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.
[Illustration: IRON-FOUNDER.]
THE IRON-FOUNDER, &c.