(a) The "bloomery" process, necessitating the employment of wood charcoal, is conducted in a "finery," originally consisting of a rectangular shallow hearth, lined with 2 - in. or 3 - in. iron plates, protected by an inside coating of "brasque" (charcoal dust), with holes in the front plate for the exit of the slag. The "twyer" or blast - pipe (tuyere) is set at an angle of 5° to 16°, and the depth of the hearth is 6 to 10 in. When the furnace is heated and ready, layers of fuel and 2 - in. or 3 - in. lumps of iron are thrown on the hearth, and the blast is put on. As the molten metal reaches the bottom of the hearth, it encounters the blast from the twyer, " the result being that the silicon in the pig is more or less oxidized to silica, the carbon to carbonic acid, and the iron to oxide of iron. The oxide of iron combines with the silica, the liquid slag thus formed dissolves a further quantity of oxide, and being intimately mixed with the carbonized iron, re - acts upon it, removing the carbon." (Prof. Huntington.) Means are taken to thoroughly break up and expose the iron to the blast from the twyer, and when the operation is judged to be complete, the metal is withdrawn as a spongy "ball" laden with slag; while still white hot, it is squeezed under a hammer to expel the slag, and welded into a rectangular " bloom," or slab, which is afterwards reheated and drawn out into a bar.
The expressed slag contains 70 to 80 Per cent. of iron protoxide, and is returned 'to the hearth with subsequent charges to help in removing their carbon. The best iron for the purpose is white pig, as it becomes pasty in melting.
Fig. 132 shows a furnace described by Dalifol, adapted for using wood charcoal: a, 4 conduits for admitting air into the furnace; b,openings for charging in the charcoal; c, 2 little plates for preventing the Bides of the furnaces from meeting when bent by the heat; or, tie bars.
(6) When grey pig - iron is used for making malleable iron, it must first be converted into white iron by the "whitening" process in a "refinery," which also removes most of the silicon. Fig. 133 shows a modern "refinery"as described by Prot. Huntington. It consists of an oblong hearth a, 3 ft. long, 2 ft. wide, 2 1/2 ft. deep (on which the fuel and pig - iron are placed), surrounded on 3 sides by iron blocks b, cooled by water, and on the fourth (in front) by a. solid cast - iron plate with a tap - hole at the bottom; c are hollow Iron columns for supporting the chimney d, about 18 ft, high; e, layers placed at an inclination of 25° to 30°; f, funnel pipes for supplying cold water to the twyers; g, overflow pipes for taking away the heated water from the twyers; A, similar pipes for performing the same service to the channels b; i, valves for regulating the blast furnished by the pipes 1/2, which is usually about 400 cub. ft. per minute at a pressure of 1 1/2 to 3 lb. per sq. in. The hearth is supplied with alternate layers of fuel (generally coke) and pig - iron, commencing and finishing with the former. The charge is subjected to the blast for about 3 hours, the first 1 1/2 hours being required to effect the melting.
The cinder and "fine " or " plate metal are tapped into a shallow cast - iron loam - lined mould, cooled by circulating water, whereby the metal is cast into a plate 10 ft. long, 3 ft. wide, and 2 in. thick, white most of the slag escapes to a second mould beyond, and the residue can be easily removed from the cool surface of the plate. The metal is further chilled by having water thrown on it, and can then be broken up readily; it should have a bright fracture and show no honeycomb.
Where supplies of wood for making charcoal are scarce, the "puddling" process is in vogue. Many forms of furnace are employed, but the main feature underlying them all is that the fuel (coal or coke) is burnt in a separate chamber, so that the heated gases only reach the metal, whereby the sulphur present is oxidised and rendered harmless before arriving at the metal. in general terios, the furnace consists of a fireplace, a hearth, and a flue leading to a chimney, bridges separating the hearth from the fireplace and flue respectively. The form and size of the hearth are governed by the character of the fuel, which is preferably dry, non - caking, and burns with a long flame. The floor of the hearth is made of l 1/4 - in. cast - iron plates, and covered with a layer of materials rich in iron oxide, such as slag and bloom scale, strongly heated, and spread about 2 in. thick. The "fettling" or lining, known as "bulldog," is obtained by roasting " tap cinder " (the slag of the puddling furnace), which is essentially a silicate of protoxide of iron, till the protoxide is converted into peroxide, and the silica is liberated.
The puddling operation is performed with a " rabble " or a " paddle," and by 2 methods, known as "dry puddling" and " pig - boiling." The former method requires white iron, wastes less metal, and consumes less fuel; but the sulphur and phosphorus are not so well removed. In the latter method, grey iron is used. The details of puddling furnaces are too diffuse and complicated for further discussion here. (See Spoils' Dictionary, article " Iron.")
(d) The Chemische Centralblatt describes the following method practised in Germany for the preparation of malleable cast - iron: - It consists of two operations: (1) pouring the cast - iron into moulds, and (2) removing part of the carbon contained in the castings. This is conducted in' so - called "temporary " furnaces, wherein the castings are brought into contact with substances containing oxygen and heated to redness. The result is the formation of a very tough material, poor in carbon. The castings, contained in cast - iron vessels, are placed on a layer of oxidizing substances, and the intervening spaces are filled up with the latter. Oxide of zinc, hammer scale, brown and red iron ores, are used for this purpose - mostly the latter. The operation lasts 24 to 36 hours, and depends on the dimensions of the iron to be tempered and the degree of tempering. With regard to the latter, the limits are within a wide range, as, with a correct formula for mixing, it is possible to absorb the whole of the carbon. It is best to use iron free from manganese and containing amorphous carbon - that is, white pig - iron. The tempered iron forms an excellent material, and compares favourably with malleable iron as to firmness.