Indian Science and Technology in the Eighteenth Century: Some Contemporary European Accounts c24.7

c24.7

5. In England the fuel now most generally used in smelting the impure iron ores of the coal fields is coke; and the ore after being first roasted to separate the volatile impurities, as much as possible, is exposed to its action in blast furnaces, generally about forty-five feet in height, but varying sometimes from thirty-six feet to even sixty feet. In the middle, these furnaces are about twelve feet in diameter, but at top are contracted to about four feet, and at bottom, where the blast of air is introduced by pipes from powerful blowing machines, the diameter is only about two feet. The pressure upon the air forced into the furnace is about three pounds upon the square inch, and the quantity of air amounts generally to as much as 4,000 cubic feet per minute. The cast iron as it forms, falls down into the bottom of the furnace; which is always hot enough to maintain it in a state of fusion; where it is protected from the action of the blast by a covering of fused slag which floats upon it. These furnaces are kept in action unremittingly, night and day, for several years together; the metal being allowed to flow out every twelve hours in quantities of about six tons at a time. The material used in building the blast furnace is principally fire brick, and a pair of furnaces cost upwards of £1,800 sterling. The proportion of coal used in making a ton of cast iron, varies very much, from three tons in Wales, to sometimes eight tons in Derbyshire; but the use of heated air in blowing the furnaces has very much increased the quantity of the products of the blast furnace, and has also diminished the expenditure of fuel, but the quality of the cast iron is said to be deteriorated. The estimated expense of making a ton of cast iron is about £3 sterling.