686 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



and arseniurets. The assays for iron aud gangne are made as usual, although the 

 solutions of ores are not evaporated to recover soluble silica and the estimation of 

 moisture is made in a very rough way. 



The specific gravity of slag is taken from day to day for each furnace by means 

 of the Jolly specific-gravity balance, already described, which is figured in Plate 

 XXXVIII. This operation is of no more advantage at smelter H than at smelter B, 

 and here, as there, the slags thrown away are the richest of the camp, both in lead aud 

 silver. 



The ore-beds are made to contain equal parts of iron and gangue, and the slags 

 thus formed are slightly acid. This plan, which is recommended in Leadville, and 

 which is gaining the confidence of smelters, should be condemned theoretically, aud 

 practice proves that theory is correct. It results, from the examination of slags made 

 in the laboratory of the Survey, that the so-called acid slags are richer in lead aud 

 silver than the more basic ones. But the chief defect of this plan is that an insufficient 

 quantity of iron is reduced, aud that very large quantities of sulphuret accretions and 

 unreduced galena are formed, interfering seriously with the working of the furnace, 

 lu the opinion of the writer the center of gravity of smelting operations, so to speak, 

 should be periodically displaced, and alternate acid and basic charges should be used, 

 for the inconveniences inherent in the use of these mixtures are precisely of an oppo- 

 site character aud calculated to counterbalance or destroy each other. At smelters C 

 aud G, where smelting is conducted on scientific principles, the mixtures are carefully 

 made to correspond tosingulo-silicate slags, which might be called neutral, so that the 

 final result is the same as the one proposed. 



The quantity of matte formed at smelter H is about 20 pounds per ton of ore, or 

 1 per cent. These mattes are roasted in heaps and resmelted; but it will be seen that 

 this mode of treatment is bad, and that much silver is lost during (he roasting. 



The method of bullion assay, which is the one in general use in Leadville, is as 

 follows : ' 



At smelter H two assay bits of lead (one from the top and one from the bottom 

 of each bar) are detached from each bar composing a car-load by the chisel represented 

 in Fig. 12, Plate XLIV. By hammering in different directions triangular bits of lead 

 are detached, such as are represented in Fig. 13. All the bits representing the car- 

 load are melted together under live charcoal in a plumbago pot. The charcoal is then 

 removed and the lead is skimmed by means of a small perforated ladle, and then 

 poured into a bar-mold. A bar about one inch thick is thus obtained (see Fig. 7, Plate 

 XLV), from which three or four assay bits are detached at a by means of an ordinary 

 chisel and hammer. Half an assay ton is weighed from each bit, and the assay is 

 made, as usual, by cupellation. 



The offices, laboratory, and Fairbanks scales occupy a detached building at the 

 entrance and rear of the works, and fuel is placed on the same level in the open space 

 at the back of the main smelting building. The ore-bins are all placed within the 

 building on the feeding-floor level, through which runs a wagon road for the distribu- 

 tion of ore from wagons. 



On the left of the furnace-room (facing the furnace) is the engine and blast room. 

 Two boilers, worked at a pressure of 80 pounds to the square inch, supply a powerful 

 engine of 100 horse-power, which drives four Baker blowers, one Blake crusher, one 



