In order that the iron may be obtained from the ore it is necessary for the oxygen to be separated from it, an operation which requires the intervention of heat, and the heat must be obtained from a fuel which consists mainly of carbon. Wood fulfils these requirements, but there is not enough wood in the whole world to smelt all the iron which we need. It was not until "pit-cole" displaced "char-cole" (to use the spelling of the period) that the iron industry began to assume its present importance.
To produce iron cheaply, therefore, ore and coal should for preference lie side by side, and in some few favoured localities that state of things exists. Generally speaking, however, the ore and the coal are not found together, with the result that one has to be taken to the other, and in practice it is usually the ore which is taken to the coal. Hence, the iron and steelworks are generally to be found on the coalfields, while the ore comes by rail or ship from, it may be, remote parts of the world.
The method by which the metal is obtained from the ore is in principle very simple. Coal and ore are mixed together in a furnace, the fire being fanned by a powerful blast of air. The result is that the bonds uniting iron and oxygen are relaxed by the heat, when the oxygen, having a preference for union with carbon rather than with iron, leaves the latter to join up with some of the carbon of the coal.
The furnace in which this operation is carried out is a tall, vertical cylinder of iron, lined with firebrick. The fire is at the bottom and the fresh fuel and ore are thrown in at the top. As the ore is
"reduced" (the chemist's term for removing oxygen from anything) the liquid iron accumulates in the lowest part of the furnace, whence it is drawn off at intervals, being allowed to run into grooves or gutters in a bed of sand, where it solidifies into what is called "pig iron."
Along with the coal and ore, there is thrown into the furnace from time to time quantities of limestone which combines with the earthy impurities with which the ore is contaminated. Together these form what is called "slag," which also exists, while in the furnace, as a liquid, but is so much lighter than the molten iron that it keeps quite separate and can periodically be drawn off through a hole higher up than that through which the iron is obtained. The slag solidifies into a hard stone which is broken up and used for making concrete and tar-paving, also for road metal.
The kind of furnace just described is, owing to the strong blast of air needed for its operation, called a "blast-furnace." One would be inclined to think that a fire so well supplied with oxygen, both from the blast and from the ore itself, would cause the fuel to be completely burnt up, yet such is not the case. The gases which ascend from the fire consist largely of "carbon-monoxide," a burnable gas with lots of heat still left in it. Years ago, and one may still see instances of it, this gas was allowed to escape at the top of the furnace, where it burnt in the form of a huge flame. In most modern furnaces, however, there is a kind of plug in the orifice at the top which, while it can be lowered in order to admit the ore and
fuel, normally prevents the escape of the gases, which are led away through pipes. In some cases the gases are burnt under boilers to provide the works with steam, in other cases they heat other furnaces for metallurgical purposes, while in yet others they are employed to drive large gas-engines to generate electricity. It is sometimes a difficulty to find useful employment for the vast quantities of this "blast-furnace gas" which are produced at a large works.
We see, then, how is obtained the pig iron from which the other kinds of iron and steel are made. It is not pure iron by any means; indeed, it is not sought to make iron pure, as is the case with most other metals, since, in its pure state, it is too soft to be of much use. All the familiar forms of iron and steel are really alloys of iron and carbon, a fact which tends to give iron its unique position among the metals, since by exceedingly slight variations in the percentage of carbon we can vary the properties of the iron to an amazing extent, thereby producing in effect a wide range of different substances each particularly suitable for a particular purpose.
To make cast iron, such as the guns of the Crimea were made of, it is only necessary to melt up some pig iron and to pour it into a mould. There is scarcely a town in which there is not an iron foundry, either large or small, and that is the work carried on there. A smaller form of the blast-furnace, known as a "cupola," melts the pig iron, and the moulds are generally made of sand. The process of pouring the melted metal into the moulds is called "casting" and the things so produced are "castings," and are said to be made of "cast" iron.