During this operation, a portion of the carbon contained in the cast iron combines with the atmospherical oxygen supplied by the bellows, and passes off in the form of carbonic oxide and carbonic acid. When the lump is coagulated sufficiently, the workman turns it over in the hearth, then increases the heat so as to melt it afresh, meanwhile exposing it all round to the blast, in order to consume the remainder of the carbon, that is, till the iron has become ductile, or refined. If one fusion should prove inadequate to this effect, two are given. Before the conclusion, the workman runs off a second stratum of vitreous slag, but at a higher level, so that some of it may remain upon the metal.
The weight of such a loupe or bloom is about 2 cwts., being the product of 2 cwts. and 7⁄10 of pig iron; the loss of weight is therefore about 26 per cent. 149 pounds of charcoal are consumed for every 100 pounds of bar iron obtained. The whole operation lasts about 5 hours. The bellows are stopped as soon as the bloom is ready; this is immediately transferred to a forge hammer, such as is represented [fig. 605.]; the cast-iron head of which weighs 8 or 9 cwts. The bloom is greatly condensed thereby, and discharges a considerable quantity of semi-fluid cinder. The lump is then divided by the hammer and a chisel into 4 or 6 pieces, which are re-heated, one after another, in the same refinery fire, in order to be forged into bars, whilst another pig of cast iron is laid in its place, to prepare for the formation of a new bloom. The above process is called by the Germans klump-frischen, or lump-refining. It differs from the durch-brech-frischen, because in the latter, the lump is not turned over in mass, but is broken, and exposed in separate pieces successively to the refining power of the blast near the tuyère. The French call this affinage par portions; it is much lighter work than the other.
The quality of the iron is tried in various ways; as first, by raising a bar by one end, with the two hands over one’s head, and bringing it forcibly down to strike across a narrow anvil at its centre of percussion, or one-third from the other extremity of the bar; after which it may be bent backwards and forwards at the place of percussion several times; 2. a heavy bar may be laid obliquely over props near its end, and struck strongly with a hammer with a narrow pane, so as to curve it in opposite directions; or while heated to redness, they may be kneed backwards and forwards at the same spot, on the edge of the anvil. This is a severe trial, which the hoop L, Swedish iron, bears surprisingly, emitting as it is hammered, a phosphoric odour, peculiar to it and to the bar iron of Ulverstone, which also resembles it, in furnishing a good steel. The forging of a horseshoe is reckoned a good criterion of the quality of iron. Its freedom from flaws is detected by the above modes; and its linear strength may be determined by suspending a scale to the lower end of a hard-drawn wire, of a given size, and adding weights till the wire breaks. The treatises of Barlow and Tredgold may be consulted with advantage on the methods of proving the strength of different kinds of iron, in a great variety of circumstances.
Steel of cementation, or blistered steel and cast steel, are treated under the article [Steel]. But since in the conversion of cast iron into wrought iron, by a very slight difference in the manipulations, a species of steel may be produced called natural steel, I shall describe this process here.
[Fig. 603.] is a view of the celebrated steel iron works, called Königshütte (king’s-forge), in Upper Silesia, being one of the best arranged in Germany, for smelting iron ore by means of coke. The front shown here is about 400 English feet long. a a are two blast furnaces. A third blast furnace, all like the English, is situated to the left of one of the towers b. b b are the charging towers, into which the ore is raised by machinery from the level of the store-houses l l, up to the mouth of the furnaces a a; c c point to the positions of the boilers of the two steam engines, which drive two cylinder bellows at f. n n n n are arched cellars placed below the store-houses l l, for containing materials and tools necessary for the establishment.
[Figs. 599.], [604.], are vertical sections of the forge of Königshütte, for making natural steel; [fig. 599.] being drawn in the line A B of the plan, [fig. 600.] a is the bottom of the hearth, consisting of a fire-proof gritstone; b is a space filled with small charcoal, damped with water, under which, at n, in [fig. 604.], is a bed of well rammed clay; d is a plate of cast iron, which lines the side of the hearth called rückstein (backstone) in German, and corrupted by the French into rustine; f is the plate of the counter-blast; g the plate of the side of the tuyère: behind, upon the face d, the fire-place or hearth is only 51⁄2 inches deep; in front as well as upon the lateral faces, it is 18 inches deep. By means of a mound made of dry charcoal, the posterior face d, is raised to the height of the face f. i, [fig. 600.], is the floss-hole, by which the slags are run off from the hearth during the working, and through which, by removing some bricks, the lump of steel is taken out when finished.