Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/315

IRON 299 the weight of the superstructure ; through the walls of the hearth are pierced conical arched openings for the tuyeres ; the throat is fitted with a valve for introducing materials from time to time, such as the cup and cone arrange ment (see 18). Just below the cup a perforation through the fur nace wall allows the gases to pass out into a down flue of rivetted boiler plate through which the gases pass (when the cone is in its normal raised position) to the boiler fires, blast superheaters, &c. ; in order to prevent the fire being extinguished when the cone is lowered, a small coal fire is kept burning under the boilers, &c. , unless the temperature of the fire place is sufficiently high to relight the gases when the cone is again raised. Even at the present day, however, many furnaces are in use of the open-mouthed class, the escaping gases simply passing into the air through the tunnel head as a mass of roaring flame, unless the temperature is too low to enable, them to ignite spontaneously, in which case they escape without flame except when accidentally or intentionally lit. The precise angle made at the boshes between the lower and upper portions of the inner furnace wall is a matter of some mo ment. If the hearth slopes too gently, the fall of the materials downwards as the reduced metal and cinder melt is apt to be re tarded, and scaffolding " to be produced. Modern furnaces usu ally have the walls of the hearth more nearly vertical than the Fig. 9. older ones, whilst the shaft instead of being conical from the boshes upwards is often cylindrical for some considerable distance, then gradually closing barrel -wise the throat ; in towards the tuy eres also are often more than four, espe cially in the larger furnaces. In some of the furnaces in use some twenty years ago and upwards, the hearth was originally built square in plan, like the still older ones of half a century back and more ; but observation of the fact that the hearth of blown-out furnaces was always corroded or fused away to an irregular circular out line, and that the same kind of effect was also produced in the inner lining of the boshes and shaft where an angle originally exist ed, led to the gradual substitution of inn^r cavities shaped in the first instance as much as possible like the ultimate shape to which the furnace be came corroded. In building a blast furnace, the " stack " or upper portion is usually constructed first on its supporting columns, the hearth I HffiMM apt to be disintegrated by the reducing action of carbon oxide on the ferruginous matter, a continual reduction of iron and reoxidation of the metal with deposition of carbon being produced in virtue of the reactions described in 19, equations (3) and (4) ; according to Pattinson s observations (Journ. I. and <!> . lust., 1876, 101) this ac tion has in several instances caused considerable injury when iron was present to a notable extent (3 or 4 per cent.). In Austria and Hun gary steatite is often used for lining the charcoal furnaces in use there ; serpentine has also been employed, but is not so advantageous on account of the difficulty in obtaining it of uniform texture and sufficientcompactness; it does not wear so well as good firebrick. The hearth and foundations frequently require to be specially prepared, consisting of a large mass of concrete, broken stone, &c., with air courses interspersed; above this is the hearth bottom, formerly made of one or more large slabs of sandstone or grit, but with the modern large furnaces of masses of firebrick or sandstone laid inverted dome- wise, or like the under portion of a barrel drain, to diminish the tendency to undermining and forcing upwards by the molten metal Fig. 10. and its outer casing being built in subsequently. The firebrick used for the purpose should be as free as possible from iron, otherwise it is Fig. 11. and slag. The crucible and hearth are then constructed of as great thickness and as infusible material as convenient, so as to increase the time requisite before renewal is necessary, owing to the solvent action continually being exerted by the cinder, especially when the furnace is working irregularly and forming a slag containing much unreduced iron (scouring cinder] which attacks the hearth lining powerfully. The tuyere holes are built in as conical perforations through the hearth wall at a level of some few feet above the bottom, the front tuyere hole being made into an arched recess (like the old " tuyere house," but on a smaller scale) reaching from the bottom to a level of 2 or 3 feet above the other tuyeres; the tymp arch usually projects a little forward from the earth wall, constituting the "fore hearth," at the base of the front of which is the dam, a block of stone or mass of firebrick pierced by a vertical cavity (tapping hole), the bottom of which is on a level with the base of the hearth, and through which the molten pig iron is drawn off from time to time, the liquid metal being dammed back by ramming a mixture of clay and sand or small coal into the hole when the furnace is not being tapped, and the stopping being withdrawn by hammering with a pointed crowbar when all is ready for tapping. The top of the dam