58 LEAD 



merely to keep up a moderate heat of the furnace during the paddling. After three 

 hours and ten minutes, the grate being charged with fuel for the third fire, the register 

 is completely opened, the doors are all shut, and the furnace is left in this state for 

 three quarters of an hour. In nearly four hours from the commencement, all the doors 

 being opened, the assistant levels the surfaces with his rake, in order to favour the 

 descent of any drops of lead ; and then spreads the slags, which are pushed back towards 

 him by the smelter. The latter now throws in a fresh quantity of lime, with the view 

 not merely of covering the lead-bath and preventing its oxidation, but of rendering the 

 slags less fluid. 



Ten minutes after the third fire is completed, the smelter puts a new charge of fuel 

 on the grate, and shuts the doors of the furnace to give it the fourth fire. In four hours 

 and forty minutes from the commencement, this fire being finished, the doors are 

 opened, the smelter pierces the tap-hole to discharge the lead into the outer basin, and 

 throws some quicklime upon the slags in the inner basin. He then pushes the slags 

 thus dried up towards the upper part of the hearth, and his assistant rakes them out by 

 the back doors. 



The whole operation of a smelting shift takes about four hours and a half, or at most 

 five hours, in which four periods may be distinguished : 



1. The first fire for roasting the ores requires very moderate firing, and lasts two 

 hours. 



2. The second fire, or smelting, requires a higher heat, with shut doors ; at the end 

 the slags are dried up with lime, and the furnace is also allowed to cool a little. 



3. 4. The last two periods, or the third and fourth fires, are likewise twosmeltings or 

 foundings, and differ from the first only in requiring a higher temperature. The heat 

 is greatest in the last. The form and dimensions of the furnace are calculated to cause 

 a uniform distribution of heat over the whole surface of the hearth. Sometimes billets 

 of green wood are plunged into the metallic lead of the outer basin, causing an ebulli- 

 tion which favours the separation of the slags, and consequently the production of a 

 purer lead ; but no more metallic metal is obtained. 



Ten cwts. of coal are consumed at Holywell in smelting one ton of the lead-ore schlich 

 or sludge ; but at Grassington, near Skipton in Yorkshire, with a similar furnace worked 

 with a slower heat, the operation taking from seven hours to seven hours and a half, 



instead of five, only 1\ cwts. of 

 coal are consumed. But here the 

 ores are less refractory, have the 

 benefit of fluor-spar as a flux, and 

 are more exhausted of their metal, 

 being smelted upon a less sloping 

 hearth. 



The ore-hearth. This furnace, 

 called by the French fourncau 

 ecossais, is from 22 to 24 inches 

 in height and 1 foot by 1^ in area 

 inside ; but its horizontal section, 

 always rectangular, varies much 



w . -or in its dimensions at different 



C, Tuyere. M, Workstone. P, Lead-pot. . /. n ooj 



levels, as shown in jiff. 1334. 



Treatment of lead oresby the Scotch furnace or ore-hearth. This furnace is generally 

 employed in the counties of Northumberland, Cumberland, and Durham for the 

 smelting of lead ores, which were formerly carried to them without any preparation, 

 but they are now often exposed to a preliminary calcination. The roasted ore yields 

 in the Scotch furnace a more considerable product than the crude ore, because it forms 

 in the furnace a more porous mass, and at the same time it works drier, to use the 

 founder's expression ; that is, it allows the stream of air impelled by the blast to diffuse 

 itself more completely across the matters contained in the furnace. 



In proceeding to smelt by means of an ore-hearth, two workmen are required to be 

 in attendance from the beginning to the end of each smelting shift, the duration of 

 which is from 12 to 15 hours. The first step in commencing a smelting shift-is to fill 

 up the hearth-bottom, and space below the workstone with peats, placing one already 

 kindled before the nozzle of the bellows. The powerful blast very soon sets the 

 whole in a blaze, and by the addition of small quantities of coal at intervals, a body of 

 fire is obtained, filling the hearth. Roasted ore is now put upon the surface of the 

 fire, between the forestone and pipestone, which immediately becomes heated red hot 

 and reduced ; the lead from it sinking down and collecting in the hearth bottom. 

 Other portions of ore of 10 or 12 Ibs. each are introduced from time to time, and the 

 contents of the hearth are stirred and kept open, being occasionally drawn out and 

 examined upon the workstone, until the hearth bottom becomes full of lead. The 



