Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne 1838 Vol.2.djvu/174

160 filtering down through all parts of the brouse into the hearth bottom, flows through the channel o, Fig. 1. into the pot f out of which it is laded into a proper mould, and formed into pigs.

The principal particulars to be attended to in managing an ore hearth properly, during the smelting shift, are these: First.—It is very important to employ a proper blast, which should be carefully regulated, so as to be neither too weak, nor too powerful. Too weak a blast would not excite the requisite heat to reduce the ore, and one too powerful, has the effect of fusing the contents of the hearth into slags. In this particular no certain rules can be given; for the same blast is not suitable for every variety of ore. Soft free-grained Galena, of great specific gravity, being very fusible, and easily reduced, requires a moderate blast; while the harder and lighter varieties, many of which contain more or less iron, and are often found rich in silver, require a blast considerably stronger. In all cases, it is most essential, that the blast should be no more than sufficient to reduce the ore, after every other necessary precaution is taken in working the hearth. Second.—The blast should be as much divided as possible, and made to pass through every part of the brouse. Third.—The hearth should be vigorously stirred, at due intervals, and part of its contents exposed upon the workstone; when the partially fused lumps should be well broken to pieces, and those which are further vitrified, so as to form slags, carefully picked out. This breaking to pieces, and exposure of the hottest part of the brouse upon the workstone, has a most beneficial effect in promoting its reduction into lead; for the atmospherical air immediately acts upon it, and, in that heated state, the sulphur is readily consumed, or converted into sulphureous acid, leaving the lead in its metallic state; hence it is, that the reduced lead always flows most abundantly out of the hearth, immediately after the return of the brouse, which has been spread out and exposed to the atmosphere. Fourth, the quantity of lime used, should be no more than is just necessary to thicken the brouse sufficiently; as it does not, in the least, contribute to reduce the ore by any chemical effect: its use is merely to render the brouse less pasty, if from the heat being too great, or from the nature of the ore, it has a