It may, by a peculiar process, be procured direct from the ore, but is generally obtained from the harder descriptions of pig-iron by a succession of processes, the object of which is to get rid of the carbon, and of the phosphorus, silica, and other impurities, which injure the iron and make it brittle.
In order to expel these foreign substances the finest qualities of wrought iron are refined and then puddled: the inferior qualities are puddled only.
Forge pig is generally used for the manufacture of wrought iron, and can be converted at once by the puddling process.
Grey iron, however, contains graphite and silicon. The latter makes it difficult to puddle, and it is often removed by the preliminary process of refining described below.
1 Matheson's Works in Iron..
2 Stoney On Strains, p. 477.
Refining consists in keeping the pig-iron in a state of fusion on an open hearth with coke, for about two hours, with a strong current of air directed upon it. It is at the same time well stirred, so that all parts of it are brought into contact with the air and oxidised.
The oxygen in the air deprives the cast iron of part of its carbon, and at the same time converts the silicon into silica, which combines with some of the oxide of iron to form a fusible slag, that runs off.
The liquid iron is then run into cast-iron moulds lined with loam, and kept cool with water circulating below them, so that it is chilled and easily broken up into what is technically known as "plate metal."
The resulting fine metal greatly resembles white cast iron in its characteristics, but the percentage of impurities will be found to have been considerably reduced by the refining process.
Puddling consists in melting the pig-iron in a reverberatory furnace, by means of which the metal is subjected to the heat of the flame and a strong current of air, and kept quite clear of the fuel.
The molten metal is at the same time well mixed with oxidising substances, such as haematite ore, oxide of iron, forge scales, etc., and sometimes with limestone and common salt. The oxygen in these combines with the remnant of carbon left in the iron, and the silicon is also oxidised, passing off in slag.
As the carbon is removed the iron becomes less fusible, and clotty lumps of pure iron appear, which are collected by the puddler and pressed together with the tool until they are formed into puddle-balls weighing about 3/4 cwt. or more.
In order to reduce the labour in puddling, rotatory furnaces and other ingenious inventions have been introduced of late years. These, however, need not be further referred to.
The lumps or balls formed in the puddling furnace are at once placed under a helve or a tilt-hammer, the blows of which force out the cinder and consolidate and weld the particles of iron together, forming it into what is called a bloom.
Inferior descriptions of iron generally have the slag removed by a squeezer, a machine something like the jaws of an alligator, after which animal it is sometimes named.
On many works the steam-hammer is used for this purpose, and it can be made to do the work very effectually. It may, however, be used to produce very inferior iron, because it can be adjusted to give the mass such very light blows that the slag is not squeezed out, but left in the iron to its very great detriment.1
Directly after this the red-hot slab of iron, or "bloom," is passed between grooved rollers, which convert it into puddled bars about 3 or 4 inches wide, 3/4 to 1 inch in thickness, and 10 or 12 feet long.
The puddled bars thus formed are wrought iron, but of the lowest class. They possess hardly any of the characteristics of the higher qualities, and require to be greatly improved by subsequent processes of piling, reheating, and rolling.