46 



PAINT TECHNOLOGY AND TESTS 



White Lead (Mild Process). The Mild Process of manu- 

 facturing white lead consists of first melting the pig lead and 

 converting it into the finest kind of lead powder, then mixing thor- 

 oughly with air and water. The lead takes up water and oxygen 

 and forms a basic hydroxide of lead. Carbon dioxide gas is 

 next pumped slowly through the cylinders which contain the 

 basic hydroxide of iead. The result is basic carbonate of lead 

 the dry white lead of commerce. The process is called " Mild " 

 because it is the mildest process possible for the manufacture of 

 white lead. It is the only method in practical operation which 



Steam Jected Pans for Drying White Lead 



does not require the use of acids, alkalis or other chemicals, 

 every trace of which should be removed from the finished product 

 by expensive purifying processes. The failure of such washing 

 and purifying means a product of inferior quality, which neces- 

 sarily reduces the durability of any paint in which it is used. 



Basic Sulphate- White Lead (Sublimed White Lead). By 

 the action of the oxygen of the air on the fume produced by the 

 roasting and subsequent volatilization of galena, this fine, white,, 

 amorphous pigment is made. On analysis, its composition 

 shows approximately 75% of lead sulphate, 20% of lead oxide, 

 and 5% of zinc oxide. It has a specific gravity of 6.2. 



