When the principal object is to obtain sound castings with a very smooth face, as for ornamental works not afterwards wrought, the soft kinds of iron containing most carbon, which are most fusible and flow easily, are principally used. But such metal would neither possess sufficient hardness, durability, nor strength, for many of the castings employed in the construction of edifices and machinery.
* A few of the modern cupolas greatly exceed the air furnaces in effect, as they are calculated to contain upwards of twelve tons of melted iron. One of these, at the works of Messrs. Nasmyths' near Manchester, is six feet two inches diameter externally, and lined with Stourbridge bricks. It has three sheet iron tuyeres, nine inches diameter at the mouth, the blasts from which enter the furnace at three points of the circle, and they may be slid like telescope-tubes to either of the four series of holes, as the furnace becomes gradually filial.
There are three other furnaces progressively smaller, arranged beside the first; all of which may be used separately or in combination according to circumstances. The blast, which is under corresponding control, is obtained from two revolving fans, five feet diameter, making above 1000 revolutions per minute.
Messrs. Acramans, of Bristol, have likewise enormous cupola power; they have a series of four cupolas, in which collectively from forty to forty-five tons of iron may be melted at one time.
In some cupolas the top is contracted by a cone made of iron plate; in Yates's patent, a brick trunk is built upon the cupola, with narrow arches crossing the trunk at right angles: this economises the heat by causing the flame and gaseous matters to be retarded, from pursuing a serpentine course in their escape.
If the cupola contained a little hard pig-iron, but were in great measure filled with old cast-iron, which had been repeatedly melted, and had become successively harder from the loss of carbon at every fusion; such castings would be brittle, and sometimes so hard as scarcely to admit of being cut, these would be equally unfit for the generality of machinery from the opposite causes.
But the same mixture of iron will be found to differ very much according to the size of the objects in which it is cast; iron, which in a plate one-fourth of an inch thick may be quite brittle and hard, will mostly be of good, soft, and useful quality, in a stout bar or plate of two or three inches thick. Thick castings are necessarily slow in cooling, and are seldom very hard, unless intentionally made so.
Between the extremes, (say three parts of pig-iron to one of old, or three parts of old iron to one of pig-iron,) various qualities may be selected; in castings for machinery the general aim is to obtain a strong, sound, and tough iron; mixtures of this nature which are used for iron ordnance, are called gun-metal amongst the gun-founders.
The fireman, or the individual having the management of the furnace, therefore always employs the scales in mingling the different kinds of iron, according to the magnitude and character of the works to be cast; and until the sorts in use are familiarly known, it is partly a matter of trial, and requires the same attention as the making of alloys properly so considered.*
* When the management of cast-iron was less efficiently understood, it was occasionally alloyed with five or six per cent, of shreds of copper, thrown into the ladle full of iron to produce a close, sound, strong metal, suitable to three-throw cranks for pumping machinery, and other purposes. It is said that ten per cent. of copper renders cast-iron malleable, and that alloyed with copper or tin it is less disposed to rust: all these alloys may be now viewed as matters of experiment alone.
It is much to be regretted that no protection has yet been found to prevent the conversion of cast-iron into plumbago, or the carburet of iron, from long immersion in sea-water, or the water of copper mines, sewers, and other places. This, which is a most serious inconvenience in dock works, sea walls and mines, arises, says Dr. Faraday, from the circumstance that the protoxide of iron, formed beneath salt waiter, is soluble, and becomes washed away, thus robbing the original mass of its iron; whereas the peroxide, or ordinary rust formed by exposure to large foundries, the plan of the building is divided into imaginary squares with a crane in the center of every square, so that the ladle is walked from one to the other, even to the far end of the shop, with great facility and expedition.
When enough iron is melted, (the common charge being two-and-a-half to four cwt., hut sometimes above twelve tons,) the cipola is tapped in front, at a hole close to the bottom which allows the whole contents to run out, either into Indies, or in very large works, into channels leading directly to the moulds.*
In pouring iron, the means of conveying the melted metal to the flasks, differ with the quantity. One man will carry from fifty to seventy pounds in a hand-ladle; three to five men will carry from two to four cwt. in a double hand-ladle, or a shank: larger quantities amounting sometimes from three to six tons are carried in the crane-ladle. These all possess one feature in common, namely their handles or pivots are placed but slightly above the center of gravity of the ladles; they may therefore be tilted very readily, as their fluid contents, in obeying the law of gravitation, are almost neutral in the operation of tilting, which they scarcely assist or retard, unless by mismanagement the ladle is over-filled, and thus rendered top-heavy.
All these ladles are coated with a thin layer of loam, and every time before use, they are brushed over with black wash, and carefully dried. The hand-ladle has a handle three or four feet long, with a crutch or cross piece at the end, which is mostly held in the left hand; frequently the contents of half a dozen or more hand-ladles are poured simultaneously into the same flask. The shank has a single handle on the one side, and one made in two branches at the other, and together they measure six to eight feet in length; the tilting is completely under the command of the one or two men at the double handle.