A blast furnace of 50 or 60 feet in height gives commonly from 60 to 70 tons of cast-iron per week ; one from 50 to 55 feet high, gives 60 tons ; two united of 45 feet produce together 100 tons ; and one of 36 feet furnishes from 30 to 40. A blast furnace should go for four or five years without needing restoration. From 31 to 4 tons of coal, inclusive of the coal of calcination, are required in Stafford shire to obtain one ton of cast-iron ; and the expense in workmen's wages is about 15 shillings British on that quantity.
Heated air is applied in some iron works.- Where this method of working the ore has been introduced, the air is blown by cylinder-bellows in the usual manner, but before entering the smelt ing-furnace it passes through pipes of cast-iron, heated to redness, which are altogether about thirty feet in length and three feet in diameter. They are usually made in three or four pieces, joined together by apertures considerably less than three feet in diameter, and placed horizontally, or in whatever manner the local arrangements about the furnace may render most convenient. A brick arch is then thrown round the pipes, leaving a free space of about eight inches, and upwards, between it and them, and two or more furnaces constructed, so as to heat the pipes in the archway, the flues playing into it, and terminating in a com mon vent at the farther extremity. They may be considered, therefore, as placed on the floor of a long and narrow rever beratory furnace, about six feet high, and nearly of the same breadth, being at the same time protected by fire-bricks, when they might be injured by the direct flame of the furnaces. The iron ore is smelted, according to this plan, with little more than half the coal necessary when the furnaces are worked with air in the usual manner ; the small coal, which is sold at an inferior price, is found quite sufficient for heating the pipes.
The number of charges in English fur naces, given in 12 hours, is different, in different furnaces, being 20, 25, and even up to 40 ; 30 is an average. Each charge is composed of from 5 to 6 cwts. of coke (or now of 3 to 4 cwts. of coal with the hot blast) ; 3, 4, and sometimes 6 cwts. of the roasted mine, according to its richness and the quality of east iron wanted ; the limestone flux is usually one third of the weight of the roasted iron stone. There are 2 casts in 24 hours ;
one at 6 in the morning, and another at 6 in the evening.
According to M. Berthier's analysis, the slag or cinder of Dowlais furnace consists of silica, 40.4; lime, 38.4; magnesia, 5.2; alumina, 11.2 ; protoxyde of iron, 3.8; and a trace of sulphur. He says that the silica contains as much oxygen as all the other bases united ; or is equivalent to them in saturating power ; and to the excess of lime he ascribes the freedom from sulphur, and the good quality of the iron produced. The specimen ex amined was from a furnace at Merthyr Tydvil. Other slabs from the same fur nace, and one from Dudley, furnished iipwards of 2 per cent of manganese. Those which he analyzed from Saint Etienne, in France, afforded about 1 per cent. of sulphur.
As the ignition in the blast furnace proceeds, and the blast let on, the metal in the ore parts with its oxygen, and sub sides to the bottom of the furnace, cov ered with a melted slag. This last is oc casionally allowed to flow off; by opening some of the side holes which were stop ped with clay, and when the bottom of the furnace becomes charged with metal, which it does after five or six hours, the iron itself is discharged, by one of these openings, into a pit of sand mixed with clay. As soon as the iron is poured out, the hole is closed, and the furnace is still kept at work, and goes on reducing iron for six months. The flux employed to assist the fusion of the ore, by vitrifying the earths aforesaid with iron, is lime stone of the best quality ; very lately, it has been proposed to use caustic lime, or that which has been burned, instead of the crude limestone. It is said to pro duce an economy of fuel.
The iron which has run out from the furnace, is cast iron, or iron with carbon intermingled with it, sometimes to the extent of 5 per cent. It has a coarse grain, and is very brittle. The mould in which the metal flows, is of a longish shape, having projecting offsets on each side, which, from some fancied resem blance to a sow and her litter, has been called pig iron.
To convert this pig or crude iron into bar iron, it has to be refined. This con sists in placing it in a furnace, like a smith's forge, or hearth, with a sloping cavity sunk a foot below the blast pipe.