[Illustration: FOUNDRY.]
Having already described the various operations of the trades employed in
building and fitting a house, we will say something of the manufacture
of those cast iron columns, girders, gratings, balconies, pipes, gutters,
air traps, coal plates, stoves, and other articles which are so necessary
to the Builder before his work can be completed. All these, as well as a
great variety of other goods made in black or bronze iron, such as gates,
bridges, pieces of furniture (like umbrella stands), iron taps, and even
pots and frying pans, are made at the _Iron Foundry_.
Iron is a metal of a bluish gray colour; but in its pure state it looks
almost white when polished, and has a brilliant lustre, while when it
is broken the broken portion looks dull and fibrous. It is the hardest
of all the malleable and ductile metals, and the most tenacious of all
metals, an iron wire of ⅟₃₆th of an inch in diameter bearing a weight of
60 pounds.
In the pure state it requires the strongest heat of what is called a wind
furnace to melt it.
Iron may be called the most precious of all metals; it is certainly the
most beneficial to man, and its uses are innumerable; indeed, there is
not a branch of human industry that could well afford to dispense with
its aid and services; nearly all the tools, implements, instruments, and
engines used by man are wholly or partly made of it, and we could better
afford to give up all the other metals than to part with this, which is
the most useful.
Iron is used in two different states, as _cast iron_ and _wrought iron_,
the differences between them depending on the proportion of carbon
combined with the metal, cast iron containing the most and wrought iron
the least.
For the production of wrought iron in the ordinary way, two distinct sets
of processes are required; first the extraction of the metal from the
“ore” that is brought up from the mine, which metal is cast iron; and
secondly the conversion of this cast iron into malleable or bar iron, by
remelting, _puddling_, and _forging_. Bar iron is turned into steel by
placing it in contact with charcoal in a peculiar kind of furnace.
When the ore is taken from the mine it is first burnt or calcined, and
then removed to a blast furnace to be smelted. These blast furnaces are
generally built of brick, and look like small towers. The ore is mixed
with limestone, which causes it to melt more easily, and the fire is
lighted with pit coal or coke. The melted metal sinks to the bottom of
the furnace in consequence of its weight, while the limestone and dross
float on the top, and are allowed to run off when they cool into a mass
of what is called “slag.”
The melted metal is run off from the bottom of the furnace, either into
moulds for some sort of castings, or into a large furrow made in a bed of
sand. This large furrow has several smaller furrows on each side of it,
and has received the name of the “sow;” the smaller furrows being called
“pigs;”—and the iron when it is formed in this shape to be afterwards
made malleable is called “pig iron.”
The pig iron is taken to other and smaller furnaces called puddling
furnaces, the bottoms of which are lined with clay mixed with the
slag just mentioned, and forming a substance which the puddlers call
“bull-dog,” though it would be difficult to discover why it received that
name.
About four hundred weight of the pigs is placed in the furnace, and as
it melts the puddler stands at the furnace mouth with a long _iron rod_
bent at the end, and stirs it about, until it comes to resemble several
great balls of iron paste. These balls are removed, and fall into iron
trucks pushed along a small railway by boys, who wheel them at once to
the “shingling hammer,” an immensely powerful hammer worked by steam, and
this beats the iron into small square bars called “blooms.”
The blooms are next carried to the rolling mill, which is a pair of great
rollers cut into grooves of various sizes, and between these grooves the
bars are squeezed, as the rollers turn round, until they become much
longer and narrower, when they are known as “forged bars.”
Some of the rolling mills, however, are plain cylinders without grooves,
and when a slab of white-hot iron is placed between these it comes out
from the pressure in a great broad sheet of metal.
These operations require great bodily strength as well as considerable
skill on the part of the workmen, who are obliged to seize the heated
metal with long tongs, and to catch it in the same way as it comes out
from the mill.
[Illustration: Casting Ladle.]
The iron which is intended for castings is melted in a “cupola furnace,”
so called on account of the dome-like shape in which it is built, which
has something to do with the more perfect heating of the metal. When the
iron is completely melted so that it will run freely, the lower portion
of the furnace is opened, and the white-hot stream is received in the
_casting ladle_, or, where it has to be carried for some distance, and
the casting is large, in great iron pails carried in a sort of frame by
two men. From the casting ladle it is at once poured into the _mould_.
[Illustration: Mould. Small Casting Ladle.]
The mould is a sort of iron box filled with a peculiar sort of sand, into
which a wooden pattern of the intended casting is pressed, and the sand
firmly rammed down and made solid. There are, in fact, two boxes of sand,
each of which is impressed with one half of the thickness of the required
casting, so that when they are brought together, and firmly fastened with
the pins, as shown in the picture, the patterns which have been taken
out have left a half of the impression in each box, each corresponding
exactly to the other. A hole in the box receives the melted metal, for
which a channel has been left in the sand, that it may freely run into
the hollows left by the pattern, and completely fill them; then, when
it has sufficiently cooled, the casting is removed, and when the rough
edges have been removed, and the irregularities trimmed off, it is ready
for use, and may be fitted to its other parts, which have perhaps been
separately cast, as in the case of garden seats, fenders, chandeliers,
umbrella stands, or ornamental girders and columns.
[Illustration: Furnace Iron. Brush. Trowel. Bellows. Foot Rule. Level.
Shovel. Spatera. Hammer. Mould Weight. Rammer.]
We have only described solid castings, but as ornamental iron work is
generally made hollow, this has to be cast in rather a different way,
though the only difference is that what are called “cores” are used.
These cores are in fact solid metal patterns made a little smaller than
the hollow left by the real pattern, and allowed to remain in the mould.
The melted iron then flows between the surface of the core and the
surface of the mould, and the casting is hollow, so that when the core is
removed the metal is only the thickness of the space left between the two
surfaces. You will see what is meant by placing a small teacup inside a
larger one, and then pouring water between them.
The tools used by the Ironfounder are not very numerous: the casting
ladles and mould have already been mentioned; the uses of the _shovel_
and the _mould weight_, the _rammer_, and the _furnace iron_ need no
description.
The designers, and pattern makers, and the mould maker have the most
important duties, and the latter will have to use a small _trowel_ and
a _spatera_ for arranging his sand and loam, a _level_ that it may be
perfectly true, and a _brush_ and a _pair of bellows_ for removing any
particles of grit from the surface of the channels where the pattern has
been impressed.
Almost all irons are improved by admixture with others, and, therefore,
when superior castings are required they should not be run direct from
the smelting furnace, but the metal should be remelted in a cupola
furnace, which gives the opportunity of suiting the quality of the iron
to its intended use. Thus, for delicate ornamental work, a soft and very
fluid iron will be required, whilst for girders and castings exposed to
cross strain the metal will require to be harder and more tenacious. For
bed-plates and castings which have merely to sustain a compressing force,
the chief point to be attended to is the hardness of the metal.
Castings should be allowed to remain in the sand until cool, as the
quality of the metal is greatly injured by the rapid and irregular
cooling which takes place from exposure to air if removed from the
moulds in a red hot state, which is sometimes done in small foundries to
economise room.
[Illustration]