Lead \ 
211 
bed of ashes removed, and the lead which has collected in 
the sump is cast into pigs. Cool ashes are next spread 
over the hot bed, and the hearth drawn and cleared from 
what remains in it, and when moderately cooled, prepar- 
ed with a bottom of ashes for a succeeding day’s work. 
The principal art in working a slag hearth is to keep a 
proper noze, and to have the hearth light and open in 
front, otherwise the blast does not work well and diffuse 
itself equally through all parts, but forces itself up behind 
and very soon destroys the pipe- stone. 
The noze is a protuberance which surrounds the orifice, 
through which the blast passes ; it is formed by the vit- 
rified slags trickling down the pipe-stone, and cooled by 
the blast as it enters the hearth. With very fusible slags it 
is sometimes difficult to get a proper noze to form, and 
with refractory slags to keep it of a moderate size. With 
too large a protuberance, the hearth works most at front ; 
with too small, chiefly at the hack. 
In general a noze may be prevented growing too large,, 
by laying the fuel principally near the pipe-stone, and oc- 
casionally forcing in a pricker through the tuyre. 
A noze may be enlarged by a contrary situation of the 
fuel, and throwing in close to the pipe-stone a few shovels 
of dust and ashes from the top of the hearth. 
The fuel used at the slag hearth is coke. 
The scoria, the refuse of the operation in the slag 
hearth, is called black slag ; it contains a portion of me- 
tallic lead which is separated by stamping and wash- 
ing. 
The lead obtained by the slag hearth is hard and sono- 
rous ; it is of an inferior quality, and unfit for many of the* 
purposes to which common lead is applied , 
