614 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



Red lead is made by heating litharge to a suitable temperature 

 alter the litharge has been cooled, ground, and purified. The heat- 

 ing is done in a reverberatory furnace where, at about 900° F., the 

 buff-colored powdered litharge takes on more oxygen, changing in 

 color to a brilliant red. This is red lead. The conversion takes 

 about 48 hours. A peculiar phenomenon is that if the temperature is 

 raised to 1,100° F. and higher, the red lead loses oxygen and reverts 

 to litharge. 



Orange mineral is made in the same way as red lead and litharge 

 except that the process starts with white lead instead of pig lead. 

 White lead is roasted in a furnace and turns to orange mineral as it 

 takes in oxygen and releases carbon and hydrogen. 



Basic lead sulphate is made from lead ore that contains zinc min- 

 erals in addition to galena. The process of manufacture consists in 

 crushing, washing, and separating the minerals from associated rock 

 by a gravity or " jigging " process, termed concentrating. The con- 

 centrates thus obtained are subjected to blast-furnace treatment in 

 which volatilization takes place, and the volatilized portion of the 

 concentrates are oxidized. The result is a white fume which is 

 carried away by a draft of air and collected in bags. Blue basic lead 

 sulphate is a by-product obtained in the above process. The lamp- 

 black which colors it is an accidental ingredient resulting from in- 

 complete combustion of carbon fuel used in the blast furnace. 



