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.
Brass moulding is carried on by means of earthen or sand moulds. The formation of sand moulds is by no means so simple an affair as it would first appear, for it requires long practical experience to overcome the disadvantages attendant upon the material used. The moulds must be sufficiently strong to withstand the action of the fluid metal perfectly, and, at the same time, must be so far pervious to the air as to permit of the egress of the gases formed by the action of the metal on the sand. If the material were perfectly air-tight, then damage would ensue from the pressure arising out of the rapid generation of gases, which would spoil the effect of the casting, and probably do serious injury to the operator. If the gases are locked up within the mould, the general result is what moulders term a "blown" casting; that is, its surface becomes filled with bubbles, rendering its texture porous and weak, besides injuring its appearance.
For a number of the more fusible metals, plaster of Paris is used. This material, however, will not answer for the more refractory ones, as the heat causes it to crumble away and lose its shape. Sand, mixed with clay or loam, possesses advantages not to be found in gypsum, and is consequently used in place of it for brass and other alloys. In the formation of brass moulds, old damp sand is principally used in preference to the fresh material, being much less adhesive, and allowing the patterns to leave the moulds easier and cleaner. Meal-dust or flour is used for facing the moulds of small articles, but for larger works, powdered chalk, wood ashes, and so on are used, as being more economical. If particularly fine work is required, a facing of charcoal or rottenstone is applied. Another plan for giving a fine surface is to dry the moulds over a slow fire of cork shavings, or other carbonaceous substance, which deposits a fine thin coating of carbon. This is done when good fine facing-sand is not to be obtained.
As regards the proportions of sand and loam used in the formation of the moulds, it is to be remarked that the greater the quantity of the former material, the more easily will the gases escape, and the less likelihood is there of a failure of the casting; on the other hand if the latter substance predominates, the impression of the pattern will be better, but a far greater liability of injury to the casting will bo incurred from the impermeable nature of the moulding material. This, however, may be got over without the slighest risk, by well drying the mould prior to casting, as would have to bo done were the mould entirely of loam.
Where easily fusible metal is used, metallic moulds are sometimes adopted. Thus, where great quantities of one particular species of casting are required, the metallic mould is cheaper, easier of management, and possesses the advantage of producing any number of exactly similar copies. The simplest example is the casting of bullets. These are cast in moulds constructed like scissors, or pliers, the jaws or nipping portions being hollowed out hcmispherieally, so that when closed a complete hollow sphere is formed, having a small aperture leading into the centre of the division line, by which the molten lead is poured in. Pewter pots, inkstands, printing types, and various other articles, composed of the easily fusible metals, or their compounds, are moulded on the same principle. The pewterer generally uses brass moulds : they are heated previous to pouring in the metal. In order to cause the casting to leave the mould easier, as well as to give a finer face to the article, the mould is brushed thinly over with red ochre and white of an egg; in some cases a thin film of oil is used instead.
Many of the moulds for this purpose are extremely complex, and, being made in several pieces, they require great care in fitting.
A few observations on the method of filling the moulds. The experienced find that the proper time for pouring the metal is indicated by the wasting of the zinc, which gives off a lambent flame from the surface of the melted metal. The moment this is observed, the crucible is removed from the fire, in order to avoid incurring a great waste of this volatile substance. The metal is then immediately poured. The best temperature for pouring is that at which it will take the sharpest impression and yet cool quickly. If the metal is very hot, and remains long in contact with the mould, what is called "sand-burning" takes place, and the face of the casting is injured. The founder, then, must rely on his own judgment as to what is the lowest heat at which good, sharp impressions will be produced. As a rule, the smallest and thinnest castings must be cast the first in a pouring, as the metal cools quickest in such cases, while the reverse holds good with regard to larger ones.
Complex objects, when inflammable, are occasionally moulded in brass, and some other of the fusible metals, by an extremely ingenious process; rendering what otherwise would be a difficult problem a comparatively easy matter. The mould, which it must be understood is to be composed of some inflammable material, is to be placed in the sand-flask, and the moulding sand is put in gradually until the box is filled up. 'When dry, the whole is placed in an oven sufficiently hot to reduce the mould to ashes, which are easily removed from their hollow, when the metal may be poured in. In "this way small animals, birds, or vegetables may be cast with the greatest facility. The animal is to be placed in the empty moulding box, being held in the exact position required by suitable wires or strings, which may be burnt or removed previous to pouring in the metal.
Another mode, which appears to be founded on the same principle, answers perfectly well when the original model is moulded in wax. The model is placed in the moulding box in the manner detailed in the last process, having an additional piece of wax to represent the runner for the metal. The composition here used for moulding is similar "to that employed by statue founders in forming the cores for statues, busts, and so on, namely, 2 parts brickdust to 1 of plaster of Paris. This is mixed with water, and poured in so as to surround the model well. The whole is then slowly dried, and when the mould is sufficiently hardened to withstand the effects of the molten wax, it is warmed, in order to liquefy and pour it out. When clear of the wax, the mould is dried and buried in sand, in order to sustain it against the action of the fluid metal.
Large bells are usually cast in loam moulds, being "swept" up, according to the founder's phraseology, by means of wooden or metal patterns whose contour is an exact representation of the inner and outer surfaces of the intended bell. Sometimes, indeed, the whole exterior of the bell is moulded in was, which serves as a model to form the impression in the sand, the wax being melted out previous to pouring in the metal. This plan is rarely pursued, and is only feasible when the casting is small. The inscriptions, ornaments, scrolls, and so on, usually found on bells, are put on the clay mould separately, being moulded in wax or clay, and stuck on while soft. The same plan is pursued with regard to the ears, or supporting lugs, by which the bell is hung.
Moulds faced with common flour turn out castings beautifully smooth and bright; the sand parts easily from the surfaces, and, as a rule, can be readily removed by the application of a hard brush. For large brass castings, quicklime is successfully used in some places; it is simply dusted on the face of the mould and smoothed down in the usual way.
Sometimes, even when the brass mixtures are good, there will be much trouble with blowing, both in dry and green moulds. This may be due to want of porosity in the sand or to insufficient heat of metal. A first-class sand is that from the Mansfield quarries, near Nottingham. It is a good plan to stir the metal with a hazel rod jus.t before pouring.
The ordinary method of casting in sand moulds applied in successive pieces, as in plaster of Paris casting, is not so much in use in Italy as what is called the "forma perduta" mode; meaning that the object is destroyed or "lost" every time. Casting from metallic or other incombustible objects is therefore impossible by this method. The object must be of wax, or something that will melt or burn out, the mould having been dried and baked. By this way very little chasing is required, but the artist has to finish his wax object (cast in a plaster mould) each time. The advantage of this method is that you get the artist's finishing of his own work instead of the chaser's, who, though he ought to be, is by no means always an artist. He can copy mechanically, but the work always loses terribly in expression and finish.
The following process is recommended by Abbass for producing metallic castings of flowers, leaves, insects, etc. The object - a dead beetle, for example - is first arranged in a natural position, and the feet are connected with an oval rim of wax. It is then fixed in the centre of a paper or wooden box by means of pieces of fine wire, so that it is perfectly free, and thicker wires are run from the sides of the box to the object, which subsequently serve to form air-channels in the mould by their removal. A wooden stick, tapering towards the bottom, is placed upon the back of the insect to produce a runner for casting. The box is then filled up with a paste of 3/4 plaster of Paris and \ brickdust, made up with a solution of alum and sal-ammoniac. It is also well first to brush the object with this paste to prevent the formation of air-bubbles. After the mould thus formed has set, the object is removed from the interior by first reducing it to ashes. It is therefore dried slowly, and finally heated gradually to a red heat, and then allowed to cool slowly to prevent the formation of flaws or cracks. The ashes are removed by pouring mercury into the cold mould and shaking it thoroughly before pouring it out, repeating this operation several times.
The thicker wires are then drawn out, and the mould needs simply to be thoroughly heated before it is filled with metal, in order that the latter may flow into all portions of it. After it has become cold, it is softened and carefully broken away from the casting.