Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne 1838 Vol.2.djvu/178

164 whole of the upper part of the hearth is filled with peat and coal, which is continually supplied, with the addition of coke as the fire gets hotter, until an intense heat is produced, and a body of fuel obtained, filling the uppper [sic] part of the hearth. Some of the grey slags from the smelting hearth, unbroken, as picked out of the brouse, are now thrown upon the top, or rather round the edges of the fire, which fuses them rapidly into a liquid glass, and any lead they contain is set at liberty; the blast at the same time tending to reduce any particles of ore which may have escaped the action of the ore hearth. The lead and the melted glass both sink down through the porous mass of cinders placed in the lower part of the hearth; the lead descending more rapidly, both on account of its greater tenuity and superior specific gravity, very soon collects below the cinders, in the metal pan placed to receive it, and filtering through under the division, b, is obtained without much impurity in the part c, Plate II. Fig. 2, out of which it is cast into pigs. The thick fluid glass, called black slag, after reaching the cast-iron bottom of the furnace, having cooled and thickened a little, does not sink further, but is made to issue through a small taphole, and flow over the cinders placed in the pan, running into the pit filled with water in a continued stream. By falling while hot into cold water, the black slag is granulated, and, as small particles of lead may be carried over with it, through inattention on the part of the workman, or otherwise, the granulated slags are carefully washed at most smelting mills before being thrown away. According to Dr. (Ann. Phil. vol. iv.) these slags consist of silex, lime, and oxide of iron, with some alumine, oxide of antimony, and oxide of lead. Their composition must, however, be various, depending upon the nature of the ore from which they are produced; in all cases they are formed from the earthy matter contained in the ore and coal, which the metallic oxides convert into a glass.

In working a slag hearth, the workman's attention is principally required to supply grey slag and fuel as it is melted down and consumed, to keep the nozzle of the bellows clear, and to guard against the metallic lead running along with the slag, into the pit of water.

Two men are generally employed to work a slag hearth, but, at some