FOUNDING, or METAL-CASTING, is the art of obtaining casts of any desired object by means of pouring melted metal into molds prepared for the purpose. It has risen to great importance in recent times, on account of the many new applications of thin. Iron-founding. brass-founding, type-founding, as well as casting in bronze and zinc, are the principal divisions of the art. The casting of the finer metals and alloys, as gold, silver, and German silver, is necessarily conducted on a smaller scale.
When the casting of an object is required, it is necessary, in the first place, to make a pattern. Suppose it to be a plain round iron pillar, such as is used for hanging a gate upon. A pattern of this is turned in some wood which can be readily made smooth on the surface, such as pine, and then varnished or painted so as to come freely out of the mold. This wooden pillar, or any similar pattern, is always made in at least two pieces, the division being lengthwise, for a reason which we shall presently see. The next step is to prepare the mold. The molds used by the iron-founder are either of sand or loam, but more generally of fine sand. Proceeding with the preparation of the mold, the founder takes a molding-box, which is composed of two open iron frames with cross-bars, the one fitting exactly on the other,. by means of pins in the upper, dropping into holes in the lower frame. One half of the box is first filled with damp sand, and the pattern laid upon it, a little dry parting sand being sprinkled on the surface. The upper half of the box is then put on, and sand firmly rammed all round the pattern. The box is then carefully opened, and, when the pattern is removed, its impression is left in the sand. The mold at this stage, however, is generally rough and broken. It is necessary, therefore, to give it a better finish, which is done by taking each half of the mold separately, repairing it with a small trowel, and reintroducing the corresponding half of the pattern till the impression is firm and perfect. Finally, the surface of the mold is coated with charcoal-dust, which gives a smooth surface to the future casting. These columns being made hollow, there is yet another matter to arrange before the casting can be made—namely, the core. In
the instance before us, it would simply lie a rod of iron, covered with straw and loam to whatever thickness the internal diameter of the column happened to require. The core of course occupies the center of the mold.
The cast iron is melted with coke in a round fire-brick 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 mal leable iron ladle, lined with clay, from which it is poured into the mold through holes called runners or .gates, When the mold 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 mold. Care must also be taken not to have the mold too damp, otherwise steam is generated, which may cause holes in the casting, and even force part of the metal out of the mold. 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 conic out of the mold in halves without tearing away the sand. When a pattern is necessa rily made in several pieces, it is drawn out of the mold 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 was to surmount the pillar, the founder would prepare the pattern of this in a more elaborate manner. He would first mold it in wax or clay, from which a cast in plaster of Paris is made; from that, again, a east 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 moldings, and the like, are made in this way, whatever metal the ultimate casting is to be produced in.