Other examples of metallic moulds might be given, but there are far more frequent cases in which one single casting is alone required; or else the number is so small, or the pieces themselves are so large or peculiar, that the construction of metal moulds would be found almost or quite impracticable, even without reference to an equally fatal barrier, the expense.

In making these single copies in the metals of considerable fusibility, plaster of Paris is sometimes employed; thus, after the printer has arranged the loose types into a page, and the requisite corrections have been made, a stereotype, or solid type, is taken of the whole as a thin sheet of metal, which serves to be printed from almost as well as the original letters; and its small cost enables the printer to retain it for future use, after the types themselves have served perhaps for a hundred similar regenerations, and are ultimately worn out.

The stereotype-founder takes a copy of the entire mass of type in plaster of Paris; this is dried in an oven, and placed face downwards within a cast iron mould, like a covered box, open at the four top corners. The mould and plaster-cast are heated to the fusing temperature of the type-metal, and gradually low-ered into a pan or bath of the same by means of a crane; the hot fluid metal runs in at the corners of the mould, and raises the inverted plaster, which latter would rise entirely to the surface but for the restraint of the cover of the mould.

Type-metal is about eleven times as heavy as water; and if the mould be immersed four inches below the surface, it is subjected to a pressure equal to that of a column of water forty-four inches high, or of above two pounds upon every square inch. The necessity of this arrangement is shown when a few ounces of type-metal are poured from a ladle on the face of the plaster; the metal looks like a dump, almost without any mark of the letters, whereas the stereotype-cast is nearly as sharp as the original type. The immersion fulfils the same end as the jerk of the hand-caster, or of the pump occasionally employed; and the long continuance of the mould in the fluid metal allows ample time for the air to escape hubbies to the surface; after which the mould is raised and cooled in a vessel of water, and the plaster is mostly destroyed in its removal.

Plaster of Paris, although it may be, and frequently is used for the fusible metals, such as lead, tin, and pewter, cannot be employed alone for iron, copper, brass, and many other metals, the intense melting heats of which would calcine the material, and cause it to crumble; even the soft metals should not be very hot, or they will make the plaster of Paris blister off in flakes or dust. We must therefore seek a substitute better capable of enduring the heat, and likewise susceptible of receiving definite forms; for which purpose damp sand, with a small natural or subsequent admixture of clay or loam, is found to be perfectly adapted.

The moulding-sand cannot however be used without external support, and which is given by shallow iron frames without tops or bottoms, called flasks, represented in figs. 160 and 161. The bottom part, 4, 5, is supposed to have been rammed full of sand, and to stand upon a flat board, 6. The model of the plain flat bar which is to be cast, is now laid on the surface of the sand, that of the round bar is embedded half way in the same, and the mould is dusted with dry parting sand.

Fig. 160.

161.

The top part of the flask 2, 3, is shown still empty, and in the act of being attached to 4, 5, by its pins, which enter corresponding holes in the latter, easily, but without shake: 2, 3, is also rammed full of sand, and covered with a top board, 1, not represented, to avoid confusion. The mould is now opened, the models are removed, and channels are scooped out from the ends of the cavities left by the models, to the hollows or pouring holes at the end of the flask; the parts are all replaced in the order 1 to 6, representad in fig. 160, and the whole are fixed together by screw clamps, so as to assume the condition of fig. 161.

The flask is now placed almost pi perpndicularly beside the pouring-trough, and the metal is poured into it from the crucible as shown in tig. 138, p. 306; hut the flask if small is put on the surface of the pouring or spill-trough, and propped up with a short bar.

This brief sketch of the entire process of moulding and casting in sand moulds, will be now followed by some remarks in greater detail: first, on the patterns of the objects to be cast; secondly, on the conditions required in the sand; and thirdly, the process of moulding simple and solid bodies. The section then following will be devoted to moulding cored works, and figures, after which a few lines will be given upon the subject of filling the moulds.