The refining of pig-metal is a modern intermediate process of conversion, which the experience of our iron masters has led them to believe is the best economy in the preparation of the metal for subsequently rendering it malleable in the puddling furnace; although it is well understood not to be indispensable to the production of wrought iron. The furnaces employed for this purpose are small buildings, termed refineries; a vertical section of one of them is given in the cut on the next page, a is a thick cast-iron trough, having three of its sides made double, with a hollow space between.
around which water is caused to flow from an external cistern b, the cooling effect of the fluid serving to prevent the fusion or other injury of the metal sides of the furnace by the intense heat; at c (in a line) are two pipes, through which air is forced from a blowing machine upon the materials in the trough; these pipes are kept cool by a constant stream of water always flowing over them, brought on by small pipes d, regulated by cocks. The bottom of the furnace is of brick, on which the fuel is laid, and over it the pigs to be refined. The building is surmounted by a wide chimney about 12 feet high, and the front, which is left open, has a projecting roof to cover the workmen who attend to it. In Wales it is usual to make these furnaces what is termed double; that is, the fire is somewhat larger though single, but the blast is double, there being usually three pipes and tuyeres on each of two. opposite sides of the furnace; each of the pipes are generally about an inch in diameter, and supplied with a blast equal to about 2 or 21/2 lbs. pressure upon the square inch.
When the operation, which usually occupies two hours, is deemed to be complete, a hole in front of the hearth is tapped, through which the liquid metal flows into a very thick oblong flat mould of cast-iron, placed over a cistern of water; this causes the metal to be rapidly chilled, which is thus brought into a cake about 2 feet broad and 20 feet long. This plate of metal is extremely brittle, and presents on its fracture a silvery whiteness: it weighs about a ton. In some refineries the furnace is lined with firebricks or stones, without water running round it, and. the refined metal is cast upon sand in shallow moulds or depressions, and water is then thrown upon the metal to cool it quickly. As inattention or neglect on the part of the refiner would be productive of serious loss to the iron master, he is always paid according to the metal produced. Great experience and practical skill are requisite qualities in a refiner; his occupation is one of great personal exertion, and he is exposed to an intense fire, that no one unaccustomed to it could even approach.
He puts the charge of pigs upon the fire, attends to the progress of the melting, supplies the fire from time to time with coke, frequently stirring it up to equalize the heat; sees that the tuyeres are in good order, and that the water circulates uninterruptedly; runs out the metal when it is ready; he removes the plate from its mould when it has a little cooled, by means of a lever, on to a truck, and wheels it out of his shed; he then prepares his mould for the next plate, removes his cinders, and repeats the operation. The quantity of refined metal thus produced by a " double" furnace in Wales, is from 60 to 70 tons per week, of 6 days of 24 hours each, the men working in turns of 12 hours at a time, and 12 hours rest. The refiner selects his materials according to the quality of iron wanted. The best quality is from the dark grey pig, or No. 3, and the inferior sorts from bright, mottled, and white in their order. The very " worst" white iron cannot be used by itself in the refinery; being almost infusible from its deficiency of carbon, it is disposed to clog and settle on the hearth. To work up such iron in the refinery, it is mixed with pigs of a " better quality," or those containing more carbon, the union conferring fusibility.
When No. 3 pigs are used, it requires about 221/4 cwt. of them to produce one ton of refined metal; the "yield" however varies, from causes before adverted to, according to the degree of carbonation of the pig metal employed, the quality of the coke, the management of the blast, and various other circumstances. This reduction of weight is however not an entire loss, as a quantity of "cinder" is produced which floats on the top of the metal, and detaches itself as it cools; this cinder contains usually about 50 per cent, of its weight of iron, which is recoverable on the blast furnace.