The mould may now be laid on a hot surface to further dry, or, better still, it may be baked in a steam or gas-oven, heated to about the same temperature as the moulding press; but in any case it should be kept flat by placing over it a piece of heavy but small-meshed wire net, and if necessary a weight is put upon this. A suitable wire net is made with iron wire of No. 16 I.W.G. ('064 in. diameter), and 6 meshes to the linear inch, and can be had from firms that furnish millers' plant. The ordinary wire gauze or net sold at the hardware 'shops,- having 6 meshes to the inch, is made of much thinner wire, and is not much used for the present purpose, as it has not sufficient rigidity to keep its shape as a slab or plate. The wire net should be in contact with the tissue paper side or face of the mould, as slight indentation on this side will do no harm, whereas any indentation on the back of the mould will show on the face of the cast; and when several moulds are piled in the oven for baking, they should be laid back to back and face to face, with a piece of sheet metal (say stout tinplate) between the backs, and one of the wire-net sheets between the faces.
The baking, or second drying, being at an end, we come to the casting, and before this is done it is a very common practice to brush some finely powdered French chalk into the mould, and then to dust out the excess by turning the mould face downwards, and gently beating the back with a fiat slice of cane. This is quite unnecessary if the mould is very dry; but by the use of French chalk the effect of any trace of moisture remaining in the mould is minimised, and, moreover, the cast separates from the mould more easily - a matter of some importance when it is wished to make several casts in the same mould.
All is now ready for laying the mould in the casting-box, the casting-box having been warmed by a gas jet underneath, or by casting a few blanks in it. The mould is laid face upwards on the horizontal slab of the casting-box (Fig. 299), the brown paper flap hanging a little over the lip of the box. The pica-high gauges are laid along the gutters formed by the clumps, the top leaf of the box is closed down and clamped by the screw, and the casting-box is swung on its axis, so as to bring the lips to the top.
When stock sizes have to be stereotyped, it is convenient to use set gauges, like Figs. 300 or 301, but in other coses it is usual to employ adjustable gauges, such as Fig. 302.
When the mould is charged with type-metal, it is necessary, in order to obtain a good cast, that the whole of the metal inside should remain fluid until the mould is completely filled with metal, as if any part solidifies before the mould is full, the cast is sure to show curved streaks where the cast has solidified, and the fresh metal has not run up so closely as to make a sound cast. This is most noticeable at the back of the cast, where the casting-box exercises the most sudden cooling action on the metal, and the object of heating the casting-box is to diminish the tendency to this sort of thing. Heating the casting-box is generally insufficient in itself, when the cast is large, unless the heat is raised to nearly the melting point of the metal - an obviously inconvenient course. It is very much more convenient and satisfactory to warm the box only slightly (say to about 100° C), and to cover the face with a non-conducting coating, which may be extremely thin; in fact it is sufficient to sponge the iron plate over with a very thin wash of jewellers' rouge (finely divided ferric oxide, or practically, much the same thing as finely divided iron rust) and water, a film of the oxide so thin as to be scarcely noticeable serving to retard the solidification of the metal during the short time required to fill the mould.
Although a thin wash of jewellers' rouge is the best coating material to employ when very delicate castings of type metal are to be made in metal moulds (as, for example, in casting the thinnest " leads "), a thicker and coarser mixture, made by stirring £ lb. red ochre into £ pint water, is often used, this being applied with a brush. London stereotypers, however, more usually lay a sheet of thin cardboard over the back plate, or a sheet of thin paper will be quite as effectual in preventing the chilling of the metal; but stereotypers generally prefer the card, as lasting longer and being easier to handle. The card, however, is liable to blister, and so cause inequalities in the thickness of the plates. In the absence of a metal casting-box, excellent work may be done by using two slabs of dry wood, held together by screw clamps.
All is now ready for the casting of the stereotype. To ascertain whether the temperature of the metal is about right, a strip of card or of old mould is immersed in it for a few seconds. If the card becomes of a medium brown, the heat is right (about 320° - 330° C), if it chars and blackens, the temperature is too high; should it merely become yellowish or light brown, more heat must be applied. When the metal is too hot, it can be rapidly brought down by stirring in some cold metal. It is important that, when poured, the surface of the metal should be clean and free from scum or oxide, as this might lodge in the cavities of the mould and render the cast unsound; and the most convenient way of cleaning the surface is to throw into the pot some powdered rosin, which melts and so far agglomerates the oxide that it can readily be removed by skimming with a perforated iron spoon. Sufficient metal is now taken out of the pot by an iron ladle - one with a flat pouring-side (Fig. 303) is often used - and the metal is poured steadily, but not so quickly as to cause splashing, into the mould.
Under ordinary circumstances, it makes but little difference whether the stream is poured against the back plate of the casting-box or against the face of the mould, although the former is the most usual course, and some persons make a point of drawing the ladle along the lips of the mould during the operation of casting.