Founding of Iron.—Owing to the immense demand for cast iron in most of our great public works, such as bridges, rail-roads, columns, girders, fences, gas and water pipes, house-building, framing of machinery, and innumerable objects of less magnitude, iron founding has become one of the most interesting and important of our national manufactures. Wherever a foundry is to be formed, a dry situation should be selected for it, as dampness would totally prevent any thing being cast with tolerable accuracy, besides rendering the founding, in such places, dangerous to the workmen employed. The floor of a building for this business should be about ten feet deep, and composed of a kind of loamy sand; and if the place selected does not afford this convenience naturally, the ground must be made hollow, and such sand brought to fill up the excavation. This loamy sand is for the purpose of burying large moulds beneath its surface, so that the metal may be conveyed to them by channels or soughs hollowed out of the sand, and through which it runs from the furnace to the mould to be cast. A foundry, or casting-house, is provided with as many air or reverberating furnaces, in addition to the blast furnaces, as is required for the extent of the works to be founded at it ; an air or reverberating furnace is only used occa sionally, either when the metal contained in the blast furnace is not sufficient, or when the quality made in it is not proper for the work about to be cast. The difference in the qualities of the metals arises from their containing too much or too little carbon, and this is corrected by the founder, who mixes them with better or worse metal till they are rendered fit for the purpose required. Cupolas, as they are called, are also wanted in a foundry, and are similar to the blast furnace, except being of somewhat smaller capacity ; they are used to melt small quantities of metal when it is wanted in haste, as the reverberatory or blast furnaces will take more time in filling the charge of metal than the cupola does, by reason of their being of larger capacity ; but the founding by cupolas requires more machinery, from which circumstance it is not so well adapted to answer the purpose of the founder as the employment of a reverbe ratory or blast furnace. A much greater stock of flasks and other implements is wanted to make the moulds with, than is required by the caster who performs his work by means of either of the other furnaces ; these kinds of furnaces are always in use at large foundries, as at such places can be employed the whole charge of metal they are capable of containing. In a foundry worked by a blast furnace, a pit is sunk at a convenient distance from it, and the moulds for all large articles, such as pipes, &c. are placed vertically in it, within reach of the crane, that they may be raised or lowered in the pit. The metal is conveyed from the furnace by a gutter or trough made in the floor of the foundry, and a small iron trough, filled with sand, conducts the fluid metal into the moulds. This method of performing foundings at large works is an improvement on the old one, (which consisted in burying the pattern in sand,) and which has caused a great saving in labour and time. The flasks for this method of casting are founded of iron. It is now a practice at most of our large foundries to substitute sand for loam castings, in cases in which there are a great number of articles of the same kind to be cast, so that the expense of the flasks becomes an object of no great importance. When it happens that the articles are intricate, the sand is wetted so much as to render it sufficiently adhesive to make it mould, and receive the form of the pattern completely ; after this is done, it is necessary to dry the mould, to prevent accidents by the explosion of the hot metal when running the cast ; for this purpose stoves are used in which an equal and moderate degree of temperature is produced, and of a capacity adequate to contain a good number of the patterns. The moulds, when ready to be dried, are placed upon a carriage adapted to the purpose, and on which they are arranged and conveyed to the oven ; and when dry, which generally happens in about half an hour, they are withdrawn, and a new set placed upon the carriage. Every foundry should be provided with one or more cranes, so placed as to be easily got at when it is required to raise, lower, or remove, any large piece of casting. The moulding of large pieces of cast-iron, when they are required to be hollow, is made in loam, and consists in laying down an iron ring upon the ground, of the diameter of the proposed calibre of the work to be cast, and which has a rod of iron in its centre ; after this is done, bricks, clay, and wet loam are mixed together, and built up within the ring and round the iron rod, of somewhat less diameter than the cylinder about to be cast, and for which this is to form the core. The whole, when built, is bound round with iron hoops to protect it, and a fire is made inside to dry it; and when properly dried, a coating of loam IN spread all over and smoothed; this coat fills up and makes it the proper size for the inside of the cylinder, and is called the core of the mould. Another cylinder is built and plastered in the same manner, ground charcoal, which is mixed up with water-like paint, and laid on with a brush ; and a thin coating of loam, mixed up with hair, is then laid over the charcoal, previously spread upon the inner cylinder. When all these are quite
dry, a man gets into the cylinder, and with a picker pulls away from the core the bricks, and then with a trowel cuts away also the loam, leaving the inside of the external cylinder, which is called the mould, quite smooth ; this part of the work is effected by the coat of charcoal, which prevents the two coats of loam from adhering together. While this is doing, a deep pit is dug, and into this the core is let down by a crane ; when this is done, the mould is lowered over the core; as soon as the adjustment is perfected, sand is thrown in and rammed round about it, to about the half of its height; after which a flat cover of dried loam is put on the top of the mould and core, and pieces of rounded wood are put into the holes, which had been before made for pouring in the metal. The plugs which keep open these holes are carefully taken out, and small channels prepared for the metal to run through from the furnace. Before the metal is run into the mould, the latter should be carefully examined, to ascertain that it is quite dry, and in other respects in a perfect condition to receive the metal. Sand, or open casting, is used for such articles as will allow of cutting into two pieces, or even more, the models of which are indented in the sand, and the metal is run in between flasks.
A patent for an improved mode of casting metallic cylinders was taken out in 1826 by Mr. William Church, of Birmingham, the object of which was to produce perfectly sound castings of uniform solidity. The process consists in exhausting the air from the moulds by means of an air-pump, and afterwards in forcing the fluid metal, from an air-tight reservoir beneath, upwards into the mould by the aid of a condensing pump. As the apparatus for this purpose may be constructed in a variety of ways, and be adapted to the description of articles to be cast, the patentee has described only one arrangement, which he adopts and recommends for the purpose of casting large cylinders, rollers, cannon, &c. The mould prepared for casting is enclosed in a cast-iron air tight casing, and suspended in a vertical position, by means of chains, to the jib of an ordinary crane, over the vessel containing the fluid metal; to the lower end of the mould, an earthen tube (the materials similar to the crucible ware,) descends and forms the channel for conveying the metal upwards into the mould at the proper period of time ; this earthen pipe is covered with a cap at its lower extremity, which is luted to it so as to be air-tight, and the material and thickness of the cap is such that it will melt a short time after being im mersed in the fluid metal. As soon as the metal has arrived at the proper temperature the suspended mould, with its appendage, as before mentioned, is lowered by means of the crane, so that the earthen tube is immersed into the liquid metal in the chest beneath; this metal chest is then closed air-tight with a flange fixed on the upper part of the earthen tube, by proper contrivances for that purpose, such as a conical rim, an elastic metal hoop and luting; the perfect closing of which is effected by the pressure of the mould in its descent to its seat on the top of the metal chest. The apparatus so far prepared is next connected by short pipes with union joints, to pipes leading from an air pump of large dimensions, which both exhausts and condenses. First, the air is exhausted from the mould, and from above the surface of the melted metal in the chest; by this time the cap of metal at the lower end of the earthen tube becomes fused, the fluid metal ascends that tube, and is then forced by the condensing operation of the air-pump into the mould above, which, being pre viously exhausted, the metal is uniformly pressed into every cavity. As the vacuum in the mould is of course imperfect, from the previous exhausting ope ration, and the remaining portion of air becomes condensed by the rising of the metal, to prevent any ill effects from its pressure, a stop cock, communicating with the exhausting end of the air-pump, is opened, by which it is withdrawn. For the purpose of rapidly cooling the mould after being filled with the liquid metal, the cast-iron mould case is surrounded with an outer case or jacket, with a vacant space between, which is charged with cold water whenever desired, as in some castings this application must be of great utility in hardening their surfaces.
In 1825 a patent was taken out by Messrs. White and Sowerby for an air furnace for melting iron, for the use of founders, in which the requisite degree of heat is obtained without the necessity of resorting to mechanical aid, but simply by the production of natural currents of air through lateral openings or es, with the further advantage of causing a body of flame to be con passages, over that portion of the metal lying in a fluid state at the bottom of the furnace, by which it is kept from solidifying prior to the whole contents being discharged. On these principles the patentees state that the furnaces may be made square, round, oval, octagonal, or any other convenient form, with pas sages for the admission of atmospheric air in various parts and in different directions, according as they may be required to direct the heat. The annexed figures are intended to illustrate one convenient mode of carrying it into effect.