FOUNDING. This important mechanical art embraces all the operations of reducing ores, and of smelting and casting metals. There are various branches of the art, and some difference prevails in the minor details of the processes, as in iron, brass, and bronze founding, in casting guns and cannon, types for printing, and bell founding. The art has been known and practised from the earliest ages.
The preparation of the principal ores is described under the names of their metals, such as COPPER, IRON, lcc. The furnaces are described under FURNACE.
Founding is practised either in casting any miantity of metal in the solid, or with a core (by means of which the metal is preserved of a determined thickness or substance), or in plain casting. Before any article can be cast in metal it is necessary that a model of it be prepared. The models must be made of various substances; clay or wax, or sand with clay, are those usually employed, but they may also be made of wood, stone, or any other material. Upon those models moulds must be made. These are commonly com posed of plaster of Paris, mixed with brick dust, sometimes sand, or sand with a mixture of cow-hair. For moulds for iron and brass work a yellowish sharp sand is preferred, which is prepared by mixing it with water and then rolling it on a flat board till it is well kneaded and fit for use. If the article is cylindrical, or of a form that admits of it, it is moulded and cast in two pieces ; these two parts are then carefully joined together, and the edges or seams carefully cleaned. For the smaller class of works, instead of running the metal at once from a large furnace, earthen crucibles are used, into which the metal is thrown in small pieces : the crucible is placed in a strong heat in a close stove, and as the metal is melted and sinks more is added till the vessel is full. It is then lifted out by means of iron instruments adapted to the purpose, and the metal is poured from it into the moulds, m which channels or ducts for receiving it have been previously made.
In noticing the different ways of casting, mention has been made of one in which a core is used. The core, as its name denotes, is a part or portion situated within the body of the cast; and its purpose is to form a centre to the work by which the thickness or substance of the metal may be regulated. In
coring, the mould is first made complete; into this, clay or wax, or any other fit substance or material, is then squeezed or pressed in a layer of uniform thickness ; in large works it is usually from half an inch to an inch thick. This layer represents the metal. The mould, if in parts, is then put together, the above mentioned layer being left within it, and into the open space in the centre a composition (usually of plaster of Paris with other sub stances mixed with it) is introduced, and made to adhere to the clay or wax, or rather is filled up to it. This is the core, and it is often made to occupy the whole interior of the mould. When this is set, or dry, the mould is taken to pieces, and the material which lies been made to represent the metal removed. The mould is then again put carefully toge• ther round its core or nucleus, the two por • tions being secured from contact by stops and keys properly arranged for that purpose. The mould and core are dried to dissipate mois ture ; and large moulds are strengthened with iron hoops. Channels or ducts are made for the entrance of the melted metal ; and others are also made for allowing the air to escape as the melted metal enters the mould ; these are called vents. With respect to placing the mould, it is only important to secure a suffi cient inclination of plane from the mouth of the furnace to the mould that the metal may run easily and uninterruptedly, and not have time to grow cool and therefore sluggish. The usual method in bronze works of large size is to bury the mould in a pit a little' be low the level of the furnace, and by ramming sand firmly round it to ensure its not being affected by any sudden or violent shock, or by the weight of the metal running into it. When everything is ready and the metal found to be in a state fit for running, the orifice or mouth of the furnace (which is usually plugged with clay and sand) is opened, when the metal descends, and in a few minutes the mould is filled. The metal is allowed to run till it overflows the mouths of the channels into the mould. The work is then left to cool, after which the mould is scraped or knocked off and the cast undergoes the neces sary processes (such as cleaning, chasing, etc.) to render it fit for the purpose designed.