LEAD.
1. Next to iron, lead is the most extensively diffused, and the most abundant metal. It is found in various combinations in nature; but that mineralized by sulphur is the most abundant. This ore is denominated galena by the mineralogists, and is the kind from which nearly all the lead of commerce is extracted.
2. The ore having been powdered, and freed, as far as possible, from stony matter, is fused either in a blast or reverberatory furnace. In the smelting, lime is used as a flux, and this combines with the sulphur and earthy matters, while the lead unites with the carbon of the fuel, and sinks to the bottom of the furnace, whence it is occasionally let out into a reservoir.
3. Lead extracted from galena, often contains a sufficient proportion of silver to render it an object to extract it. This is done by oxydizing the lead by means of heat, and a current of air. At the end of this operation, the silver remains with a small quantity of lead, which is afterwards separated by the process of cupellation. The oxyde is applied to the purposes for which it is used, or it is reduced again to a metallic state.
4. The lead mines on the Mississippi are very productive, and very extensive. The principal mines are in the neighborhood of Galena, in the north-western part of Illinois, and these are the richest on the globe. The lead mines in the vicinity of Potosi, Missouri, are also very productive. About 3,000,000 pounds are annually smelted in the United States.
5. Lead, on account of its easy fusibility and softness, can be readily applied to a variety of purposes. It is cast in moulds, to form weights, bullets, and other small articles. Cisterns are lined, and roofs, &c., are covered with sheet lead; and also in the construction of pumps and aqueducts, leaden pipes are considerably used. The mechanic who applies this metal to these purposes, is called a plumber.
6. Lead is cast into sheets in sand, on large tables having a high ledge on each side. The melted lead is poured out upon the surface from a box, which is made to move on rollers across the table, and is equalized, by passing over it a straight piece of wood called a strike. The sheets thus formed, are afterwards reduced in thickness, and spread to greater dimensions, by compressing them between steel rollers.
7. Leaden pipes may be made in various ways. They were at first formed of sheet lead, bent round a cylindrical bar, or mandrel, and then soldered; but pipes formed in this manner, were liable to crack and break. The second method consists in casting successive portions of the tube in a cylindrical mould, having in it a core. As soon as the tube gets cold, it is drawn nearly out of the mould, and more lead is poured in, which unites with the tube previously formed. But pipes cast in this way are found to have imperfections, arising from flaws and air bubbles.
8. In the third method, which is the one most commonly practised, a thick tube of lead is cast upon one end of a long polished iron cylinder, or mandrel, of the size of the bore of the intended pipe. The lead is then reduced, and drawn out in length, either by drawing it on the mandrel through circular holes of different sizes, in a steel plate, or by rolling it between contiguous rollers, which have a semicircular groove cut round the circumference of each.
9. The fourth method consists in forcing melted lead, by means of a pump, into one end of a mould, while it is discharged in the form of a pipe, at the opposite end. Care is taken so to regulate the temperature, that the lead is chilled just before it leaves the mould.
10. Shot is likewise made of lead. These instruments of death are usually cast in high towers constructed for the purpose. The lead is previously alloyed with a small portion of arsenic, to increase the cohesion of its particles, and to cause it to assume more readily the globular form. It is melted at the top of the tower, and poured into a vessel perforated at the bottom with a great number of holes.
11. The lead, after running through these perforations, immediately separates into drops, which cool in falling through the height of the tower. They are received below in a reservoir of water, which breaks the fall. The shot are then proved by rolling them down a board placed in an inclined position. Those which are irregular in shape roll off at the sides, or stop, while the spherical ones continue on to the end.
