Reduction of Iron and Silver Ores. 219 



In the European and some of the South American silver smelting 

 works, limestone, and the slags of former operations, are used as a 

 flux. In the Mexican smelting houses, carbonate of soda is gener- 

 ally employed, as the flux. Carbonate of soda occurs in such abun- 

 dance over extensive districts of country in Mexico, as to be consid- 

 ered of little value. The price, at the smelting works, is only about 

 thirty cents per quintal. It is well adapted to the smelting of muri- 

 ate of silver, and this is an abundant ore in the Mexican mines. 



The poorest ores are first smelted, with one of the fluxes above 

 described, in the common high furnace. The sulphurets contained 

 in the ore, and perhaps some metals in a reguline state with the silver, 

 collect in the crucible of the furnace. This mass of mixed sulphu- 

 rets and metals is called " matt." 



When the crucible is full of the fluid matt, the latter is drawn off 

 into a cavity adapted to receive it. As the matt cools, so as to have 

 a crust form over its surface, the crust is taken off* in successive plates, 

 until all the matt is in this form, the object of which is, to have as much 

 surface exposed as possible in the subsequent roasting. 



The matt, in plates, is then roasted slowly, the objects of which are, 

 to volatilize the sulphur, arsenic, &:c., and to oxidize the iron, and oth- 

 er metals, so that in the succeeding fusion, the metallic oxides may 

 be dissolved by the earthy glasses. 



In the second fusion, the roasted matts of the first fusion are smelt- 

 ed with richer ores, and with slags of a previous third fusion. The 

 resulting matts are roasted as before, and for the same purposes. 



In the third fusion, the roasted matts of the second fusion are 

 smelted with the richest ores. 



The matt, resulting from this fusion, is drawn off into a cavity on 

 the outside of the furnace containing melted lead. The fluid matt, 

 having a less specific gravity than the lead, floats upon its surface, so 

 that it is necessary to stir them well together, that the lead may dis- 

 solve all the silver. The matt, as it cools, is taken off in plates as be- 

 fore, and roasted and smelted with the poorest ores of a new batch. 



The silver is afterwards separated from the lead by cupellation. 

 The argentiferous lead is kept in a state of fusion in a cupelling fur- 

 nace, with a constant current of air passing across its surface. The 

 lead is converted into an oxide, (the litharge of commerce,) and the 

 silver finally remains, nearly in a state of purity. 



The reason that the slags are used as a flux is, that they still con- 

 tain a small portion of silver, which is obtained, chiefly, by successive 



