After being baked, the mould is placed, with its face downwards, upon a smooth plate of iron, called a itoating-plate, which lies at the bottom of a cast-iron box rather larger than the mould. The box is then covered in by a lid, the under surface of which is made perfectly flatand which has the corners cut off, to allow the melted metal to ' I enter the box. The cover is firmly held down by a scree, which is attached to an apparatus by which the box is suspended from a crane. It should be observed that the casting-box and plate are heated to the same temperature as the mould, before it is inserted. The box is then swung by the crane over the metal-pit, which hi an open iron vessel containing a largo quantity of melted metal, resembling in its compositiou that used for casting types; and it is lowered into the metal in a nearly horizontal position, being a very little inclined, to facilitate the escape of air from the mould and box. The melted metal runs in at the corners of the box ; and, by its greeter specific gravity, floats up the plate with the mould, forcing the htter tightly against the lid of the box. By this contrivance the metal is forced, by hydrostatic pressure, into every part of the mould, iu the margin of which notches are cut to allow free passage for the metal between it and the floating-plate. After remaining immersed in the metal for about ten minutes, the box is gently raised, and removed by the crane to a trough in which its lower part is rapidly cooled by contact with cold water. While the box is cooling, the caster pours in a little metal at the corners, to fill the space left by the contraction of the metal, and so to keep up the necessary pressure upon the cast. When cold, the contents of the box are removed in a mass, from which the superfluous metal is broken off by blows from a mallet. The plaster mould is then broken away from the cast, the face of which is a fee ' simile of the types and engravings from which the mould was taken. I As the mould is destroyed by this process, it is necessary, when several stereotype plates of the same page are required, to take a distinct plaster mould for each.
Another plan, which has been tried for the production of stereotype plates too large for casting in the ordinary way, but which has not proved very successful, is to place the mould in a flat iron box, having a trough-shaped mouth nt one end, and to pour in the metal with the mould-box, which must be previously heated, in an inclined position. The method of striking the moulds, and from them the plates them selves, by letting the original or pattern types or blocks fall upon a macs of soft metal in a half-melted state, is practised in Fretless and has been repeatedly tried in this country: it is especially adapted for the production of copies of wood engravings, but it is very liable to injure a delicately engraved block. In this, as in the ordinary casting process, the original engraved blocks should be smeared with some substance that may prevent the cast from adhering to them.
A process of stereotyping, which was several years in attaining its present practical efficiency, has been introduced from Paris. On receiving the type, moistened eboets of unsized tissue paper are laid over it, ono above the other, to the number of six or seven, slightly pasted together, and upon these is laid a stouter paper; the whole is then struck with a large flat brush till the softened paper has received a perfect impression of the type. This being done, the paper
is removed, placed upon a hot iron table, a blanket laid over it, and screwed down moderately tight, in order to harden it and extract the moisture. The whole operation (paper having been prepared by wetting n few hours in advance) occupies less than an hour. From this mould a stereotype cast can he taken by merely pouring in the molten metal on It when placed in a receptacle, in which a simple contrivance at once regulates the thickness and the size of the plate, that is, how much or how little margin is left outside the type for the purpose of fastening it to the blocks. A couple of hours suffices for the whole process ; and it is now adopted upon several newspapers and other publications of largo circulation, as many casts an ho taken from one mould. A plurality of machines or presses are thus enabled to be set to work for the production of the requisite supply.
Another considerable advantage is, that the moulds may be pre served, so as to be used whenever wanted. It is only necessary that they should be stored so as to be neither too warm, nor too damp, nor too dry, as the paste will ferment, or the sheets crack and separate. With care however they may be kept safe for a considerable time, and thus the locking up of a large amount of metal may be avoided.
The alloys used for various kinds of stereotyping differ slightly in their composition, but generally consist of type metal.
Stereotype-plates need careful examination and picking, to remove the imperfections in the casting. Small hollows, such as the loops of an a, an e, or an o, are liable to be filled up with metal, owing to blebs of air in the mould, and the fine white lines in wood-engravings are sometimes filled up. Such matters should be corrected by the picker, who should also cut down, with suitable tools, such blank spaces as might be liable to soil in printing. Before printing, also, defective letters or words which cannot be corrected by the picker should be cut away, and types inserted in their place. These types are soldered into holes drilled through the plate ; their stems being sawn off flush with the back.
Although the plates are cast of as equal a thickness as possible, they require, before printing from, to be accurately flattened at the back by means of a peculiar kind of lathe, in which a steel cutter, or knife, mounted in a slide-rest, shaves off the metal from the back of the plate in concentric circles, until it is made perfectly even. They are then mounted upon blocks of wood or metal, to raise them to the same height as common types. The tendency of wood to warp when exposed to changes of temperature, or to occasional wetting, has led to many projects for mounting stereotype-plates upon blocks of cement, or upon metallic mounts which might be applicable to plates of various sizes. When wooden blocks are used, the plates are usually secured to them by clips at the edges, and sometimes by screws.