The English furnaces for smelting zinc ores are sometimes quadrangular, sometimes round; the latter form being preferable. They are mounted with from 6 to 8 crucibles or pots (see [fig. 1231.]), arched over with a cupola a, placed under a conical chimney b, which serves to give a strong draught, and to carry off the smoke. In this cone there are as many doors c, c, c, as there are pots in the furnace; and an equal number of vents d, d, d, in the cupola, through which the smoke may escape, and the pots may be set. In the surrounding walls there are holes for taking out the pots, when they become unserviceable; after the pots are set, these holes are bricked up. The pots are heated to ignition in a reverberatory furnace before being set, and are put in by means of iron tong machinery supported upon two wheels, as is the case with glass-house pots, e, is the grate; f, the door for the fuel; g, the ash-pit. The pots h, h, h, have a hole in the centre of their bottom, which is closed with a wooden plug, when they are set charged with calamine, mixed with one-seventh of coal; which coal prevents the mixture from falling through the orifice, when the heat rises and consumes the plug. The sole of the hearth i, i, upon which the crucibles stand, is perforated under each of them, so that they can be reached from below; to the bottom orifice of the pot, when the distillation begins, a long sheet-iron pipe k, is joined, which dips at its end into a water-vessel l, for receiving in drops the condensed vapours of the zinc. The pot is charged from above, through an orifice in the lid of the pot, which is left open after the firing, till the bluish colour of the flame shows the volatilization of the metal; immediately whereupon the hole is covered with a fire-tile m. The iron tubes are apt to get obstructed during the distillation, and must therefore be occasionally cleared out with a redhot rod. When the distillation is finished, the iron pipes must be removed; the coaly and other contents of the pot cleared away. A pot lasts about four months upon an average. Five distillations may be made in the course of 14 days, in which from 6 to 10 tons of calamine may be worked up, and from 22 to 24 tons of coals consumed, with a product of two tons of zinc. The metal amounts to from 25 to 40 per cent. of the ore.
1, 2, is the level of the upper floor; 3, 4, level of the lower ceiling of the lower floor.
[Fig. 1232.], ground plan on the level of 1, 2: only one-half is here shown.
The zinc collected in this operation, is in the form of drops, and a very fine powder, mingled with some oxide. It must be melted in an iron pot or boiler, set in a proper furnace; and the oxide is skimmed off the surface, to be returned into the crucibles. The metal is, lastly, cast into square bars or ingots.
The crucibles are discharged at the end of each operation, by withdrawing the condenser, breaking with a rake the piece of charcoal which shuts their bottom, and then emptying them completely, by shaking their upper part. In replacing the condenser-pipe k (see second pot from the right hand, [fig. 1231]), the flange at its top is covered with a ring of loam-lute, pressed against the conical bottom of the crucible, and secured in its place by means of two parallel rods o, o, which can be clamped by screws projecting horizontally from the vertical tunnel. See their places, indicated by two open dots near o, o.
A smelter and two labourers are employed in conducting a furnace; who make, with a mixture of equal parts of fire-clay, and cement of old pounds finely ground, the pots or crucibles, which last about four months. Five charges are made in 15 days; these work up from 6 to 10 tons of calamine, consume from 22 to 24 tons of coals, and produce 2 tons of zinc, upon an average. The following estimate of prices was made a few years ago:—
| 3 tons of calamine, at £6. | £18 | 0 | 0 |
| 24 ditto coal, at 5s. | 6 | 0 | 0 |
| A smelter, at 2 guineas a week | 2 | 2 | 0 |
| Two labourers, each at 4s. per day | 2 | 16 | 0 |
| Incidental expenses | 1 | 0 | 0 |
| £29 | 18 | 0 |
The calamine of Alston-moor, used at Sheffield, is not so rich; it produces at most only 25 per cent. of zinc. The coals are laid down at a cost of 5s. 8d. per ton; and the calamine laid down there 5l.; whence the zinc will amount to 32l. 14s. per ton. The considerable importations of zinc from Belgium and Germany, for some years back, have caused a considerable fall in its price.