238 MINING INDUSTRY. 



part of the ore produced from the various mines is worked in such. " Cus- 

 tom mills" are those that receive the ore from the producer, work it at a fixed 

 price per ton for treatment, and return to the customer a certain percentage of 

 the value of the ore, the latter having been previously determined by assays. 

 This price for working ores during two or three years past has varied between 

 $13 and $15 per ton, including the cost of hauling the ore from the mine to the 

 mill. Quite lately it has been reduced somewhat, though during 1869 the 

 average was but little, if at all, below $13. Since the beginning of the pres- 

 ent year, according to late advices from Virginia City, some of the mills of the 

 district have reduced the price to $9 per ton. Some of the large producing 

 mines also own mills, but seldom of capacity sufficient to meet all their 

 demands. Thus the Savage mine in the year ending July 1, 1868, treated 

 16,810 tons of ore in its own mills and 67,815 tons in custom mills. The 

 ores are carefully sampled, both before delivery to the mill and after 

 crushing at the mill, as will be explained in more detail below, and 

 a return of sixty-five per cent, of the assay value is required of the 

 mill owner. Falling short of this in bullion, the mill is bound to make 

 good the deficiency, while any excess obtained by so much of the process 

 of treatment as has been already described in this chapter belongs also to 

 the mine or customer. Some mills, as may be seen by the tabular statement 

 further on, make returns as high as seventy per cent, or even more, but, as 

 might naturally be expected, the average return of the mill to the mine does 

 not much exceed the requirement. The residue, or "tailings," of the ore, after 

 it has passed through the separators, belongs to the mill, and this is in many 

 cases made to yield a good result on reworking. It is, therefore, the interest of 

 the mill owner, when working at a fixed price per ton, to treat as many tons 

 as possible and to return no more bullion in excess of the required standard 

 than may be necessary in order to maintain a good reputation among com- 

 peting mills. Sixty-five per cent., therefore, does not fairly indicate the per- 

 centage of value actually extracted from the ore, and is not a fair criterion by 

 which the efficiency of the process may be judged. 



Sampling the Ores. — As the mill is required to return, to the mine a 

 certain percentage of the value contained, it is necessary to have the ore 

 carefully sampled and assayed beforehand in order to get at a basis of set- 



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