Page:The American Cyclopædia (1879) Volume IX.djvu/413

IRON MANUFACTURE 399 1J hour. The yield of blooms is about 87 per cent, of the pig iron, and the consumption of charcoal one ton to a ton of blooms. The iron is subsequently heated in a gas furnace and forged out into bars. All the Swedish bar iron used in Sheffield for conversion into steel is made by this process, except that from Dannemora, which is made by the more expensive Walloon process. The latter consumes three tons of charcoal to a ton of blooms, with a yield of but 80 per cent. Formerly the process of re- fining pig iron previous to its conversion into wrought iron was extensively employed, but at present this practice is falling into disuse. It consists in exposing the molten pig iron in an elongated rectangular hearth to blasts of air from two or three tuyeres on each side. The operation is in all respects similar to that already described, but is interrupted be- fore all the carbon is removed, and while the metal is still molten. The metal is then run out on an iron plate, where it solidifies in plates about 3 in. thick. Gray pig is used in the refining or running-out fire ; and the change that is effected con- sists mainly in the removal of the greater part of the silicon and a little of the car- bon, the resulting refined or " fine " metal being white and more easily and quickly converted into wrought iron than the original gray pig. Puddling consists in melting down pig iron on the bed of a reverberatory furnace heated by flame, and stirring it actively in an oxidizing atmosphere, until it has become malle- able. This process was first successfully introduced by Henry Oort in England in 1784. Although others, notably the brothers Cranage in 1766 and Peter On- ions in 1783, had plainly indicated in pat- ent specifications the essential operations in puddling, yet to Cort belongs the credit of having made the process a practical success. For some time puddling was performed on a sand bottom, which made the operation tedious, and caused great waste of iron by the formation of an ex- cessive amount of cinder. In 1818 Samuel B. Rogers introduced iron bottoms, by the use of which the production of the furnace was great- ly increased, and the waste of iron lessened. Before the puddling furnace is used, the iron bottom is covered with a thick layer of cin- ders or scrap wrought iron, and exposed for a long time to an oxidizing atmosphere, so as to form a refractory lining. The essential parts of a puddling furnace are the fireplace, hearth, and flue. The fireplace is from one third to two thirds the size of the hearth. The draught of the fire is effected by a high chimney, and often there is a blast under the grate. Either bituminous coal or anthracite may be used as fuel. The hearth is generally rectangular or oval. It is formed of a bottom plate and sides of cast iron, and ends of fire brick, called the fire and the flue bridges. The 439 VOL. ix. 26 side castings are hollow, as are also the bridges, to allow of a circulation of air for cooling. There are recesses in the side castings, in which is put the fix or fettling, which is either rich iron ore or roasted cinder. The waste heat passing off in the flue is usually utilized to heat steam boilers. Fig. 5 shows a vertical and horizontal section of a puddling furnace with steam boiler. Sometimes puddling fur- naces are built of double the usual size, with a working door on each side. In these the charge is twice the usual amount, and two puddlers work together. There is a great variety of puddling furnaces, built mainly with a view to economize fuel or to better regulate the com- bustion. Gas furnaces have never come into very general use for puddling. The pig iron used for puddling is mostly the lighter gray FIG. 5. iron and white iron. The highly graphitic va- rieties are apt to be rich in silicon, which makes a large amount of fluid cinder, prolongs the operation, and causes great loss of iron. Gray iron requires a high temperature for fusion, but becomes thinly fluid, while white iron is sluggish when melted. The former therefore generally yields an iron of better quality, as it can be more thoroughly worked. The pud- dling process may be divided into four stages : 1, melting down the charge ; 2, incorporation of the fettling ; 3, boiling ; and 4, uniting the soft iron to balls. When the charge is com- pletely melted, it is stirred to render it uni- form. The temperature is then somewhat low- ered in order to allow the cinder which has formed on the surface and the oxide of iron of the fettling to be thoroughly incorporated with the iron. The temperature is then raised again,