



^ 2. The body, a fine powder which makes the paint opaque. 

 Since the powder does not shrink, it also " fills " the paint and 

 prevents the formation of minute pores which otherwise would 

 appear in the oil after drying. White lead (see below) is the most 

 common material for the body but zinc oxide and othe/ sub- 

 stances are also used. 



3. Except in the case 



Various oxides, such as minium, colored salts, and lakes (p. 476) 

 are used as coloring matters. 



The oil does not " dry " by evaporation but gives a resin by 

 oxidation (see p. 41). Linseed oil and hemp oil are commonly 

 used. They contain glyceryl esters (p. 432) of unsaturated acids, 

 such as that of linoleic acid, (CsUfCC^CnHaOs). The unsaturated 

 part of the molecule takes up the oxygen. By previously boiling 

 the oil with manganese dioxide and other oxides, it is rendered 

 more active, and " dries " more quickly. 



Plumbers use a cement made of minium and linseed oil, in which 

 the former oxidizes the latter, without access of air being neces- 

 sary, to make joints tight. 



White Lead. White lead is a basic carbonate 2PbC03 2 Pb- 

 (OH) 2 . It is a heavy, white, insoluble, amorphous substance. 

 "Mixed with linseed 'oil, it forms a white paint valued for 

 its " body " or covering power (opacity). Its disadvantage is 

 the darkening, due to formation of the black lead sulphide PbS, 

 which is produced by the hydrogen sulphide in the air (see p. 223). 

 Its poisonous character is also objectionable. 



The old Dutch process for making white lead is still used exten- 

 sively. Gratings (" buckles ") of lead are placed above a little 

 vinegar in small pots. The pots are covered with boards, on 

 which manure or spent tan bark is spread. Other tiers of pots, 

 boards and bark are placed on the first, until the shed is full. Car- 

 bon dioxide, warmth and moisture are furnished by the decaying 

 bark. The gratings, by the end of three months, are converted 



