684

THEORY AND PRACTICE OF ENGINEERING.

Book II.

mixture of two parts raw coal and one part coke is preferred. At those of Woombridge inShropshire , the proportion is half coal and half coke ; but even this is a great advantage.The iron produced by the hot-blast is not only less expensive, but for some purposes it isinfinitely superior, for instance in fine castings, ornamented on the surface with delicatefigures: when poured into the moulds it will enter into every line, however fine, almostlike a Daguerreotype. For all articles, on the contrary, that are made by passing wrought-iron through the rollers, a stronger iron is better adapted. '

In the districts of Staffordshire and Shropshire about one-third of the furnaces are blownby the hot-blast, and two-thirds by the cold blast: that part of a furnace which first requires tobe repaired is the hearth, or lower part of the interior, into which the iron glides down fromthe melted materials. Die hearth is made of sandstone, which resists heat; the most notedquarries of which in Staffordshire are at Gornal, about two miles from Dudley , andbelong to the geological formation called the millstone grit. A furnace may require to berenewed* in four years, and sometimes it may last seven or eight. A new furnace takes con-siderable time to dry thoroughly, and afterwards to heat, anti then to be gradually chargedwith materials, ironstone, limestone, coke, or coal, so as to bring it into a proper state forthe making of iron. When it is intended to discontinue a furnace, it must be blown out, asit is called, for if the blowing were suddenly to cease, the melted and half-melted materialswould all vitrefy into one solid mass, and adhere to the sides of the furnace, which wouldinvolve the taking it down : hence it becomes necessary to continue putting on fuel andblowing until the whole contents has descended in a melted state to the bottom, and been letoff. A furnace in full operation is charged by a set of hands, consisting of men, young people,and boys : the hoys fill coke into baskets or barrows, and ironstone and limestone into whatare cal ed boxes, though they resemble baskets. The young persons and men convey thesematerials to the filling place at the top of the furnace, and a certain proportion of each isthrown in, according to the orders given from time to time ; to ascertain the proper quan-tities, an acquaintance with the peculiar qualities of those found in the district is necessary.A skilful and trustworthy person is required to superintend the weighing of the ironstoneand limestone, for which proper machines are provided ; for the coal or coke the eye issufficient. There are generally two furnaces together, sometimes three, and when one ischarged the people proceed to the other ; they have never many minutes to rest, until after4 or 5 o’clock in the afternoon, when the furnace is usually quite full; the blast is thenstopped for a time, until the melted iron and cinder be let off'. In about ten hours, some-times a little more, a hole is bored in the sand and clay at the bottom of the hearth ; theliquid iron flows out, and runs into a broad mould, with a number of smaller on one side,prepared for it in sand, on the floor in front of the hearth ; these moulds are called the sowand pigs, and in conformity with this expression the iron is called pig-iron, and also crude-iron. Sand is sprinkled over it to prevent its cooling too rapidly, which would injure itsquality. The cinder or liquid mass, composed of the clay and lime, with silex and a por-tion of iron, is then let off*, and flows round a piece of iron, by which it is held fast whencooled, and to which a crane pulling a chain is attached ; the whole mass is hoisted upon awaggon, and carried off’from the surface to the further part of the cinder hill. If intendedto he used as road materials water is thrown on it before it is quite cooled, and it readilybreaks. The cinder has to be let off several times in the course of the twelve hours, gene-rally every hour and a half or every two hours; in some furnaces it is allowed to keepcontinually running off. The furnace-master observes from time to time the appearance ofthe melted cinder, and from it he is able to ascertain the condition of the furnace, and giveshis orders accordingly, as to the proportion of the several materials. The people are re-lieved every twelve hours; the day set takes the nightwork every alternate week; the changeis effected by the set at work during the day on the Sunday continuing all night till(i o'clock on the Monday morning, that is called the double turn.

Moulding and Casting _The pig-iron is found to be of various qualities, dependent on the

quantity of carbon which has entered into combination with it during the process of smelt-ing. The iron called No. 1. in commerce is highly carbonated, the most fusible of all. andmost fluid when melted, and therefore the best adapted for fine castings, giving a smoothsurface, and filling up the finest parts of the figure moulded. That called No. 2. is lessfluid when melted, but better adapted for articles requiring strength and durability. TheNo. 8. is used for castings where very great strength is demanded ; it may also he made intobar-iron. In order to be made into articles of east-iron, the pig-iron has to he melted asecond time. Moulds in the form of the articles to be cast are made of a mixture of sandand clay in boxes, laid on the floor of the foundery : the iron is melted in a furnace,let out into large pans, and then carried and poured into the moulds and left to cool.A great deal of casting is made from the iron as it comes from the blast furnace, as water-pipes, rails for tramways, broad flat pieces of iron for the flooring in front of the ironfurnaces, See. See.

defining of Iron . — The furnace is generally small, being about 8 feet square at the basein the inside ; the bottom is of hearth brick, and the front, hack, and sides are of cast-iron.