Founding

Founding is the art of casting molten metal in moulds. It is not applicable to all metals, requiring that the temperature of melting shall not be inconveniently high, that the molten liquid shall flow readily into the mould and fill up all the corners, and that it shall disengage air-bubbles and solidify into a uniform homogeneous mass. Such properties distinguish the different compounds of iron with carbon, and, while giving the name to cast-iron, they render wrought-iron incapable of such treatment and steel either the one or the other, according as the proportion of carbon it contains renders it similar to cast-iron or to wrought-iron. Copper and aluminium cannot he cast, but bronze or gun-metal, which are both alloys of copper and tin, make excellent castings. So also will the bronzes containing phosphorus, aluminium, or ferro-raanganese, which are respectively named phosphor-bronze, aluminium-bronze, and manganese-bronze. The other characteristic properties of these alloys render their capability of being cast highly useful in engineering. Cast-iron parts are formed by melting the pig-iron (q.v.) in a cupola, heated strongly by means of a forced draught, running the metal out of the bottom of the furnace through small gates into a clay-lined wrought-iron ladle, and pouring it into moulds of the requisite shape and size. Patterns are made of yellow pine, or in special cases of mahogany or metal; they are made about one-eighth of an inch to the foot larger than the required casting to allow for contraction of the metal on cooling. Moulds are then prepared from the patterns in loam or foundry sand; hollows in the castings require special cores of loam in the moulds. The molten iron is then poured in, and, after it has solidified, the sand is cleared away and the casting removed. Although castings may be much more complex in form than wrought work, yet if they are badly designed stresses may be produced in them when they cool down from the liquid condition, and they may be so weakened as to fracture with much readiness. On this account sharp angles are to be avoided, all corners being much stronger when well rounded. Thin parts in continuation of thicker parts are liable to solidify first, and may be fractured later on when the thicker material contracts and solidifies. Much depends then on good design, though many difficulties are overcome by skilful founding. Very rapid cooling of the molten metal is effected by the use of moulds of cast-iron lined with loam. The result of this is that separation of graphite from the iron is prevented; it becomes extremely hard and much more brittle. Such is the nature of chilled castings. A reverse effect is produced by keeping the casting for a day or two at high temperature in contact with iron-oxide; the surface then partakes of the nature of wrought-iron, and the casting will stand blows much better than before. Bubbles of gas in the molten liquid must be eliminated before the metal cools. Small holes are made in the mould to allow their exit. Whitworth introduced casting of steel under pressure to prevent the formation of bubbles, and to render the metal denser and stronger. Bronze-founding has been practised for ages. The Colossus of Rhodes was a brass casting, about 100 feet high. A specimen of bronze-work still exists at Constantinople, that was cast some 500 years B.C. It is a pillar formed of three twisted serpents, originally 20 feet high. Modern specimens of bronze-founding show no great advance in excellence of workmanship. The most famous are the "Bavaria" national statue at Munich, 67 feet high; the "Arminius," 90 feet; the "Vierge du Puy," 51 feet; the "Germania," 112 feet; and Bartholdi's "Liberty," 156 feet. The general method of casting is closely similar to that employed in iron-founding. Small bronzes are most beautifully produced by the cire-perdue process, known to the ancients. The essential principle is that of using a wax-covered pattern, which can receive all the refinements from the artists' hands. It is embedded in a clay mould; the wax is removed by heating and allowing it to escape, and the molten bronze is then poured into the hollow thus left.