SILVER MINING IN COLORADO. 607 



here in the crushing of the ore, the dressing or concentration of that part 

 which is not ah'eady sufficiently rich in lead; the roasting or calcining of the 

 charge; the smelting for the production of metallic lead, and, finally, the 

 cupellation, by which fine silver is obtained. The Brown mill is provided with 

 a Blake's rock-breaker for the first crushing of the ores. That portion which 

 is rich enough in lead to be smelted without further concentration is sufficiently 

 reduced by this machine for further treatment ; the remainder, which needs 

 to be dressed, is delivered to the stamps. Of these there are 20, weighing 

 400 to 500 pounds each, falling 10 inches, between 50 and 60 times per 

 minute. The ore is crushed wet and discharged through coarse screens that 

 are made of punched sheet-iron plates. The crushed material is then dressed 

 in a round huddle of the common English form. This is the only means of 

 concentration employed in this mill, and is said by Mr. Watson, the manager, 

 to work very satisfactorily. The ore crushed in the stamps, a large portion 

 of which only contains, on the average, about 20 ounces of silver to the ton, 

 is so concentrated as to yield one ton of dressed ore from six tons of raw 

 material. The dressed product is then mixed with the richer ore of the first 

 class, so that, when ready for further operations, it contains about 200 ounces 

 of silver. The ore is then calcined or roasted, by which process a portion of 

 the sulphur is expelled and a partial oxidation of the metals effected. The 

 furnace for this purpose may be a simple reverberatory, as in the case of the 

 works at Georgetown, or of some other suitable form. The furnace at the 

 Brown mill is a double hearth, or two hearths, one above the other, each 20 

 or 25 feet long, 10 feet wide, and covered by an arch 15 or 18 inches high. 

 The material is introduced upon the upper hearth at one end, near the stack, 

 and, by means of side doors, is gradually moved along toward the other end, 

 where it drops through to the hearth below, and is then moved, in contrary 

 direction, toward the bridge, where it is discharged. The entire capacity of 

 the furnace is 4 tons per day. Each charge of about one ton requires 

 twenty-four hours to pass through the entire length of the furnace, and one 

 charge is supplied and delivered every six homrs. 



The smelting furnace employed in both these establishments is the com- 

 mon lead furnace used in England. The hearth is about 11 feet long by 8 

 feet wide. The charge consists of about 3,000 pounds of roasted ore, which 



