Lead \ 
209 
natcd by a quantity of oxyd of lead produced by the 
action of the blast ; they contain also particles of metallic 
lead dispersed through their substance, and not unfre- 
quently 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 smelting. 
As it is necessary to bring these slags to a perfect fu- 
sion to separate the lead, a furnace capable of producing 
a more intense heat than the ore-hearth is requisite. The 
plate contains 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- stone 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 ( h ) 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 tue-iron . The pipe- stone (c) is bed- 
ded in tempered clay on the back ; it is a block of free- 
stone about 15 or 18 inches square, and 30 long, hollow- 
ed out on the underside to fit the tuyre. Two old bear- 
ers (dd) 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 (e e), about 15 by 18, and in length equal to the 
height of the pipe-stone, are placed on end ; the front is 
built entirely with old castings, the lower one resting on 
the ends of the side bearers. 
Voi. Ill 
Dd 
