Mode of working furnace.—When the furnace is finished, the bottom and sides of it for 2 feet up the square funnel, receive a lining of common bricks upon edge, to prevent the stone from shivering or mouldering when the fire comes in contact with it. On the front of the furnace is erected a temporary fire-place, about 4 feet long, into the bottom of which are laid corresponding bars. The side walls are made so high as to reach the under surface of the tymp-stone. A fire being kindled upon the bars, the whole cavity of the fur nace serves as a chimney ; the draught and heat is therefore considerable. In the course of three weeks, the furnace is freed from damp, and ready to receive the materials ; the fire-place is then removed, hut the interior brick lining remains till the operation of blowing commences. Some loose fuel is then thrown upon the bottom of the furnace, and a few baskets of coke, which are allowed to become thoroughly ignited before more are added, and the fur nace is then gradually filled. The furnace represented is capable of holding 99,000 lbs. of coke, equivalent to 198,00016s. of coal. This quantity of mate rials is continually burning for several years together, without intermission ! A renewal of the total quantity occurs about every third day. There are, neverthe less, still larger furnaces in Wales. The first charges which a furnace receives contain but a small proportion of iron-stone compared to the weight of the coke, which is afterwards increased to a full burden. The descent of the bur den is facilitated by opening the furnace below, two or three times a day, throwing out the cold cinders, and admitting, for an hour at a time, a fresh body of air. This operation is repeated till the approach of the iron-stone and cinder, which is always announced liy a partial fusion, and the dropping of lava through the iron bars, introduced to support the incumbent materials while those in the bottom are carried away. The filling above is regularly continued; and when the furnace at the to has acquired a considerable degree of beat, it is then judged time to introduce the blast ; the preparations of which are the follow ing :—The dam-stone is laid in its place firmly imbedded on fire-clay ; the dam-plate is again imbedded in this with the same cement, and is subject to the same inclination. On the top of this plate is a slight depression, of a curved form, towards that side farthest from the blast, for the purpose of concentrating the scoriae, and allowing it to flow of in a continued stream, as it tends to sur mount the level of the dam. From this notch to the level of the floor a decli vity of brick-work is erected, down which the scoriae of the furnace flows in large quantities. The opening betwixt the dam and side walls of the furnace, called the fauld, is then built up with sand, the loose bricks are removed. and the furnace bottom is covered with powdered lime or charcoal dust The ignited cokes are now allowed to fall down, and are brought forward with iron bars, nearly to a level with the dam. The space between the surface of the cokes and the bottom of the tymp-plate is next rammed hard with binding sande ; and these cokes, which are exposed on the outside, are covered with coke dust. These precautions being taken, the tuyere hole is then opened and lined with a soft mixture of clay and loam; the blast is commonly in troduced into the furnace, at first with a small discharging pipe, which is afterwards increased as occasion may require. In two hours after blowing, a considerable quantity of lava is accumulated ; iron bare are then introduced, and perforations made in the compressed matter at the bottom of the furnace ; the lava is ad mitted to all parts of the hearth, and soon thoroughly heats and glazes the Ku faces of the fire-stone. Shortly after this it rises to a level with the notch in the dam-plate, and by its own accumulation, together with the forcible action of the blast, it flows over. Its colour is at first black ; its fracture dense, and very ponderous ; the form it assumes in running offsia flat and branched, sometimes in long streams, and at other times less extensive. If the preparation has been well conducted, the colour of the cinder soon changes to white ; and the metal, which is in the state of an oxide formerly coloured, will be left in a disengaged state in the furnace. When the metal has risen to nearly a level with the dam, it is then let out by cutting away the hardened loom of the fauld, and conveyed by a channel made in sand to its proper destination • the principal channel, or runner, is called the sow, the lateral moulds are Called the pigs; hence the name of the raw commercial article, "pig-iron," of which many hundred thousands of tons are annually made.
Phenomena attending the production of the deferent qualities of Pig-metal. When fine (No. I,) or super-carbonated crude iron is run from the furnace, the stream of metal, as it issues from the fauld, throws off an infinite number of brilliant sparkles of carbon. The surface is covered with a fluid pellicle of carburet of iron, which, as it flows, rears itself up in the most delicate folds; at first the fluid metal appears like a dense ponderous stream, but as the colla teral moulds become filled, it exhibits a general rapid motion, from the surface of the pigs to the centre of many points. Millions of the finest undulations move upon each mould, displaying the greatest nicety:and rapidity of movement, conjoined with an uncommonly beautiful variegation of colour, which language is inadequate justly to describe. Such metal, in quantity, will remain fluid for twenty minutes after it has run from the furnace, and, when cold, will have its surface covered with beautiful carburet of iron, already mentioned, of an uncommonly rich and beautiful appearance. When the surface of the metal is not carburetted, it is smooth, like forged iron, and always convex. In this state iron is too rich for melting without the addition of coarse metal, and is unfit to be used in a cupola furnace for making fine castings, where thinness and a good skin are requisite. ' No. 4, or oxygenated crude iron, when issuing from the blast furnace, throws off from all parts of the fluid surface a vast number of metallic sparks, and, while running, it is covered with waving flakes of an obscure smoky flame, accompanied with a biasing noise. This is a slight skytch of the appearances of the two extreme qualities of crude iron, when in a state of fusion. There still remain two intermediate stages of quality to be described; these are, carbonated, and carbo-oxygenated iron, that is, No. 2 and No. 3 of the manufacturers.
No.2, or carbonated iron, exhibits, like No. 1, a beautiful appearance in the runner and pig. The breakings of the fluid, in general, are less fine, the agitation less delicate, though the division of the fluid is equal, if not beyond, that of the other.
No.3, or carbo-oxygenated iron, runs smoothly, without any great degree of ebullition, or disengagement of metallic sparks. The partings upon its surface are longer, and at greater distances from each other than in the former varieties ; the shape they assume is either elliptical, circular, or curved. In cooling, this metal acquires a considerable portion of oxide. An infallible criterion of the quality of the iron in the furnace is afforded by the colour of the scorite upon the working bars, which are from time to time inserted to keep the furnace free from lumps, and to bring forward the scoriae. When No. 1, or super-carbonated iron, is on the hearth, the vitriol crust upon the bars will be of a black colour and smooth surface, fully covered with large and brilliant plates of plumbago. As the quality of the metal approaches to No. 2, (carbonated,) the carburet upon the norm decreases both in point of quantity and size. When carbo oxygenated (No. 3) is in the furnace, the working bars are always coated with a lighter coloured scoria than when the former varieties exist; a speck of plumbago is now only found here and there; and when the quality of the metal is oxygenated (No. 4), not only have the plates of carburet disappeared, but also the crally colour on the external surface of the scorite. It may perhaps be proper here to mention, that although for convenience the manufacturer has, from a just estimation of the value of the metal in a subsequent manufacture, affixed certain numbers for determinate qualities of iron, yet it is difficult to say at what degree of saturation of carbon each respective term commences ; suffice it then to say, that the two alterative principles, oxygen and carbon, form two distinct classes—that in which oxygen predominates, and that in which carbon predominates.