As a carburet, or actual cast-iron, must be formed in these processes, and, as the separation of carbon at the bottom of a deep cylinder, and where the metal would probably be covered by a vitreous liquid, is difficult, the iron might sometimes resist the efforts made to render it malleable, and run from the furnace in a liquid form. It might therefore have readily occurred, that it would be less costly to finish the process in a forge. The stuckoffen were therefore converted into flossoffen, or melting furnaces, whence the liquid carburet was withdrawn, and afterwards converted into bar iron. Such was probably the cause that led to the original discovery of cast iron, a discovery that cannot be traced further back than the end of the fifteenth century.
The uses of cast iron for purposes to which wrought iron is inapplicable, and the readiness with which it is fashioned, by pouring it into moulds, led to the increase of the size of the flossoffen, and in the power of the blowing apparatus, which has caused the introduction of the blast furnace. This forms the basis of the methods by which iron in all its forms is chiefly prepared at the present day, and is hence worthy of particular consideration.
The difference between the blast furnace proper, and the ancient fires from which it gradually took its rise, consists wholly in its superior height, and in the greater power of the blowing machines, by which its combustion is supplied with air.
This increase of height adds to the mass of the contained combustible,—additional air is therefore required for effecting its complete inflammation, and the joint effect is, that a much higher temperature is generated. By this, the earthy matters either contained in the ores, forming portions of the combustible, or added as fluxes, are rendered fusible at a less expense of oxide of iron; the carburet formed, becomes more fluid, and the product is more likely to assume the character of grey pig-iron.
Charcoal, as in the other processes, was the fuel originally employed, and is still principally used in most countries. But coal deprived of its volatile parts, and charred or converted into coke, has been substituted in some regions, as will hereafter be stated. Each of these combustibles requires a furnace of appropriate character, and demands a difference in the mode of management.
A blast-furnace is a hollow chamber enveloped, generally speaking, in a mass of masonry, of the form of a truncated pyramid. The chamber is composed essentially of three parts; the upper has the figure of a truncated cone, whose greatest base is lowest: this may be called the body of the furnace; the middle portion has also the figure of a truncated cone, whose greater base is uppermost, and is common to it and the upper portion: this contraction is called the boshes of the furnace; the lower position is called the hearth, and is usually enclosed on three sides by walls of refractory substances, on the fourth it is bounded by two stones, one serving as a lintel, which is called the tymp, the other resting on the foundation, and known by the name of the dam. Such at least is the shape of the blast furnaces in common use, and which will suffice for our present purpose.
The blast is introduced into the hearth, at a small distance above the level of the upper edge of the dam, and is now generally performed by means of two tuyeres; in the more ancient furnaces, there was but one. The furnace being completely dried, a fire is lighted in the hearth, and fuel gradually added, until the whole is filled to the trundle head, which is the open and lesser base of the truncated cone that forms the body of the furnace. The blast may then be applied, slowly and gently at first, and increasing gradually, until it reach its maximum of intensity. As the blast proceeds, the charcoal gradually burns, and descends; its place is supplied at top by fresh fuel, by ore, and by the earthy matter used as a flux. This is styled charging the furnaces. The earlier charges often contain no ore, but are wholly composed of charcoal and flux, and, in all cases, the proportion of ore and flux is at first small, and is gradually augmented. The charges are made as often as the mixed mass in the furnace descends sufficiently low to admit the quantity that is chosen as the proper amount. The charcoal is thrown in first, and the ore and flux are spread and mixed upon its surface. The principles which govern the amount of the charge, are as follows:—
"The volume of the charges depends upon the capacity of the furnace. If they be too large, they cool the upper part of the furnace, which will cause great inconveniences, particularly if zinc exist in the ore. On the other hand, small charges of charcoal will be cut or displaced by the ore, which will occasion a descent by sudden falls, in an oblique direction, or in a confused manner. It follows that the volume of the charge, although proportioned to the volume of the furnace, must be augmented: when the charcoal is light and susceptible of being displaced; and with the friability, the weight, and the shape of the fragments of the ore."
"The heat, considered in any given horizontal section of the furnace, will be intense in proportion to the thickness of the layer of charcoal that reaches it. It follows, that the fusible ore requires smaller charges of charcoal than one that is more refractory. If the beds of charcoal and mineral are too thick, the upper part of the furnace will not be sufficiently heated. Hence it is obvious, that there must be a maximum and minimum charge for every different dimension of furnace, and for every different species of ore and fuel." Karsten.
The charge of charcoal being determined upon such principles, it is added by measure, and always in equal quantities, while the proportion of ore and flux is made to vary, not only by a gradual increase at the beginning of the operation, but according to the working of the furnace. The manner in which the furnace is working can be inferred, even before its products are ascertained, by the appearance of the flame at the trundle-head, and at the tymp, by the manner in which the charge descends, and more surely still, by the appearance of the scoriæ. By a strict attention to these circumstances the proportion of the charge of ore may be regulated. A fortnight usually elapses from the time of the first charge until it reaches a regular state of working, and variations will occur even after that period, in consequence of the greater or less moisture of the combustible and minerals, the continual wearing away of the sides of the furnace, the variations in the state of the atmosphere, and in the play of the blowing machines, the greater or less attention of the workmen, and numerous other accidental circumstances.