372 On Smelting of Lead. 



the hearth and the produce of lead ; those which are free 

 from sulphur, and which leare but little residuum after com- 

 bustion, are the best filled for smelting. 



The lead, which is separated directly from the ore, is 

 called ore lead, or common lead, to distinguish it from that 

 which is the result of a subsequent process. 



Slag-hearth Smelting. 



The slags or scoria separated in the process of ore-hearth 

 smelting, consist of the infusible part of the ore, the ashes 

 of the coals, peats, &c. semi-vitrified and agglutinated by a 

 quantity of oxide of lead produced by the action of the blast j 

 they contain also particles of metallic lead dispersed through 

 their substance, and not unfrequently unreduced ore. 



These scoria, which are technically named gray slags, 

 vary considerably in the quantity of lead they contain, but 

 the poorest hold a sufficient quantity to pay the expense of 

 smelling. 



As it is necessary to bring these slags to a perfect fusion 

 to separate the lead, a furnace capable of producing a more 

 intense heat than the ore-hearth is requisite. Plate X con- 

 tains plans, sections, &c. of the slag hearth, in which the 

 same letters are applied to the same parts in the different 

 figures. Fig. 1 is a perspective view of the hearth; Fig. 2 

 a plan ; and Fig. 3 a perpendicular section ; {a) a cast-iron 

 plate, which forms the bottom of the hearth : an old work- 

 slone is generally used for this purpose ; what it wants in 

 dimensions is supplied by other old castings, refuse of the 

 ore- hearth : the bottom is laid in fine dust which has been 

 damped a little, and well rammed ; on the bottom is placed 

 the back (/') which is formed of three or four old bearers laid 

 on each other; on the centre of the back is placed the tuyre, 

 or as it is generally called the iue-iron. The pipe-stone (c) 

 is bedded in tempered clay on the back; it is a block of 

 free-stone about 15 or 18 inches square, and 30 long, hol- 

 lowed nut on the underside to fit the tuyre. Tvvo old bearers 

 {d d) about 18 inches apart, and placed at right angles with 

 the back, against which their ends butt, form the lower part 

 of the sides: on these, two blocks of free-stone (ee), about 

 l.") by IS, and in length equal to the height of the pipe- 

 stone, are placed on end; the front is i)uilt entirely with old 

 castings, the lower one resting on the ends of the side 

 bearers. 



The spaces between the back of the chimney, the side 

 and the cheek stone, are filled with old castings, bricks, or 

 pieces of stone, and the joints filled up with dust or ashes; 



the 



