Chap. II.'
COMPOSITION AND USE OF MINERALS.
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ami the bosh-stones are always made from a coarse-gritted freestone, whose fracture presentslarge rounded grains of quartz, connected by a cement less pure. The height of the entirefurnace is 55 feet, and its diameter at bottom 38 feet.
The cost of a pair of blast furnaces in the Dudley district is about 1800/. ; when 40 feetin height they require for their construction 160,000 bricks, 3900 fire-bricks, and 8 ‘25 boshes.A furnace 50 or 60 feet in height will produce 60 or 70 tons of cast-iron per week ; from50 to 55 feet high, 60 tons; ahd two of 45 feet in height together, 100 tons. 3^ tons ofcoal, including the coal of calcination, are required to obtain a ton of cast-iron, and theworkmen’s wages amount to about 15 shillings; 14^ tons of coke, 16 of roasted ore, and6i| tons of limestone, being thrown into the furnace every 24 hours, allow a run of 7 tonsof pig-iron every 12 hours. In many parts of Wales these furnaces are of much largerdimensions ; that at the Plymouth iron works at Duffryn, near Merthyr , is 18 feet in diameterin the boshes, and 10 feet atthe top or filling place, theheight being 40 feet; itcontains 7000 cubic feet,and when at work 150 tonsof ignited materials for thesmelting of the iron. Insome of the largest furnaces20,000 cubic feet of atmo-spheric air is forced intothem every minute, by apressure of l.^lb. upon eachsquare foot : their formvaries in different districts,and when built against atide bill they have a flat wall, and the blast is admitted at the front.
The air apparatus is put in motion by a steam-engine, and the noise may be heard formany miles ; the rod is made to force down the piston to the blowing cylinder, E, whentwo valves open, which allows the air to pass above the piston ; by drawing it backthe valves are shutby the pressure ofthe air above them,which being broughtinto a smaller spaceforces open thevalve : as the pis-ton rises air rushesinto the vessel, andns the piston begins
to descend, the valve is shut close by the force of the air. The heavy weights upon thepiston now press it down, and force the air through the opening, and along the pipewith great violence into the furnace, with a constant and unintermitting stream. Theair in the furnaces becomes very much heated, and passes up with great force throughthe materials, which by the heat are gradually melted, and slowly subside downwards,whilst fresh materials are put on at the top ; about 36 hours is the time that it takes fora charge to get down to the hearth. Instead of the regulator, just described, several furnaceshave a large vessel or a reservoir, made of plates of wrought-iron in the form of a cylinder,rounded at each end; the usual length is about 20 feet, and the diameter 8 feet. Theair is forced into this precisely in the same way as in the vessel, and passes from it in acontinuous and powerful stream into the blast furnace. The diagram shows only oneair-blast directed towards the furnace, but in practice there is one from the other side, andgenerally another from the back of the furnace ; the melted metal is let out from the front;a roof over head shelters the men engaged under it in the operation of casting; this iscalled the casting-house.
The hot blast has been used with effect in Staffordshire and Shropshire ; the generalprinciple is, that instead of forcing the air into the blast furnace at the temperature of theatmosphere, it is previously raised to from 600° to 700° Fahrenheit by traversing a numberof tubes which pass through a large fire made in a building at the back of the furnace. Inpractice the air is considered sufficiently hot when a jet striking against a piece of lead willmelt it and cause it to fall in drops.
The effects of this invention in the iron trade have been very great: the operations of thefurnace go on more rapidly, a greater quantity of iron is made from the ironstone, and thereis a saving in fuel and limestone; raw coals may also be used instead of coke, whichdiminishes the expense.
In Staffordshire some hot-blust furnaces are worked with raw coals only, but in others a
Fig. 588.
PLANS OF IRON FURNACES,