1. Bar-iron is manufactured from pig-iron, from _blooms_, and directly
from the ore; the process is consequently varied in conformity with
the state of the material on which it is commenced.
2. In producing bar-iron from pigs, the latter are melted in a furnace
similar to a smith's forge, with a sloping cavity ten or twelve inches
below, where the blast-pipe is admitted. This hearth is filled with
charcoal and dross, or scoria; and upon these is laid the metal and
more coal. After the coal has become well ignited, the blast is
applied. The iron soon begins to melt, and as it liquefies, it runs
into the cavity or hearth below. Here, being out of the reach of the
blast, it soon becomes solid.
3. It is then taken out, and fused again in the same manner, and
afterwards a third time. After the third heat, when the iron has
become solid enough to bear beating, it is slightly hammered with a
sledge, to free it from the adhering scoria. It is then returned to
the furnace; but, being placed out of the reach of the blast, it soon
becomes sufficiently compact to bear the _tilt-hammer_.
4. With this instrument, the iron is beaten, until the mass has been
considerably extended, when it is cut into several pieces, which, by
repeated beating and forging, are extended into bars, as we see them
for sale. The tilt-hammer weighs from six to twelve hundred pounds,
and is most commonly moved by water power.
5. For manufacturing bar-iron directly from the ore, the furnace is
similar in its construction to the one just described, and the
operations throughout are very similar. A fire is first made upon the
hearth with charcoal; and, when the fuel has become well ignited, a
quantity of ore is thrown upon it, and the ore and the fuel are
renewed as occasion may require. As the iron melts, and separates from
the earthy portions of the ore, it sinks to the bottom of the hearth.
The scoria is let off occasionally, through holes made for the
purpose. When iron enough has accumulated to make a _loop_, as the
mass is called, it is taken out, and forged into bars under the
tilt-hammer.
6. This way of making bar-iron is denominated the _method of the
Catalan forge_, and is by far the cheapest and most expeditious. It is
in general use in all the southern countries of Europe, and it is
beginning to be extensively practised in the United States. When a
Catalan forge is employed in making _blooms_, it is called a
_bloomery_.
7. The blooms are about eighteen inches long, and four in diameter.
They are formed under the tilt-hammer, and differ in substance from
bar-iron in nothing, except that, having been imperfectly forged, the
fibres of the metal are not fully extended, nor firmly united. The
blooms are manufactured in the interior of the country, where wood is
abundant, and sold by the ton, frequently, in the cities, to be
converted into bar or sheet iron.
8. These blooms are converted into bar-iron, by first heating them in
an air-furnace, by means of stone coal, and then passing them between
chill cast iron rollers. The rollers are filled with grooves, which
gradually decrease in size from one side to the other. When the iron
has passed through these, the bloom of eighteen inches in length, has
become extended to nearly as many feet. The bar thus formed, having
been cut into four pieces, the process is finished by welding them
together laterally, and again passing them between another set of
rollers, by which they are brought to the form in which they are to
remain.
9. Blooms are also laminated into two sheets, on the same principle,
between smooth rollers, which are screwed nearer to each other every
time the bloom is passed between them. Very thin plates, like those
which are tinned for the tin-plate workers, are repeatedly doubled,
and passed between the rollers, so that in the thinnest plates,
sixteen thicknesses are rolled together, oil being interposed to
prevent their cohesion. The last rollings are performed while the
metal is cold.
10. Rolled plates of iron are frequently cut into rods and narrow
strips. This operation is performed by means of elevated angular rings
upon rollers, which are so situated that they act reciprocally upon
each other, and cut like shears. These rings are separately made, so
that they can be removed for the purpose of sharpening them, when
necessary. The mills in which the operations of rolling and slitting
iron are performed, are called rolling and slitting mills.