This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
When brass is ready to be poured, the zinc on the surface begins to waste with a lambent flame. When this condition is observed, the large cokes are first removed from the mouth of the pot, and a long pair of crucible tongs are thrust down beside the same to embrace it securely, after which a coupler is dropped upon the handles of the tongs; the pot is now lifted out with both hands and carried to the skimming place, where the loose dross is skimmed off with an iron rod, and the pot is rested upon the spill-trough, against or upon which the flasks are arranged.
The temperature at which the metal is poured must be proportioned to the magnitude of the work; thus, large, straggling, and thin castings require the metal to be very hot, otherwise it will be chilled from coming in contact with the extended surface of sand before having entirely filled the mould; thick massive castings, if filled with such hot metal, would be sandburnt, as the long continuance of the heat would destroy the face of the mould before the metal would be solidified. The line of policy seems therefore to be, to pour the metals at that period when they shall be sufficiently fluid to fill the moulds perfectly, and produce distinct and sharp impressions, but that the metal shall become externally congealed as soon as possible afterwards.
For slight moulds, the carbonaceous facings, whether meal-dust, charcoal, or soot, are good, as these substances are bad conductors of heat, and rather aid than otherwise by their ignition; it is also proper to air these moulds for thin works, or slightly warm them before a grate containing a coke fire. But in massive works these precautions are less required; and the facing of common brickdust, which is incombustible and more binding, succeeds better.
The founder therefore fills the moulds having the slightest works first, and gradually proceeds to the heaviest; if needful, he will wait a little to cool the metal, or will effect the same purpose by stirring it with one of the ridges or waste runners, which thereby becomes partially melted. He judges of the temperature of the melted brass principally by the eye, as, when out of the furnace, and the very hot surface emits a brilliant bluish-white flame, and gives off clouds of white oxide of zinc, a considerable portion of which floats in the air like snow, the light decreases with the temperature, and but little zinc is then fumed away.
Gun-metal and pot-metal do not flare away in the manner of brass, the tin and lead being far less volatile than zinc; neither should they be poured so hot or fluid as yellow brass, or they will become sandburnt in a greater degree, or, rather, the tin and lead will strike to the surface. Gun-metal and the much-used alloys of copper, tin, and zinc are sometimes mixed at the time of pouring; the alloy of lead and copper is never so treated, but always contains old metal, and copper is seldom cast alone, but a trifling portion of zinc is added to it, otherwise the work becomes nearly full of little air-bubbles throughout its surface.
"When the founder is in doubt as to the quality of the metal, from its containing old metal of unknown character, or if he desires to be very exact, he will either pour a sample from the pot into an ingot-mould, or extract a little with a long rod terminating in a spoon heated to redness. The lump is cooled, and tried with a file, saw, hammer, or drill, to learn its quality. The engraved cylinders for calico-printing are required to be of pure copper, and their unsoundness, when cast in the usual way, was found to be so serious an evil that it gave rise to casting the metal under pressure.
Some persons judge of the heat proper for pouring by applying the skimmer to the surface of the metal, which, when very hot, has a motion like that of boiling water; this dies away and becomes more languid as the metal cools. Many works are spoiled from being poured too hot, and the management of the heat is much more difficult when the quantity of metal is small. In pouring the metal, care should be taken to keep back the dross from the lip of the melting-pot. A crucible containing the general quantity of 40 lb. or 50 lb. of metal can be very conveniently managed by one individual, but for larger quantities, sometimes amounting to 1 cwt, an assistant aids in supporting the crucible by catching hold of the shoulder of the tongs with a grunter, an iron rod bent like a hook.
Whilst the mould is being filled, there is a rushing or hissing sound from the flow of metal and escape of air; the effect is less violent where there are 2 or more passages, as in heavy pieces, and then the jet can be kept entirely full, which is desirable. Immediately after the mould is filled, there are generally small but harmless explosions of the gases, which escape through the seams of the mould; they ignite from the runners, and burn quietly; but when the metal blows, from the after-escape of any confined air, it makes a gurgling, bubbling noise, like the boiling of water, but much louder, and it will sometimes throw the fluid metal out of the runner in 3 or 4 separate spurts; this effect, which mostly spoils the castings, is much the more likely to occur with cored works, and with such as are rammed in less judiciously hard, without being, like the moulds for fine castings, subsequently well dried. The moulds are generally opened before the castings are cold, and the founder's duty is ended when he has sawn off the ingates or ridges, and filed away the ragged edges where the metal has entered the seams of the mould; small works are additionally cleaned in a rumble, or revolving cask, where they soon scrub each other clean.
Nearly all small brass works are poured vertically, and the runners must be proportioned to the size of the castings, that they may serve to fill the mould quickly, and supply at the top a mass of still fluid metal, to serve as a head or pressure for compressing that which is beneath, to increase the density and soundness of the casting. Most large works in brass, and the greater part of those in iron, are moulded and poured horizontally.
The casting of figures is the most complex and difficult branch of the founder's art. An example of this is found in the moulding of their ornaments in relief. The ornament, whatever it may be - a monumental bas-relief, for instance - is first modelled in relief, in clay or wax, upon a flat surface. A sand-flask is then placed upon the board over the model, and well rammed with sand, which thus takes the impress of the model on its lower surface. A second flask is now laid on the sunken impression, and also filled with sand, in order to take the relief impression from it. This is generally termed the cope or back mould. The thickness of the intended cast is then determined by placing an edging of clay around the lower flask, upon which edging the upper one rests, thus keeping the two surfaces at the precise distance from each other that it is intended the thickness of the casting shall be. In this process, the metal is economized to the greatest possible extent, as the interior surface, or back of the casting, is an exact representation of the relief of the subject, and the whole is thus made as thin in every part as the strength of the metal permits. Several modifications of the process just described are also made use of, to suit the particular circumstances of the case.
What has been said, however, is a detail of the principle pursued in all matters of a similar nature.