In conducting this latter operation, the lead is again placed first in the pot, melted, and well skimmed, taking especial care that no zinc is allowed to contaminate it. When quite clean, the proper proportion of antimony is added, that being, for every 100 lb. of lead, 18 lb. for the plaster process, but only 12 lb. for the paper process. If the molten alloy, after being well stirred to mingle the two metals, be found to exhibit a tendency to adhere to the sides of the pot and to the tools plunged into it, this may be taken as a sign of poor quality and the necessity for adding more antimony. Having secured a good quality of metal, it will be fit for casting at about 600° F. (316° C). Experienced workmen estimate the temperature by holding the hand at a distance above the pot, instead of having recourse to a pyrometer, but a novice would need to resort to the more scientific method until accustomed to judge of the heat.
In making stereotyping metal in the foundry, a quantity should be mixed at one time, as the plaster process demands much material always in use. The manufacture of the alloy considerably interferes with the casting, and the dipping should never be commenced without a sufficient supply being available. In most foundries where mixing is carried on to any extent, a separate pot is provided for the purpose, taking care that it is well closed in by an iron hood, before described, just sufficient opening being left for the long iron for mixing.
The broken metal, if of good quality, should present a sparkling appearance; if it is dull, sufficient antimony has not been added, and plates made from such metal will lack sharpness of outline.
The first step is to subdivide the forme, if possible, so as to have plates of minimum size. The forme is laid on the imposing surface, unlocked, slightly damped, and re-imposed in smaller chases with type-high clumps to replace the furniture round the pages, noting that the lower side of the clumps must come next to the type. Every precaution must be observed to prevent types falling out, and to ensure the matter being securely locked up and level.
Low spaces and quads must all be raised to the level of the height of the shanks of the letters prior to moulding; therefore it is desirable to employ high-spaced founts when plaster casts are to be taken. Besides being originally more costly, however, the high spaces would be a source of much trouble if the type should be required for working from.
"Filling-up" is effected by pouring plaster having a pasty consistence over the surface of the forme, and rubbing it down by hand. When this has been evenly and thoroughly performed, and before the plaster has completely set, the whole is gone over with a moderately stiff brush, to remove the plaster from the beards of the letters.
The pages having been examined for imperfections, the forme is set to dry thoroughly in a rack specially provided. If a number of formes are to be cast, it is well to fill them all at one time. '1 he plaster being dry, the forme is laid on the imposing surface, which must be perfectly clean, and the face is again brushed, so that any small detached crumbs of plaster may be cleared away.
Before oiling, it is absolutely essential that the face of the type be both clean and dry. When this is the case, carefully apply some olive oil on a soft brush, sometimes adding a small proportion of turps if the oil is very thick. The oil must adhere in every part, or the cast will come away in an imperfect condition - pieces of plaster remaining attached to the unoiled spots. The oil answers a double purpose, preventing the adhesion of the mould to the type as well as hindering the moisture of the fresh composition from affecting the plaster already used in filling-up the forme.
On the locking-up furniture round the type are placed pieces of tin about 1 1/2 in. wide and of various lengths, destined to provide a perfectly flat surface for the casting frame, and to stop the thin plaster from running. The casting frame is adjusted in position round the pages, after the sides have been oiled. Then, in an iron or tin pot kept for the purpose, is mixed a sufficiency of plaster to a creamy consistence; this is poured upon the face of the type, and carefully forced in by a pad of folded blanket, called a "dabber." In this way the air is expelled from between the plaster and the face of the type. Then more pasty plaster is added, and well rubbed in by hand, to ensure the plaster occupying the smallest interstices, when the surface of the type will be completely covered with a film of plaster.
Next a further quantity of the composition is mixed somewhat thicker, and enough of it is poured on the forme to entirely fill the casting frame. As the plaster hardens very rapidly, every means must be taken to prevent small lumps from forming, both on the hands and in the mixing pot; after each operation, the hands and pot should be well washed. Whilst the plaster remains liquid, the surface is scraped with a straight-edge level with the top of the casting frame, after which the mould is allowed to stand for 5 minutes; by this time, it will have partially hardened, when the buck may be scraped again. Should the mould not be of uniform thickness throughout, it is apt to be cracked by the pressure of the molten metal in the dipping pan. Success is almost entirely dependent upon the quality and manipulation of the plaster.
When the mould has stood some 15-20 minutes, it should have become sufficiently firm for lifting from the type. Forked tools with short handles are used for this purpose, one in each hand, the points being carefully inserted between the casting frame and the chase. The operation demands the gentlest care; if force is exerted unevenly, small portions of plaster will break off and spoil the mould. After detaching one end of the frame, the other end is loosened in the same manner; then the whole can be lifted off the forme, being supported by the protruding bevel of the casting frame. After removal of the mould, the type should present a perfectly clean face, not a particle of plaster appearing among the type.