604 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



former arc the nearest approach to pure ores and the smelting of 

 them involves roasting, that is, heating at a high temperature in the 

 presence of air, sufficient to oxidize and drive off the sulphur from 

 the galena and free a great proportion of the lead. The furnace 

 used for smelting this class of ores is known as the Scotch hearth 

 and while in the early days, when the lead industry centered entirely 

 in Missouri, it was the chief form of smelting furnace, its use to-day 

 is greatly restricted, due to the increasing complexity of the ores and 

 their decreasing purity. 



The ores of the second class, comprising almost two-thirds of the 

 lead ores mined, have other valuable minerals intimately associated 

 with them as well as nonmetallic compounds which are even more 

 refractory, so that heating to a very high temperature and in the 

 presence of a flux is required to separate the lead from the associated 

 substances. Blast furnaces are used for smelting this class of ores 

 and the flux generally used is a mixture of sand, iron ore, and lime- 

 stone. The nonmetallic substances in the ore have a greater affinity 

 for the silica in the sand than they have for the lead so that in the 

 course of smelting new relationships are formed, the lead is freed 

 and flows out of the furnace pure, except for the other metals con- 

 tained. These are eliminated by refining. Smelting is a delicate 

 operation requiring an intricate manipulation of the mixture of 

 fluxes and ores composing the furnace " charge," so that after the 

 mixture has passed from top to bottom of the furnace all of the im- 

 purities have parted company with the valuable metals and gathered 

 together as a "slag" while the lead and other metals have joined 

 together as a separate body. The " slag " being lighter floats on the 

 molten metals with the result that when the furnace is "tapped" 

 the " slag " flows off first in one direction and the molten metals fol- 

 low in another. 



Silver, copper, antimony, arsenic, and bismuth are often present 

 in lead in small yet sufficient quantities to be objectionable, and re- 

 fining comprises the removal of these metals, so far as practicable. 

 Silver and copper are eliminated by the use of zinc, the zinc being 

 melted into the lead in a large iron pot and the mass made red hot. 

 In this state the copper and silver combine with the zinc forming a 

 new compound which rises to the top of the molten mass as a scum. 

 The scum is then distilled in a retort which process eliminates the 

 zinc and leaves a residue from which the copper and silver are re- 

 covered. Antimony, arsenic, bismuth, and the minor nonmetallic 

 impurities are eliminated after the removal of the silver and copper 

 by making use of the fact that, at a red heat and in the presence of 

 air, they are oxidized and converted into new compounds which sepa- 

 rate from the lead. The practical application of this truth is the 

 refining process generally known as " poling," deriving its name 



