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calcination, be rendered less capable of being reduced to a metallic state.
The roasting of ores of iron is performed by kindling piles, consisting of strata of fuel and of ore placed alternately upon one another, or in furnaces similar to those commonly employed for the calcination of lime-stone. Some authors advise the addition of a calcareous earth to sulphureous ores during the roasting, that the sulphur may be absorbed by this earth when converted into quicklime. But we may observe, that the quicklime cannot absorb the sulphur or sulphureous acid, till these be first extricated from the ore, and does therefore only prevent the dissipation of these volatile matters; and, secondly, that the sulphur thus united with the quicklime forms a hepar of sulphur, which will unite with and dissolve the ore during its fusion, and prevent the precipitation of the metal. The next operation is the fusion or smelting of the ore. This is generally performed in furnaces, or towers, from twenty to thirty feet high; in the bottom of which is a basin for the reception of the fluid metal. When the furnace is sufficiently heated, which must be done at first very gradually, to prevent the cracking of the walls; a quantity of the ore is to be thrown in from time to time, at the top of the furnace, along with a certain quantity of fuel and limestone, or whatever other flux is employed. When the fuel below is consumed by the fire, excited by the wind of the bellows, the ore, together with its proportional quantity of fuel and of flux, sink gradually down, till they are exposed to the greatest heat in the furnace. There the ore and the flux are fused, the metallic particles are revived by the fuel, are precipitated by means of their weight through the scoria formed of the lighter earthy parts of the flux and of the ore, and unite in the basin at the bottom of the furnace, forming a mass of fluid metal covered by a glassy scoria. When a sufficient quantity of this fluid metal is collected, which is generally twice or thrice in twenty-four hours, an aperture is made, through which the metal flows into a channel or groove made in a bed of sand; and from thence into smaller, lateral, or connected channels, or other moulds. There it is cooled, becomes solid, and retains the forms of the channels or moulds into which it flows. The piece of iron formed in the large channel is called a sow, and those formed in the smaller channels are called pigs. Sometimes the fluid iron is taken out of the furnace by means of ladles, and poured into moulds ready prepared, of sand or of clay, and is thus formed into the various utensils and instruments for which cast iron is a proper material.
The scoria must be, from time to time, allowed to flow out, when a considerable quantity is formed, through an aperture made in the front of the furnace for that purpose. A sufficient quantity of it must, however, be always left to cover the surface of the melted iron, else the ore which would fall upon it, before the separation of its metallic from its unmetallic parts, would lessen the fluidity and injure the purity of the melted metal. This scoria ought to have a certain degree of fluidity; for, if it be too thick, the revived metallic particles will not be able to
overcome its tenacity, and collect together into drops, nor be precipitated. Accordingly, a scoria, not sufficiently fluid, is always found to contain much of the metal. If the scoria be too thin, the metallic particles of the ore will be precipitated before they are sufficiently metallised, and separated from the earthy and unmetallic parts. A due degree of fluidity is given to the scoria by applying a proper heat and by adding fluxes suited to the ore.
Various substances are added to assist the fusion of iron ores which are difficultly fusible. These are, 1. Ores of a fusible quality, or which, being mixed with others of a different quality, become fusible: accordingly, in the great works for smelting ores of iron, two or more different kinds of ore are commonly mixed, to facilitate the fusion, and also to ameliorate the quality of the iron. Thus an ore yielding an iron which is brittle when hot, which quality is called redshort, and another ore which produces iron brittle when cold, or cold-short, are often mixed together; not, as sometimes supposed, that these qualities are mutually destructive of each other, but that each of them is diminished in the mixed mass of iron, as much as this mass is larger than the part of the mass originally possessed of that quality. Thus, if from two such ores the mass of iron obtained consists of equal parts of cold-short and of red-short iron, it will have both these qualities, but will only be half as cold-short as iron obtained solely from one of these ores, and half as red-short as iron obtained only from the other ore. 2. Earths and stones are also generally added to facilitate the fusion of iron ores. These are such as are fusible, or become fusible when mixed with the ore, or with the earth adhering to it. Authors direct that, if this earth be of an argillaceous nature, limestone or some calcareous earth should be added; and that, if the adherent earth be calcareous, an argillaceous or siliceous earth should be added; because these two earths, though singly infusible, yet, when mixed, mutually promote the fusion of each other; but as limestone is almost always added in the smelting of iron ores, and as in some of these, at least, no argillaceous earth appears to be contained, we are inclined to believe, that it generally facilitates the fusion, not merely by uniting with those earths, but uniting with that part of the ore which is most perfectly calcined, and least disposed to metallisation; since we know that by mixing a calciform or roasted ore of iron with calcareous earth, without any inflammable matter, these two substances may be totally vitrified. Calcareous earth does indeed so powerfully facilitate the fusion of iron ores, that it deserves to be considered whether workmen do not generally use too great a quantity of it in order to hasten the operation. For, when the scoria is rendered too thin, much earthy or unmetallised matter is precipitated, and the cast iron produced is of too vitreous a quality, and not sufficiently approximated to its true metallic state.
Some authors pretend that a principal use of the addition of limestone in the smelting of iron ores is to absorb the sulphur, or vitriolic acid, of these ores: but, as we have already observed, a hepar of sulphur is formed by that mixture of
calcareous earth and sulphur, which is capable of dissolving iron in a metallic state; and thus the quantity of metal obtained from an ore not sufficiently divested of its sulphur, or vitriolic acid (which, by uniting with the fuel, is formed into a sulphur during the smelting), must be considerably diminished, though rendered purer, by the addition of calcareous earth: hence the utility appears of previously expelling the sulphur and vitriolic acid from the ore by a sufficient roasting. 3. The scoria of former smeltings is frequently added to assist the fusion of the ore; and, when the scoria contains much iron, as sometimes happens in ill-conducted operations, it also increases the quantity of metal obtained.
The quantity of these fusible matters to be added varies according to the nature of the ore; but ought in general to be such, that the scoria shall have its requisite degree of thinness, as above mentioned.
The fuel used in most parts of Europe for the smelting of ores of iron is charcoal. In several works, in England and Wales, iron ore is smelted by means of pit-coal previously reduced to cinders, or cokes, by a kind of calcination similar to the operation for converting wood into charcoal, by which the aqueous and sulphureous parts of the coal are expelled, while only the more fixed bituminous parts are left behind. The quality of the iron depends considerably upon the quality and also upon the quantity of the fuel employed. Charcoal is fitter than cokes for producing an iron capable of being rendered malleable by forging. The quantity of fuel, or the intensity of the heat, must be suited to the greater or less fusibility of the ore. Sulphureous, and other ores easily fusible, require less fuel than ores distinctly fusible. In general, if the quantity of fuel be too small, and the heat not sufficiently intense, all the iron will not be reduced, and much of it will remain in the scoria, which will not be sufficiently thin. This defect of fuel may be known by the blackness and compactness of the scoria; by the qualities of the iron obtained, which, in this case, is hard, white, light, intermixed with scoria, smooth in its texture, without scales or grains, rough and convex in its surface, and liable to great loss of weight by being forged; and, lastly, it may be known by observing the color and appearance of the drops of metal falling down from the smelted ore, and of the scoria upon the surface of the fluid metal, both of which are darker colored than when more fuel is used. When the quantity of fuel is sufficiently large, and the heat is intense enough, the iron is darker colored, denser, more tenacious, contains less scoria, and is therefore less fusible, and loses less of its weight by being forged. Its surface is also smoother and somewhat concave; and its texture is generally granulated. The scoria, in this case, is of a lighter color and less dense. The drops falling down from the smelted ore, and the liquid scoria, in the furnace, appear hotter and of a brighter color. When the quantity of fuel is too great, and the heat too intense, the iron will appear to have a still darker color, and more conspicuous grains or plates, and the scoria will be lighter, whiter, and more spongy. The drops falling from this melted ore, and the
fluid scoria, will appear to a person looking into the furnace through the blast hole to be very white and shining hot. The quantity of charcoal necessary to produce 5 cwt. of iron, when the ore is rich, the furnace well-contrived, and the operation skilfully conducted, is computed to be about forty cubic feet; but is much more in contrary circumstances.
The time during which the fluid metal ought to be kept in fusion before it is allowed to flow out of the furnace must also be attended to. How long that time is, and whether it ought not to vary according to the qualities of ores and other circumstances, we cannot determine. In some works the metal is allowed to flow out of the furnace every six or eight, and in others only every ten or twelve hours. Some workmen imagine that a considerable time is necessary for the concoction of the metal. This is certain, that the iron undergoes some change by being kept in a fluid state; and that, if its fusion be prolonged much beyond the usual time, it is rendered less fluid, and also its cohesion, when it becomes cold, is thereby greatly diminished. The marquis de Courtivron says that the cohesion may be restored to iron in this state by adding to it some vitrifiable earth, which he considers as one of the constituent parts of iron, and which he thinks is destroyed by the fusion too long continued. That the fusibility of cast iron does depend on an admixture of some vitrifiable earth appears probable from the great quantity of scoria forced out of iron during its conversion into malleable or forged iron, and from the loss of fusibility which it suffers nearly in proportion to its loss of scoria. The quantity of iron daily obtained, from such a furnace as is above described, is from two to five tons in twenty-four hours, according to the richness and fusibility of the ore, to the construction of the furnace, to the adjustment of the due quantity of flux and of steel, and to the skill employed in conducting the operation.
The quality of the iron is judged of by observing the appearances during its flowing from the furnace, and when it is fixed and cold. If the fluid iron, while it flows, emits many and large sparkles; if many brown spots appear on it while it is yet red hot; if, when it is fixed and cold, its corners and edges are thick and rough, and its surface is spotted, it is known to have a red-short quality. If, in flowing, the iron seems covered with a thin glassy crust, and if, when cold, its texture is whitish, it is believed to be cold-short. M. Reaumur says that darkcolored cast iron is more impure than that which is white. No certain rules for judging of the quality of iron before it is. forged can be given. From brittle cast-iron sometimes ductile forged iron is produced. Cast-iron with brilliant plates and points, when forged, becomes sometimes red-short and sometimes cold-short. Large shining plates, large cavities called eyes, want of sufficient density, are almost certain marks of bad iron; but whether it will be cold or redshort cannot be affirmed till it be forged. Whiteness of color, brittleness, closeness of texture, and hardness, are given to almost any cast-iron by sudden cooling; and we may observe that, in
general, the whiter the metal is, the harder it is also, whether these properties proceed from the quality of the iron or from sudden cooling; and that, therefore, the darker-colored iron is fitter for being cast into moulds, because it is capable in some measure of being filed and polished, especially after it has been exposed during several hours to a red-heat in a reverberatory furnace, and very gradually cooled. This operation, called by the workmen annealing, changes the texture of the metal, renders it softer and more capable of being filed than before, and also considerably less brittle. M. Reaumur found that, by cementing cast-iron with absorbent earths in a red heat, the metal may be rendered softer, tougher, and consequently a fit material for many utensils formerly made of forged iron.
In Navarre, and in some of the southern parts of France, iron ore is smelted in furnaces much smaller, and of a very different construction from those above described. A furnace of this kind consists of a wide-mouth copper cauldron, the inner surface of which is lined with masonry a foot thick. The mouth of this cauldron is nearly of an oval or elliptic form. The space or cavity contained by the masonry is the furnace in which the ore is smelted. The depth of this cavity is equal to two feet and a half: the larger diameter of the oval mouth of the cavity is about eight , feet, and its smaller diameter is about six feet; the space of the furnace is gradually contracted towards the bottom, the greatest diameter of which does not exceed six feet: eighteen inches above the bottom is a cylindrical channel in one of the longer sides of the cauldron and masonry, through which the nozzle of the bellows passes. This channel, and also the bellows pipe, are so inclined that the wind is directed towards the lowest point of the opposite side of the furnace. Another cylindrical channel is in one of the shorter sides of the furnace, at the height of a few inches from the bottom, which is generally kept closed, and is opened occasionally to give passage to the scoria: and above this is a third channel, in the same side of the furnace, through which an iron instrument is occasionally introduced to stir the fluid metal, and to assist, as is said, the separation of the scoria from it. The greatest height of this channel is at its external aperture on the outside of the furnace, and its smaller height is at its internal aperture; so that the instrument may be directed towards the bottom of the furnace; but the second channel below it has a contrary inclination, that, when an opening is made, the scoria may flow out of the furnace into a basin placed for its reception. When the furnace is heated sufficiently the workmen begin to throw into it alternate changes of charcoal and ore previously roasted. They take care to throw the charcoal chiefly on that side at which the wind enters, and the ore at the opposite side. At the end of about four hours a mass of iron is collected at the bottom of the furnace, which is generally about 6 cwt.; the bellows are then stopped; and when the mass of iron is become solid the workmen raise it from the bottom of the furnace, and place it, while yet soft, under a large hammer, where it is forged. The iron produced in these furnaces is of the
best quality; the quantity is also very considerable, in proportion to the quantity of ore, and to the quantity of fuel employed. In these furnaces no limestone or other substauce is used to facilitate the fusion of the ore.
The iron thus produced by smelting ores is very far from being a pure metal; and though its fusibility renders it very useful for the formation of cannon, pots, and a great variety of utensils, yet it wants the strength, toughness, and malleability which it is capable of receiving by further operations.
Cast-iron seems to contain a large quantity of vitreous or earthy matter mixed with the pure iron; which matter is probably the chief cause of its fusibility, brittleness, hardness, and other properties by which it differs from forged iron. The sulphur, arsenic, and other impurities of the ore, which are sometimes contained in cast-iron, are probably only accidental, and may be the causes of the red-short quality, and of other properties of certain kinds of iron but the earthy matter above mentioned seems principally to distinguish cast-iron from forged or malleable iron; for first, by depriving the former of this earthy matter, it is rendered malleable; and secondly, by fusing malleable iron with earthy and vitrifiable matters, it loses its malleability, and is restored to the state and properties of cast-iron.
The earthy vitreous matter contained in castiron consists probably of some of the ferruginous earth or calx of the ore not sufficiently metallised, and also of some unmetallic earth. Perhaps it is only a part of the scoria which adheres to, and is precipitated with, the metallic particles from which it is more and more separated, as the heat applied is more intense, and as the fusion is longer continued. To separate these impurities from cast-iron, and to unite the metallic parts more closely and compactly, and thus to give it the ductility and tenacity which render this metal more useful than any other, are the effects produced by the following operations:-The first of these operations is a fusion of the iron, by which much of its impurities is separated in form of scoria; and by the second operation a further and more complete separation of these impurities, and also a closer compaction of the metallic particles, are effected by the application of mechanical force or pressure by means of large hammers. Some differences in the construction of the forge or furnace, in which the fusion or refining of cast-iron is performed, in the method of conducting the operation, and in other circumstances, are observed to occur in different places.
The following new method of shingling and manufacturing iron must not be passed unnoticed. The ore, being fused in a reverberatory furnace, is conveyed, whilst fluid, into an air-furnace, where it is exposed to a strong heat till a bluish flame is observed on the surface; it is then agitated on the surface till it loses its fusibility, and is collected into lumps called loops. These loops are then put into another air-furnace, brought to a white or welding heat, and then shingled into half-blooms or slabes. They are again exposed to the air-furnace, and the half-blooms taken out and forged into anconies,