582 MINING INDFSTEY. 



The working or skimming door, for the removal of the slag, is at the end near 

 the stack, which latter stands at the corner of the structure, connected by a 

 flue with the interior of the furnace. The skimming plate, which forms the 

 sill of the door, is of cast iron, 9 inches thick and 8 or 9 feet long. In one 

 side of the furnace is the feeding door, and on the opposite side is the tap. 

 The bottom is slightly concave, or sloping toward the tap. The concavity is 

 but a few inches below the skimming plate. The bottom is made by first 

 laying, upon the prepared foundation, a flat floor of fire-brick, about tv/o feet 

 below the surface of the hearth. This is covered with a layer of finely- 

 pounded flint, which, after being strongly heated during several days, is again 

 pounded, and then covered by another layer of same material, mixed with 

 slag The sides of the furnace are 12 inches thick; the top is one course of 

 brick, set on end. The whole structure is tied together by means of timbers 

 connected by iron rods. The stack is 54 feet high, having a section at top of 

 27 inches square, increasing slightly toward the bottom. 



The charge, weighing about two tons, is composed of different grades of 

 calcined ores and tailings, with, sometimes, a little raw ore, or some rich slags 

 of previous melting, so mixed as to adjust to each other the proportions of 

 silica, iron, copper, sulphur, and other component parts, as may be desired. 

 No other material is usually mixed with it. Six or seven hours, and some- 

 times more, are required for the smelting of each charge. When finished, the 

 slag is drawn out through the skimming door by means of iron rakes, and is 

 cast in sand molds of convenient size for handling. These are broken up and 

 carefully inspected, as a certain portion on the bottom usually carries some 

 particles of matt with it, and must be remelted; that which is suflSciently poor 

 is thrown away. The matt resulting from the charge remains in the furnace, 

 and another charge is introduced to increase the quantity of matt or metal, 

 which is usually tapped off" after about a ton has accumulated; it is finally drawn 

 oflf through the tap, and cast in sand molds. If not sufficiently rich for shipment, 

 it is broken up, and the concentration carried still further by repeated smelting. 

 Usually from four to five charges, or from eight to ten tons of ore, produce 

 one ton of matt. Under favorable conditions each furnace may yield one ton 

 of matt per day, but it is not commonly quite so much. This matt, if made 

 from the best ore, is rich enough to ship after the first melting, but the 



