The ore is estimated to yield, in the large way, fifty per cent. the first smelting. A considerable portion of what is put in, however, does not become completely desulphurated, and is found in the bottom of the furnace after cooling. This is chiefly the smallest lumps, which have fallen through the apertures that burn between the logs, before they were thoroughly roasted, and thus, getting out of the way of the heat, lie entangled with the ashes. Some lumps, which are too large, also escape complete desulphuration, and either remain unmelted, or else, when the fire is raised, melt altogether into a kind of slag, and produce little or no metallic lead. This constitutes what are called the lead-ashes. The larger pieces, consisting of ore but partially desulphurated, are carefully picked out from among the ashes, and added at the next smelting in the log furnace; while the remainder is thrown by in heaps for further examination.
The lead-ashes are still rich in lead, and, when a sufficient quantity has accumulated from repeated smeltings, it is taken off to a proper place contrived for the purpose, and separated from the cinders, wood-ashes, and other adhering impurities. This is done by washing the whole in buddles, one set below another, in the manner of the potter, when it is necessary to search his clays. The ashes, which consist of clotted lumps of a moderate hardness, are first pounded to a gross powder, and then introduced into the water through a sieve. The wood-ashes and other impurities, being lighter, swim on the top, and, by letting off the water, are thus carried away. Fresh water is added, the ashes briskly stirred with a hoe, and the water again let off, carrying a further portion of impurity with it. By repeating this operation several times, the lead-ashes are brought to the required degree of purity. Thus washed, they are carried to a furnace of a different construction, called the ash furnace (see Plate II.), and undergo a second smelting.
Plate II., Figure 1. A Perspective View of the Ash Furnace.
a, the ash-pit, 2 feet wide, 6 feet long, and 20 inches in height.
b, the mouth of the fire-arch, a foot square.
c, the mouth of the flue, where the charge is put in.
d, the iron pot for the lead to flow in, when the furnace is tapped.
Figure 2, is a longitudinal section through the furnace, at right angles with the front, showing the curve of the arch, flue, &c.
a, the ash-pit.
b, the grates, 10 inches square, and 3 feet long; these are pieces of hewn stone.