The next process consists in depriving the iron of its remaining carbon and oxygen (or as far as that may be practicable), by which it is rendered malleable and is constituted wrought-iron. For this purpose the brittle plate of white refined iron is broken up into small pieces, and brought to the puddling furnace, sometimes called the balling furnace, from the circumstance of the iron being therein made up into balls, about the size and shape of quartern loaves of bread. The furnace differs but little from an ordinary reverberatory furnace, as will be seen upon reference to the subjoined engravings, in which Fig. 1 exhibits a vertical and longitudinal section, and Fig. 2 an horizontal section or plan. As a strong flaming fuel is required in this" operation, coal is employed in preference to coke, the gaseous portion of the former being rendered available as fuel. At a is the ash-pit; b a series of loose grate-bars, on which is deposited the coals introduced through an aperture c kept closed, except during the charging or stoking, by heaping up the small coal against it externally, where it is formed into a cast-iron receptacle for the purpose, resembling a coal scuttle; at d is the fire-bridge, frequently made hollow by an iron casting, as shown in the drawing, and encased with firebricks; this hollow casting is connected externally with a vertical pipe to convey off the air, which is caused to pass through the fire-bridge; and by the heat it thus abstracts, the fire-bridge is prevented from becoming fused by the intense action of the flame.

At e is the basin of a bed of sea sand spread into a concave shape; it is supported upon a stratum of fire-bricks, which lie on solid masonry f beneath. On this bed the refined metal is placed and exposed to the heat of the flames, which are made to impinge downward upon the metal in a constant stream, owing to the curved form of the roof, against which the current of air that passes through the grate-bars strikes; from the metal the flames again ascend, passing over a dam g, through the throat h, into the chimney i, which is carried up from 30 to 40 feet high, and is provided with a damper at the top, by which the puddler regulates the degree of draft, and consequently the heat of his furnace; for during some parts of his operations, he requires a more intense heat than at others. The draft through these furnaces is frequently so great as to cause the flames to ascend above the chimney top, and to make the damper, situated there, red hot. At j is a sliding door, suspended by a chain, and raised or lowered by a lever k; at the bottom of the door is a small aperture (as shown in Fig. 1) employed as a spy-hole, and for the introduction occasionally of the puddler's tools without opening the door.

There is also usually another small aperture at f, lined with a cast-iron box or case, and divided by a partition into two cavities, through which the "shingler" puts his iron bars to be heated, as will be hereafter explained. The dam g before mentioned is deeply grooved or furrowed (as seen in Fig. 2) to allow the cinder or slag to run off into a pit n, whence it is discharged through an opening o out of the furnace. The chimney is bound together at different points with iron, and the external sides of the body of the furnace is encased in iron plates, bolted through the brick-work. All these precautions are, however, insufficient to prevent the early distortion of the fabric, and which would without them soon fall into ruins by the destructive effects of the fire. The employment of a sand bottom to the furnace, it is proper to mention, is now getting into disuse in many parts, where, in lieu of the sand, a thick cast-iron plate forms the bottom, on which is strewed a coat of the oxide or cinder made in the puddling process. The puddler is assisted in all his operations by an inferior workman, termed his "under-hand," who is not recognised by the master, but paid by the puddler.

The metal being put into the furnace, the door is shut down and carefully closed to prevent the admission of air, as any admission of air otherwise than through the grate-bars, tends to derange or moderate the proper current. In about half an hour the metal begins to fuse, at which time the severe labours of the puddler begin. His first business is so to dispose the pieces of melting metal, that those which are the least exposed to the action of the flame may be drawn more immediately under its influence, and that the whole quantity may be brought to the same fluid state, as nearly as possible at the same time. If it is not so managed, that which is first melted begins to burn and waste before the other is ready, and the yield is therefore inferior than when all goes on well together. When the whole is melted, the puddler, sometimes with a tool turned at the end like a hoe, and sometimes with a flat one, stirs it about diligently in all directions, exposing every part of it in turn to the action of the flame. In doing this he is obliged to be constantly changing his tools, which soon become red hot, and are plunged, as they are withdrawn, into a vessel of water to cool them. The liquid mass heaves and boils as it is being stirred, showing the escape of elastic fluid.

By degrees the metal loses its fluid property, and assumes the appearance of a loose granulated mass, the external particles of which appear in vivid combustion, whilst the main portion of it is less brilliant; these appearances indicate that the metal is "coming round to nature," as the puddler terms it; and to accelerate this process, he frequently throws some water from a scoop on to the metal, and occasionally adds a portion of the black oxide of iron formed in the subsequent processes of the forge, by which the weight of his product of metal is increased. He continues to move the mass about vigorously till it becomes so thick and tenacious as to stick together and form into lumps. At this time the puddler with great dexterity, exposed to a severely scorching heat, and a light of the most dazzling brilliancy (which no ordinary person can even look at but for an instant), separates the metal into masses resembling in size and figure quartern loaves of bread, but weighing from 50 to 80 lbs. each, called " puddled balls," and having arranged his batch of metal upon their vitrified bed, they are left there exposed to the continuation of the heat, until they can be successively extracted and delivered by the under-hand to the shingler.

By this mode of puddling, a loss of time is incurred, by the heating of the fresh metal for the next batch, amounting to nearly half an hour, besides the waste of fuel during that period. To obviate these losses, various contrivances have been resorted to for heating the fresh charge of metal, whilst the previous batch of puddled balls are being shingled or rolled. The simplest and most improved plan deserves mentioning. It is merely to make the body of the furnace longer than usual, and to have a second door between that where the puddler works and the chimney. This affords sufficient room for the succeeding charge of metal, which is at the same time so near the workman that when he has completed the heat and sent the balls to the shingler, the newly-prepared charge is just in the right state for him to bring forward and recommence his operations upon. By the puddling furnaces first described, but seven heats could be completed in the turn of twelve hours; but with the improved furnace, nine heats are effected in the same time by one puddler, the results being two-ninths more of iron, without any additional quantity of coals.