Page:The American Cyclopædia (1879) Volume IV.djvu/89

 CASTING 81 when compared to that required for pieces of machinery. It may be made of moulding sand or of loam. If of the former, the pattern, which of the exact size and form of the future bell, laid, mouth downward, upon a turn-over sard, and dusted with coke dust or charcoal. . drag of sufficient depth is then placed upon
 * ie board and rammed full of sand. The bot-

board is then clamped on, and the flask is irned over and a cope adjusted and rammed " of sand, the necessary holes being provided, The cope is then taken off, the pattern amoved, the mould properly dusted with part- ig sand, and the parts replaced and clamped together, when the piece is ready for casting. For large bells the method pursued is similar that for large cast-iron cylinders. A sweep constructed having a templet which may be to sweep the interior of the cope or the irface of the core. The core is built of brick and loam upon an iron plate, and swept in the same manner as the core of a cylinder, and the cope, made of brick and loam also, and bolted, which is to form the exterior of the bell, is swept like the cope of a cylinder. The surfaces are then washed with charcoal or plumbago mixture, and dried in the oven in the usual way. The parts are then sunk in a pit upon a bed plate, secured, and the pit well rammed with sand, pouring and vent holes having been provided. The top of the cope is covered with loam through which holes are made connect- ing with those in the cope, and channels are formed in it to receive the metal. The core must be so constructed that it will yield when the metal shrinks on cooling, which is the case with bronze as with cast iron. If rupture is not produced by rigidity of the core, the metal will have a strain of tension which will be likely to cause fracture when the bell is struck. It may be constructed with bands of straw wound around it, over which loam is spread before finishing with the templet. The Messrs. Meneely of West Troy, N. Y., use perforated cases (fig. 19), upon which the core as well as the cope is formed. The cone upon which the core is made be- fore being spread with loam is wound with ropes of straw, which yield sufficiently for the contraction of the bell. The outer case is spread with loam on the interior, the holes with which it is pierced allowing of the expulsion of ex- panding air and gases. For casting of heavy guns, see CANNON. Casting in the Fine Arts. Before treat- ing of the casting of statuary and other FIG. 19. Meneely's Cases. articles of sculpture in metals, it will be proper to describe the more simple methods of casting them in plaster of Paris. This substance possesses peculiar properties which give it a wide application in nearly all the arts. Gypsum, from which plaster of Paris is made by calcination, is a hydrated sul- phate of lime containing about 20 per cent, of water by weight. Where a large, but not necessarily definite quantity of this water is driven off by a gentle heat, the gypsum, after being ground and sifted, becomes the beau- tiful white article known as plaster of Paris. This substance possesses the property of re- combining with the same amount of water which the heat had driven off. When mixing it for use, however, a much larger quantity of water is used, sometimes twice or three times as much ; but for making strong moulds, as little is used as will answer to render the mix- ture fluid. A definite quantity of water enters into chemical combination, and the rest is held in the pores of the plaster when it sets, most of it afterward passing away by evaporation. A proper mixture of plaster and water pos- sesses the property of running into the minu- test parts of a mould, so that the finest lines may be copied with considerable approach to perfection. For this reason it is often used in electrotyping, for taking casts, upon which the metal is subsequently deposited by galvanic action ; and also for taking casts of leaves of plants, and other articles of a similar character. The mixture sets in 20 or 30 minutes after being stirred, depending upon the quality and quantity of the plaster used, and upon the pres- ence of other substances, such as lime, alum, or cream of tartar, which may have been added to it. The casting of models in bass relief is quite a simple process, and is performed in the following manner. The mould is made by simply laying the model, which is usually made upon a plate of glass or a slate, upon a table and pouring over it the mixture of plaster and water. If the model is in clay, it should be wet with water, and the slate slightly greased with lard oil. It is not necessary to build a bar- rier about the edge of the slate, for the mixture poured on with a cup or a large spoon, first over the object, soon becomes thick enough to allow of its being spread over the slate without running off. The mould may be made from one half to one and a half inch in thickness. In from 50 to 70 minutes it will set firmly enough for removal, which may be done by carefully introducing the blade of a stout knife under its edge, and gently raising it with the assistance of the hand. If the model is not undercut, the mould may be used in one piece for casting several copies. If, however, projections are left, the mould must be broken away from the cast, or else it must be composed of more than one piece so that it may be drawn. When a mould is broken off it is called a waste mould, and when in parts it is called a piece mould. When a model is undercut it may be made flush by