Founding, or METAL-CASTING, is the art of working metals by pouring them, while in a fluid condition, into moulds in which they solidify and harden into the form of the mould which they fill. In general the terms are restricted to metal-work; but glass, plaster, cement, wax, gutta-percha, &c. are cast in moulds precisely in the same manner as the metals.
Casting is the most important of all the operations by which metals are fashioned into useful or ornamental forms; but there are certain metals which cannot be in practice treated by this process. It is essential that the metal employed should be meltable at a temperature which can be reached in foundry practice, that it should flow freely, and should fill the mould well without forming air-bubbles in the mass. Malleable iron cannot be treated by casting, on account of the excessive temperature at which it becomes fluid; and neither copper nor aluminium become sufficiently limpid to flow and fill a mould properly. Iron-founding, in which the special form of pig-iron known as 'foundry pig' is used, is the greatest of metal- casting industries; after which come brass-founding, bronze-casting, zinc, German silver, gold and silver casting, and typefounding and stereotyping. The last two will be dealt with under their own proper heads, and the casting of the minor metals will receive such notice as they require in connection with the metals themselves.
The general process of founding or casting embraces numerous and complex operations connected with (1) the preparation of a mould or form into which the molten metal is run, (2) the melting of the metal or alloy, and (3) the pouring of the molten metal into the mould, in which it solidifies and hardens. According to the form, size, and intricacy of the object to be cast, and the nature and value of the metal used, the variations of working are very many, and it will not be possible here to describe more than two or three typical processes.
Bronze-casting is the most ancient of all metalurgical arts, and an outline of its development may serve to illustrate the processes employed in dealing with that alloy. The earlier forms of bronze axe used by prehistoric man were solid castings made in moulds, which consisted of two flat stones, having in the faces of each hollowed-out depressions which corresponded to two equal halves of the axe. The stones, when placed accurately face to face, had thus between them a hollow space which represented the form of a perfect axe, and into that hollow from the outer surface a channel or 'gate' was formed, by which the molten metal was poured in, till the hollow or mould was filled up. In later times, with the view of saving valuable metal, the practice was introduced of forming a core within moulds of large objects, around which the bronze was poured, which thus formed only an outer surface or skin. Iron appears to have been the material at first selected as suitable for such cores. But at a very early period the Greek sculptors discovered the art of casting their figures, &c. so that the core around which the metal flowed was removable, thus forming hollow castings. By whatever process the early artists worked they were able to produce castings which for beauty of surface, intricacy of pattern, size of casting, and economy of metal cannot be excelled with all the resources of modern ingenuity. Their process was probably that employed to the present day for the finest products of bronze-casting, known as cire perdue. In this process a rough model of the statue or other object to be cast is formed slightly smaller than the work to be cast. This model is coated over with beeswax; and to the wax surface the most finished form and touches of the artist are given. The whole surface then receives two or three coatings of a potter's slip or fine cream of ground brick, clay, and ashes, which forms a closely adherent skin around the wax. Then the mould is built up of clay, packed around all the parts to form a solid mass, which is clamped up within a strong iron framework. At certain intervals iron pins are stuck through the clay and wax into the central core, and suitable 'gates' are made for pouring in the bronze and allowing the wax to escape. The clamped-up mass is now placed in the furnace, and slowly heated up till all the wax runs out, and the clay of both mould and core are dried and sufficiently baked. The space at first occupied by the wax is now vacant, and it only remains to pour in molten bronze to occupy the space and assume the form and the thickness of the original wax. In modern bronze-casting for which the cire perdue process is not employed, objects of large size, such as heroic figures, are cast in sections, which have afterwards to be fused together at their edges. Models in plaster of the separate pieces are first made, from which moulds in sand and plaster are obtained. Within these moulds cores are cast, and these when baked and dry are uniformly scraped down all over according to the thickness desired in the finished object. The cores so reduced are again placed and supported within the moulds, and thus there is a space between mould and core into which the molten bronze is poured. The various pieces are subsequently brazed together and finished. Some of the recorded works of early Greek sculptors were cast of gigantic proportions. The Colossus of Rhodes is variously stated at from 90 to 120 feet in height. There yet remains in the Hippodrome at Constantinople a pillar of bronze in the form of three twisted serpents, cast 479 B.C., which in its perfect condition was 20 feet high. Schwanthaler's figure of 'Bavaria' in Munich, finished in 1850, stands 67 feet in height, and contains nearly 63 tons of bronze. 'La Vierge du Puy,' by Bonnasieux, is 51 feet high, and contains about 100 tons of bronze. For Bartholdi's 'Liberty' and other famous bronze figures, see COLOSSUS.
Iron-founding, from the nature of its operations and the gigantic scale on which it is carried on, may be regarded as a separate and distinct industry. The large-grained, gray pig-iron, on account of the highly fluid condition into which it can be brought, is the material principally employed; but according to the size of the castings required and the purposes for which they are to be used mixtures of finer grained pig are also melted up. The metal is melted with coke in a round firebrick furnace called a cupola, the heat being urged by means of a powerful blast created by fanners revolving at a high speed. The molten metal is run from a tap at the bottom of the furnace into a malleable iron ladle lined with clay, from which it is poured into the mould through holes called runners or gates. When the mould is newly filled numerous jets of blue flame issue from as many small holes pierced in the sand. These perforations are necessary for the escape of air and other gases produced by the action of the hot metal on the mould. Care must also be taken not to have the mould too damp, otherwise steam is generated, which may cause holes in the casting, or even force part of the metal out of the mould. The casting remains covered up for a time in order to cool slowly, and is then removed by breaking away the sand and drawing out the core.
In the case of a fluted or otherwise ornamented pillar the pattern would require to be in at least four pieces instead of two, because it is only a plain pattern that will come out of the mould in halves without tearing away the sand. When a pattern is necessarily made in several pieces it is drawn out of the mould bit by bit, to the right or left as the case may be, and so parts from the sand without breaking it.
Suppose that a small ornamental vase were to surmount the pillar, the founder would prepare the pattern of this in a more elaborate manner. He would first mould it in wax or clay, from which a cast in plaster of Paris is made; from that again a cast is taken in an alloy of tin and lead, which, after being sharply chased and divided into the required number of pieces, is used as a pattern to cast from. All ornamental patterns, such as figures, scrolls, leaves, enriched mouldings, and the like, are made in this way, whatever metal the ultimate casting is to be produced in.
Very large engine cylinders, pans, and such vessels are cast in loam-moulds, which are built of brick, plastered with loam, then coated with coal-dust, and finally dried by means of a fire. This method is adopted with large plain objects where a pattern would be expensive, and when few castings of one kind are required. Iron moulds, coated with blacklead or plumago, are now used for water and gas pipe founding, which forms an extensive special industry. They are greatly more expensive than any other kind, but they enable the founder to dispense with a pattern, as, when once made into the required form, they are not destroyed like moulds of sand or loam at each casting.
For chilled castings, also, metal moulds or moulds partly metal and sand are employed. In these the molten iron solidifies and cools with great rapidity, owing to which it acquires extreme hardness, a quality of peculiar value for shot and for many purposes where wear-resisting properties are of more importance than toughness. In malleable castings the opposite effect of rendering the metal soft, tenacious, and partially malleable is obtained. For this end the finished casting is embedded in powdered hematite ore or other iron oxide, packed in an iron chest, and exposed for two or three days to a red heat in a furnace. Thereby the cast-iron parts with its carbon, and assumes the softness, tenacity, and infusibility of malleable iron. The process may be continued till the effect is produced throughout the mass acted on, or it may be stopped when merely a skin is decarbonised, leaving the core of unaltered cast metal.
In iron-casting, and especially in steel-casting, bubbles of carbonic oxide and other gases are formed, which partly escape and partly remain imprisoned in the mass, tending to render it vesicular and less sound than it might otherwise be. Various means have been adopted to promote the escape of imprisoned gases, and thereby to render the casting more homogeneous and strong. In Krupp steel the addition of silico-spiegeleisen to the cast metal acts favourably by stopping the evolution of the gases. To force out the gases the late Sir Joseph Whitworth in casting steel adopted the expedient of casting under pressure. While the metal was in a fluid condition he applied pressure of as much as from 6 to 9 tons per square inch over its surface, and by preventing the formation of air-bubbles he thereby obtained a remarkable increase in the strength of the castings so operated on.
The variety of articles produced by founding or casting are very numerous. Besides others incidentally alluded to, we may mention cylinders, cisterns, paper-engines, beams, boilers, pumps, and the heavy parts of machinery generally, gates, railings, lamps, grates, fenders, cooking-vessels, and the like in iron; cannon, many portions of machinery, and numerous ornamental objects in brass, bronze, and the more costly metals. See Mullin, Modern Moulding and Pattern Making (1886); Overman, The Moulder's and Founder's Guide (1880); Wylie, Treatise on Iron Founding (1884), &c.