42 iUTI,LETIN 42, UNITED STATES NATIONAL MUSEUM. 



worked along, as the roasting proceeds, to the end uearest to the tire, 

 where the heat is generally snfficient to fuse the roasted ore. 



Having a good supply of oxidized ores of a suitable character, and 

 roasted sulpliide ores, they are subjected to fusion in a shaft furnace 

 for the extraction of the lead and silver in the form of an alloy. Great 

 care is necessary in mixing the different ores and tiuxes in order to pro- 

 duce a proi)er charge for the furnace. The objects of the fusion are to 

 remove the silver and lead from the ore in the form of metal, and to 

 separate the earthy material in the form of a slag, sufficiently free from 

 the metals to be rejected. 



The ores and fluxes are mixed in such a manner that a suitable sili- 

 cate slag shall be formed by the silica and the basic constitutents of the 

 mixture. This slag must be readily fusible at the temperature of smelt- 

 ing, so as to flow freely froii the furnace, and it must be fluid enough 

 to allow the bullion to settle through it to the bottom of the furnace. 

 It must not contain too much of either lead or silver, as, when such is 

 the case, it has to be returned to the furnace and be resmelted. 



The gangue of the ore being mainly quartzose or acid, the fluxes must 

 be basic and are for the most part limestone and oxides of iron and 

 manganese. In some localities these latter are frequently found car- 

 rying a small amount of silver, which will be extracted during the oper- 

 ation. 



Having determined upon a certain mixture for smelting the various 

 materials must be suitably mixed before being charged into the furnace. 

 This is generally accomplished by making up beds of alternate layers 

 of the dift'erent materials and then taking off vertical sections of these 

 beds and charging them into the furnace with a suitable amount of fuel. 



The furnaces in which the fusion takes place vary greatly in the 

 details of their construction, but they consist essentially of a stack with 

 a flue at the top, connecting with a suitable draft stack, through dust 

 chambers, and closed at the bottom. At a suitable distance from the 

 top is a door for the introduction of the charge, and at the bottom are 

 openings for the introiluction of air, and for the withdrawal of the bul- 

 lion and slag ; in most cases the metal and slag are drawn off separately, 

 but in some cases they are drawn oft' together. Besides the waste slag 

 and the valuable bullion, two intermediate by-products are frequently 

 formed. The first of these is called matte and consists of sulphides, 

 mainly of iron, formed from the sulphur remaining in the ore as charged, 

 and generally containing a valuable amount of silver. When the ore 

 mixture contains copper it is found mostly in the matte and may occur 

 in sufficient amount to be valuable. Where small amounts of matte are 

 produced it is either thrown away or allowed to accumulate until a con- 

 siderable amount is on hand, when, as also where large amounts are 

 produced, it is roasted and forms a valuable flux, on account of the 

 oxide of iron it then contains. The second by-product is called speiss, 

 and it is found in smelting ores containing arsenic. It is essentially an 

 arsenide of iron. 



