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 occasionally, 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 reverberatory 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, etc. 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 is 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 but without hoops, whose diameter is the same as the outside of the cylinder to be founded. When this is finished, it is covered over with charcoal dust, or 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.