266 MINING INDUSTEY. 



stirring door; d, the discharge door; G^ the grate; C, the bridge; jP, the 

 flues; P, the ashpit; J, the hopper. The charge consists of 1,000 pounds of 

 ore, which is mixed with six per cent, of salt, the latter being added to the 

 charge in the hopper by which the furnace is supplied. The charge is heated 

 very gently at first, the temperature being gradually raised, until at the end 

 it is subjected to a high heat. Usually six hours are required for the roast- 

 ing. The charge is constantly stirred, and once or twice during the opera- 

 tion it is turned; that is, the portion of the charge remote from the bridge is 

 caused to exchange place with that which is near. 



The operation effected by thus roasting with salt consists, very briefly 

 expressed, first, in the oxidation of the metallic compounds, converting the 

 sulphurets, in which form the silver chiefly exists in the ore, to sulphates; 

 and the subsequent decomposition of these combinations by the salt, with the 

 formation of the chlorides of the metals. Sometimes an addition of limestone 

 is made to the charge, for the purpose of decomposing the chlorides of cop- 

 per, zinc, &c., thus preventing, to some extent, their subsequent amalgama- 

 tion in the barrel, and obtaining bullion of a purer quality. 



Each furnace, roasting four charges of 1,000 pounds each, or two tons, 

 in twenty-four hours, consumes one cord of wood. Two stirrers are employed 

 on each twelve-hour shift, making four men in twenty-four hours. One man - 

 is required to receive and attend to the ore on the cooling floor, after its dis- 

 charge. The same man can attend to more than one furnace. 



The roasted ore is passed again through a screen, having 1,600 holes to 

 the square inch, in order to remove from it any lumps that may have formed 

 by caking in the furnace, or coarse particles that may have escaped the bat- 

 tery-screen. It is then elevated to a large hopper, placed above the amalga- 

 mating barrels, to which latter it is thence supplied by means of smaller 

 hoppers, one of which is suspended over each barrel. 



Barrel Amalgamation.— The barrels are 4 or 5 feet in length and 

 diameter. They are usually made of soft pine. Figs. 3 and 4, on Plate | 



XXiy, show a vertical section and end view of an amalga,mating barrel, 

 formerly used at the Gould and Curry mill. The ends of the barrel are made 

 of plank, nicely fitted together and joined with a tongue of hard wood. The 



f 



