When matter has been set up in type, it is often desirable to reproduce it in a form more convenient for handling, while at the same time liberating the type for further use. Hence has arisen the art of stereotyping, or reproducing the type surface in solid plates of metal. There are two methods in which this is performed, differing essentially in the material forming the moulds. They are known respectively as the Plaster and the Paper processes.
In this, the older but less adopted process, the moulds are formed of plaster of Paris. Among the advantages of this method are that the castings produced have much sharper and deeper outlines; on the other hand, the mould is destroyed in releasing the casting, and the operation occupies a longer time and is more expensive.
The metal-pot, of a convenient size for immersing the dipping pans, is best fixed against a wall, to facilitate handling the pans by means of a crane. The oven for baking the mould may adjoin the melting pot, and be fitted with several shelves. A good arrangement is an ordinary low brick furnace surmounted by a square oven about 3 ft. wide and 4-5 ft. high, bricked in, and having the furnace flue carried round the back and sides. The door covers the whole front of the oven, and an iron shelf 8-10 in. wide, is fixed beneath it on a level with the bottom shelf, for convenience in sliding the articles in and out. The floor of the oven should be reserved for heating pans and plates before casting, and never for baking the plaster moulds, as its temperature is unequal, and would cause uneven shrinkage and consequent destruction of the mould.
The plates are cast in dipping pans (Fig. 175), 3-4 in. deep, oblong, and with sloping sides, on which are sockets a to admit the clamps by which the pans are swung from the crane. The cover b may be flat or slightly domed, the corners c being cut off to admit the metal. The lid is held in place by the screw d and the hinged clamp e. The floating plate, of 1/2-in. iron, fits loosely into the dipping pan.
The trough for cooling the dipping pan and its contents is placed beside the metal-pot, in a position to admit of the crane easily depositing its charge. It should be about 4 ft. long and 2 ft. wide, and stand slightly below the top of the metal-pot; 4 iron bars, 3/4 in. thick and 2 in. wide, are fixed across, sufficiently near each other to allow of 2 dipping pans being placed on them at one time. A few pieces of thick flannel, or similar substance, secured round the bars will admit of the moisture being communicated gradually to the hot pan. Some molten metal frequently falls into the trough from the corners of the dipping pan during suspension, and from the ladle while filling; the trough should be cleaned up at intervals, and the cleanings thrown into the metal-pot, but not while the metal contained in it is heated. A good plan is to sweep the foundry and clean out the cooling trough every night, and to transfer the sweepings to the metal-pot, ready for melting on the following morning.
The plaster matrix is taken from the forme in a moulding frame, which in appearance bears a close resemblance to an ordinary chase; but the 4 sides facing the type are bevelled inwards, so that when the plaster hardens it is equally supported on all sides.
The formes are placed for moulding on an iron surface fixed against a wall. It should be long enough for several moulds to be prepared in immediate succession. The iron surface may be replaced by a slab of stereo metal, 1 in. thick, well planed on the surface, and arranged along an ordinary wooden balk; in this latter case, the surface of the slab should be frequently examined, to ascertain whether there are any indentations, which would necessitate their being either replaned or discarded, as the type might sink into the cavities when planed down, and render the mould imperfect.
The dipping pan, after cooling, is placed on a block 4 ft. high and 3 ft. wide, where the mould is knocked out of the pan, the corners of the cast are detached by the mallet, and the plate is thus set free.
Brushes are needed for cleaning the type, removing the plaster from the surface, and oiling; small steel straightedges, for taking off the superfluous plaster from the back of the newly-made mould; chisels, for raising the moulding frames from the forme, and releasing the plate from the metal; a strong barrel, with lid, for the storage of the plaster in a dry place; and several tin cans for mixing the composition.
Several recipes for the composition of alloys for casting stereotype plates will be found in 'Workshop Receipts,' Third Series, p. 33. In general terms it consists of lead with about 12-18 per cent. of antimony added to produce the necessary degree of hardness. It may be bought in blocks ready for use, which is the better plan in all but large establishments, as some skills required to ensure making a good quality of metal, and without that quality satisfactory work is impossible.
The furnace and pot for melting the metals and making the alloy, as well as for heating the metal ready for casting, should be completely encased in a hood of sheet iron, with a flue leading from the top, which flue may be utilised for conveying a certain amount of heat to the drying oven. The hood must have a door in the front to permit the metal to be stirred, skimmed, and ladled out.
Before making the alloy, the lead is melted alone first, and thoroughly freed from the dirt and dross which collect on the surface, by means of a skimmer, consisting of a disc of perforated sheet iron with a rim and short handle. The addition of a small quantity of oil or grease to the molten metal will much facilitate the liberation and removal of the dross. When the lead is perfectly clean, it is cast into blocks ready for remelting to make the alloy.