The second operation is executed by cleaning the basin, removing the slags, covering the basin anew with 2 or 3 brasques, (coats of pounded charcoal), and piling up to the right and the left, two heaps of charcoal dust. Into the interval between these conical piles two or three baskets of charcoal are cast, and on its top some cakes of the reduced crude metal being laid, the blast is resumed. The cakes, as they heat, undergo a sort of liquation, or sweating, by the action of the earthy glasses on the unreduced black oxide present. Very fusible slags flow down through the mass; and the iron, reduced and melted, passes finally through the coals, and falls into the slag basin below. To the first parcel of cakes, others are added in succession. In proportion as the slags proceeding from these run down, and the melted iron falls to the bottom, the thin slag is run off by an upper overflow or chio hole, and the reduced iron kept by the heat in the pasty condition, remains in the basin: all its parts get agglutinated, forming a soft mass, which is removed by means of a hooked pole in order to be forged. Each lump or bloom of malleable iron requires 3 hours and a half for its production.
The iron obtained by this process is in general soft, very malleable, and but little steely. In Corsica four workmen are employed at one forge. The produce of their labour is only about 4 cwt. of iron from 10 cwt. of ore and 20 of charcoal, mingled with wood of beech and chestnut. Though their ore contains on an average 65 per cent. of iron, only about 40 parts are extracted; evincing a prodigious waste, which remains in the slags.
The difference between the Corsican and the Catalonian methods consists in the latter roasting the ore at a distinct operation, and employing a second one in the reduction, agglutination, and refining of the metal. In the Catalonian forges, 100 pounds of iron are obtained from 300 pounds of ore and 310 pounds of charcoal; being a produce of only 33 per cent. It may be concluded that there is a notable loss, since the sparry iron ores, which are those principally smelted, contain on an average from 54 to 56 per cent. of iron. The same ores smelted in the ordinary blast furnace produce about 45 per cent. of cast iron.
On the Continent, iron is frequently refined from the cast metal of the blast furnaces by three operations, in three different ways. In one, the pig being melted, with aspersion of water, a cake is obtained, which is again melted in order to form a second cake. This being treated in the refinery fire, is then worked into a bloom. In another system, the pig iron is melted and cast into plates: these are melted anew in order to obtain crude balls, which are finally worked into blooms. In a third mode of manufacture, the pig-iron is melted and cast into plates, which are roasted, and then strongly heated, to form a bloom.
The French fusible ores, such as the silicates of iron, are very apt to smelt into white cast iron. An excess of fluxes, light charcoals, too strong a blast, produce the same results. A surcharge of ores which deranges the furnace and affords impure slags mixed with much iron, too rapid a slope in the boshes, too low a degree of heat, and too great condensation of the materials in the upper part of the furnace; all tend also to produce a white cast iron. In its state of perfection, white cast iron has a silver colour, and a bright metallic lustre. It is employed frequently in Germany for the manufacture of steel, and is then called steel floss, or lamellar floss, a title which it still retains, though it be hardly silver white, and have ceased to be foliated. When its colour takes a bluish-gray tinge, and its fracture appears striated or splintery, or when it exhibits gray spots, it is then styled flower floss. In a third species of white cast iron we observe still much lustre, but its colour verges upon gray, and its texture is variable. Its fracture has been sometimes compared to that of a broken cheese. This variety occurs very frequently. It is a white cast iron, made by a surcharge of ore in the furnace. If the white colour becomes less clear and turns bluish, if its fracture be contorted, and contains a great many empty spaces or air-cells, the metal takes the name of cavernous-floss, or tender-floss. The whitest metal cannot be employed for casting. When the white is mixed with the gray cast iron, it becomes riband or trout cast iron.
The German refining forge.—[Figs. 601], [602.] represent one of the numerous refinery furnaces so common in the Hartz. The example is taken from the Mandelholz works, in the neighbourhood of Elbingerode. [Fig. 602.] is an elevation of this forge. D is the refinery hearth, provided with two pairs of bellows. [Fig. 601.] is a vertical section, showing particularly the construction of the crucible or hearth in the refinery forge D. C is an overshot water-wheel, which gives an alternate impulsion to the two bellows a b by means of the revolving shaft c, and the cams or tappets d f e g.
D, the hearth, is lined with cast-iron plates. Through the pipe l, cold water may be introduced, under the bottom plate m, in order to keep down, when necessary, the temperature of the crucible, and facilitate the solidification of the loupe or bloom. An orifice n, [figs. 601], [602.], called the chio (floss hole), allows the melted slag or cinder to flow off from the surface of the melted metal. The copper pipe or nose piece p, [fig. 600.], conducts the blast of both bellows into the hearth, as shown at b x, [fig. 602.], and D g p [fig. 600.]
The substance subjected to this mode of refinery, is a gray carbonaceous cast iron, from the works of Rothehütte. The hearth D, being filled and heaped over with live charcoal, upon the side opposite to the tuyère x, [figs. 601], [602], long pigs of cast iron are laid with their ends sloping downwards, and are drawn forwards successively into the hearth by a hooked poker, so that the extremity of each may be plunged into the middle of the fire, at a distance of 6 or 8 inches from the mouth of the tuyère. The workman proceeds in this way, till he has melted enough of metal to form a loupe. The cast iron, on melting, falls down in drops to the bottom of the hearth; being covered by the fused slags, or vitreous matters more or less loaded with oxide of iron. After running them off by the orifice n, he then works the cast iron by powerful stirring with an iron rake (ringard), till it is converted into a mass of a pasty consistence.