The coke and iron-stone having been duly prepared, the next consideration is the nature and extent of the flux required to separate the metal by fusion from those of its combinations which the previous process was inadequate to perform. These differ in every degree and form, according to the nature of the ore, and as their consideration at length would occupy a greater space than the assigned limits of this work will afford, we purpose availing ourselves of a few extracts from the valuable papers by Mr. David Mushet, inserted in Tillock's Philosophical Magazine; and must refer those of our readers who may desire more information to the original source for it. " To deprive an ore of its iron" (says Mr. Mushet) " so that no portion of it shall escape in the scoriae unrevived, two things are indispensable. First, the metal contained in the ore must be presented to a portion of fuel sufficient to take up the oxygen of the metal. Second, as this revivification goes on in the manner of a metallic perspiration upon the softened surface of the ore, another agent ought to be present to facilitate the separation by uniting with the earthy parts of the ore, forming a thinly divided lava, no longer capable of retaining the globules of metal, or of preventing the congenial affinity of the carbonic principle from taking full effect for the improvement of the quality of the iron." - "Experience has shown that an excess of any particular earth may be corrected by applying one opposite in its effects; and that the addition of lime, in various proportions, if found in most cases to answer the desired end." - "If the various classes so iron ore are fused in contact with charcoal, without the addition of a flux or solvent, the result is, from calcareous iron-stone, a larger portion of iron proportioned to the intrinsic richness of the ore, than from argillaceous iron-stone: and from the latter a larger produce of iron, than from an ore whose chief mixture was silex; the scoriae produced from the respective operations always demonstrate, from the colour and opacity, the probable quantity of iron which still remains to be taken up." There are some calcareous iron-stones which contain lime almost sufficient to form the necessary scoriae, the colour of which, when freed from the metal, possesses a considerable degree of transparency.
When a number of these stones are used in the blast furnace, a much less quantity of calcareous earths are necessary. It sometimes happens at iron works, whose chief supply is derived from a calcareous field of iron-stone, that by using a great proportion of an individual ore, surcharged with lime, the operations of the furnace are obstructed, and consequences entailed fatal to the interest of the manufacturer. From an excess of pure calcareous earth being present in the furnace, the scoriae, thick and curdled, becomes attached to the sides and bottom of the furnace; the quantity hourly increases, till it has accumulated to such a degree as to intercept the ascent of the blast, and the descent of the materials. The remedy suggested by Mr. Mushet for these inconveniences, is, reducing the quantity of lime direct, or by an admixture of clay or sand, whether combined with iron or not. An excess of clay in the argillaceous ores has the same prejudicial effects which have led to their rejection, though it has been owing to improper application. The fusibility of lime and clay individually is much facilitated by the addition of sand.
In all cases where these earths exist in excess in the ores, they ought either to be combined in the blast furnace with siliceous iron-stone, or treated with a lime-stone containing a considerable portion of sand. When a scarcity of lime exists in the blast furnace, and a superior quantity of clay and silex is combined with the iron-stones, the lava will flow from the furnace comparatively cold, tenaceous, and of a brown or pale dirty green colour, containing iron; when the mixture is just, the colour of the scoriae is pure white, enamelled with a variety of blue shades, waving, circular, or formed in straight delicate lines, arising from a peculiar existing modification of a minute portion of the metal. Where nature has bestowed mixtures productive of every quality of crude iron, the proper management of ores would become simple and easy: just combinations supersede the necessity of changing the quality of the lime-stone added for a flux, or of having recourse to various qualities of it, in order to assist or correct the deficiency of the native mixture.
Wherever the ores are of a structure thus deficient, it then becomes the province of the manufacturer to ascertain the mixture of the individual ores which compose his supply, and to restore that equilibrium of parts by the proper application of superadded earths which experiment and observation have proved to determine a certain quality of iron. Many years ago, Messrs. Hill and Co. of the Plymouth works near Merthyr, took out a patent for the use of the "cinder" produced in the refining and puddling operations, as a substitute for a portion of mine in the smelting process. This cinder or scoriae is an oxide of iron combined with but little foreign matter, and usually contains from 50 to 70 per cent, of metal. All previous attempts to smelt it economically had failed. Messrs. Hill and Co. mixed with it such a portion of argillaceous matter as to assimilate it to the natural ore of their district, and by this combination succeeded in its perfect reduction. The patent was soon after invaded; and, upon an action being brought for infringement of right, the defendants proved a prior application of the process.
The patent was thereby quashed, and the iron masters, in consequence, generally adopted the process; but, from injudicious management, much bad iron was made, which got it into disrepute. With proper care, and in small proportion, the cinder is now used advantageously at most of the iron works, in the making of forge-pigs, under the subsequent process of puddling. It if usual, in most furnaces, to make the coke always a fixed quantity, and to proportion the ironstone and the lime or other flux, to the quantity of iron to be made, and the working order of the furnace; and in proportion to the latter additions, the furnace is said to carry a greater or less burthen. Some furnaces carry so little burthen as to produce only about 13 tons per week, while there are others which yield as much as 70 tons per week. In these latter, the ore is in much greater proportion to the coke than the former, but the product is inferior. The burthen is varied according as the iron is required to possess more or less carbon; thus in making No. 1, or best iron, (which contains the greatest proportion of carbon, the burthen must be considerably less than that required to make less carburetted iron, or what is called white-iron, or forge-pig.
A general idea of the proportions of the materials from an argillaceous ore, containing about 27 per cent, of iron, with a strong carbonaceous coal, and a good limestone, consisting of shells, is afforded by the operation of a furnace under Mr. Mushet's direction. The furnace works with a bright tuyere, and receives from the blast about 2500 cubic feet of air per minute, through a circular aperture 2f inches in diameter. The quantity of calcined ore for the manufacture of good melting iron is upon a par with the coke; and forge-pig, or the least carburetted variety, six of coke to seven of ore. The lime-stone, unburnt, under the same circumstances, is to coke as 4 to 11; and for melting metal, retains a similar ratio.