CHAPTER XXV 
METAL CASTING 
Cuttlefish moulds—Materials for moulds—The piece mould—The core— Casting a relief—Loam or casting sand—Flasks—Casting on the descending or the ascending principle—The pour—The vents. 

The casting of ingots of simple form is considered in Chapter II. Casting in cuttlefish-bone and by piece mould are dealt with here, a description of the waste wax process being reserved for the following chapter. 

Casting in cuttlefish is a process sometimes employed for the reproduction of small reliefs or other articles which have no undercutting. The model must not be in any soft material such as modelling-wax, though sealing-wax may be used. With these limitations it is a very convenient method for reproducing quite small work. Let us therefore consider it before we go on to the more important processes. 

Take a sound cuttlefish bone and cut it across into two equal parts. One side of the bone is softer than the other. Grind the soft side of each piece quite flat on the rub stone. Flatten also one of the outer sides of the bone, near the thicker end. The reason for this will be given later. Fix three small wooden pegs with conical or pointed tops in one of the flattened faces of the mould. These pegs should be placed as far apart as they may conveniently be. When the two halves of the mould are pressed together the pointed tops of the pegs make marks which ensure the proper regis tration of the two pieces whenever they are again put together. Lay the pattern on one face of the mould, say, an inch from the top edge. Press the pattern a little way into the face of the bone. It will give sufficiently to allow this. Now place the second piece of bone exactly over the first, and press it down on to the pegs and the model. Between hands and knees press the two parts firmly together. When the two faces have met, take a rough file and true up the outer edges of the mould while its two halves are in contact. Open it and lift out the model, a sharp impression of which will remain. Make a funnel-shaped opening from the im pression to the top edge of the mould. This opening is called the " pour "—it is the way in for the molten metal. Make also a number of small grooves across the face of the mould, radiating from the impression. These grooves are called " vents." Through them the air escapes as the hot metal enters the mould. The grooves need not reach quite so far as the outer edge of the mould, or rather, they should not be wide enough to allow the metal to run right through them and escape. If they are as thick as a knitting-needle they will do. Next paint the face of the mould, where it will be touched by the heated metal, with a strong borax solution. This may be borax ground up on the slate as for soldering, or ground up with paraffin. When this has sunk in, go over the surface with a solution of silicate of soda (water glass)—half water and half silicate of soda. You may, if you wish, put on the borax and the silicate of soda solutions together, half of each. Their purpose is to toughen the face of the mould, so that it may stand the heat of the molten metal better. You may now replace the model very carefully in the mould, seeing that it registers exactly. But slightly oil its surface, or dust it over with French chalk first, lest it stick to the mould. Then put the second part of the mould -in place and press the two halves together. This should give you a very sharp impression, see Fig. 287. Remove the model, tie the two parts of the mould together with binding wire, and dry thoroughly. Take a piece of charcoal and make a hollow in one face of it large enough to hold the metal which you wish to use for the cast. Tie this piece of charcoal against the flattened outer side of the cuttlefish mould, the upper surface of the two being level. Scrape a little channel between the hollow in the charcoal and the funnel-shaped pour in the mould. All being ready, melt the metal in the charcoal hollow. Get it quite hot, and as soon as the brightness disappears and a film is about to form on it, tilt the block and mould so that the metal runs into the latter. You may open the mould immediately the metal has set. If, however, you heat the metal in a crucible, first set the mould in a box of dry (hot) sand. 

Cuttlefish bone is unlike the other materials from which moulds are made in that the recess for the metal is made at 
once,—simply by pressure. When other materials are employed they are either painted or poured over the work 
in a more or less liquid state, or used in granular form and pressed against the pattern. With this exception, then, 
the qualities necessary for any material that is to be used for a mould are : (1) That it be of such consistency that it 
may be spread over the model in a very finely divided state— so that it may go into every crack, following the form very closely; and give a smooth surface where necessar};1. And (2) that it be able to stand the heat of the molten metal without melting, powdering or splitting. Moulds have to be heated to a bright-red heat to expel the moisture before the metal can be poured in, for any water present then will be converted into steam, which will force its way out somehow, even through the liquid metal. You may thus with a damp mould have a serious explosion, with molten metal thrown all about. The surface of the mould must be quite sound after this heating, or you will get a rough surface to the cast. In foundries where very large work is cast in sand, and it would not be practicable to dry the mould in this thorough way, the mould is " vented " for the escape of the gas by making an elaborate system of small holes through the sand with a wire. But in this work a very fine surface is not required. For fine metal­work it is of course necessary. 

Now if the work is a relief, without any undercutting, it is possible to make one mould do for the front. But as open moulds do not work very well, a slab of some material will be required for the back. Moreover if the model is not to be destroyed, and the work is in the round and undercut anywhere, the mould must be in several pieces. You could not otherwise remove the model. We will therefore discuss nextthe makingofapiecemould. Letus supposethatyou are to reproduce a statuette of a draped figure, Fig. 291. First look carefully over the model and decide where the joints in the mould must come. Each piece of the mould must be of such a shape that it is possible to withdraw it without any undercut piece catching. You must be able to withdraw each part of the mould in turn from the model, yet when all are in place the cast must be completely covered. Plaster casts of antique statues are generally sold with the seams which mark the position of joints in the mould still visible. An examination of one or two of these casts will teach you a good deal as to the planning out of the different pieces which go to make the mould. When a mould is in many pieces it is usual to make an outer or mother mould in two or three pieces to keep all the others in place. To return to our work.


One of the first things you have to decide is the manner in which you will arrange the pour (the place where the metal is poured into the mould) and the vents. In this case we will suppose that the metal is to enter the mould at the base. A suitably placed conical opening must therefore be provided when you make the mould. To settle the position and number of the vents you must, in imagination, follow the course of the metal and note every place where the air in the mould would be trapped by the molten metal. At each of these places you must leave an opening for the escape of the air. The metal will follow the air through the openings; so they must point upwards and extend up to the level of the pour. The seams in the mould will, of course, allow a good deal of ah to escape. Hav ing ' decided upon their position, paint the statuette all over with linseed oil, lard, clay water or some other substance which will keep the mould from sticking to the plaster. Then take some modelling clay or wax roll it into a rod and flatten it into a strip i inch thick. Smooth it all along with the flat side of a knife, and cut the stripinto bands | inch wide. Take one of these bands and pressit edgewise against the statuette, bending the strip all round the space which you have decided shall be covered by the first piece of the mould. The position of the first two pieces is shown by a thick dotted line in the illustration *ig. 291. The first space should, as a rule, be near the' base of the statuette if you are moulding it in an uprightposition; for this first piece will make a support for the next pieces higher up the figure. When you have bent the strip right round, see that it touches the figure all alongwithout gaps. Support the back of the strip by small pieces of clay placed at intervals, then cover the rest of the figure with a cloth or wet paper to avoid splashing it. You are now ready for the moulding material. 


Moulds are made in many materials: of loam, sand plaster, bnckdust, pumice powder, fireclay, soapstone or of other ingredients in various proportions. Each moulder has his own preferences. Large works are generally cast in earthen moulds, i. e. of loam or sand, smaller ones in mixtures of the other materials. Let us suppose that you are to use " compo "-a mixture of plaster of Paris and bathbrick dust. The plaster should be such as will set quite hard in five minutes if mixed as described in Chapter III. The bathbrick dust is just the ordinarypowdered bathbrick, used for cleaning knives. Half plasterand half bathbrick is a good proportion. For large works the proportion may be one-third plaster, two-thirds bath brick. Plaster alone is liable to crumble away if made red hot, but if mixed with a little potassium sulphatealum, soda or borax, it will stand the heat well Cellini sometimes used a compo made from two parts plaster one part powdered brick and one part powdered bone. Both bnckdust and plaster should be free from lumps When you have mixed and thoroughly beaten up the liquid compo takealittleinyourhandandthrowitontothat part ofthe figure which is within the clay band. Take care toleave no bubbles or gaps. Then as the compo gets stiffer, build upthis piece of the mould till it is half an inch thick. For a large piece the thickness may be increased to three-quarters of an inch. Make the outer surface quite smooth. When the compo has set, lift the piece off and trim up all its edges. The compo will set hard in a few minutes, so do not mix more at a time than you can use. 

Cut some shallow, conical holes at the edges to make " keys " with which the adjacent pieces of the mould may interlock. The projections on the one piece of the mould fit into the hollows in other pieces and keep all the parts from slipping out of place later on. Grease or claywash the back and edges of the piece just made—lard does well for this. Then replace the piece. Take another clay or wax band, and outline with it another space on the figure; the edge of the piece already made serving in place of a band on that side. Mix some more compo and fill in as before. When this piece has set, trim up all the edges except that which fits against the piece first made—that side should require no trimming. Cut keys in the edges.Claywash or grease, and replace the piece. Go on in this way till the whole figure is covered with small pieces which fit together like a puzzle. When the figure is thus covered in, make sure that the backs of all the pieces are smooth, that there are no undercut places which would prevent the mother mould from coming off. If there are any such re cesses fill them up temporarily with clay. See that the backs of all the pieces have been greased. Then, following exactly the same method make the mother mould in two or three pieces. Afterwards remove it and lay the small pieces, each in its place, within one or other of the parts of the mother mould. If you now put these together you will have a hollow inside of exactly the shape of the statuette. 


If this space were filled with metal you would have a solid casting. But, except for very small articles, castings are generally made hollow. The thickness of a metal cast varies, of course, with the size of the work and with its form. Thus, for a statue standing on one foot the metal in the standing leg and foot would be made thicker than if the weight came on both. Three-eighths of an inch is about the thickness that a life-sized figure would be cast, though for such a work in the pose mentioned above, the thickness at the foot would be considerably greater. 

A mass or " core " of a similar material to that of which the mould is made is therefore to be fixed inside, supported on iron wires, leaving a space of just the thickness that the metal is to be, between core and mould. To make the core. First take some lengths of iron wire and fix them inside the mould in such a way that they will support the core (when it has been poured round them) in the centre of the hollow. The wires must be fixed to the mould, so scrape small holes to receive their extremities. You will, of course, arrange that these come in places which will not disfigure the work much; for when the figure has been cast you may remove the core and the iron wires, plug up the holes left by them, and true up the surface of the work again. The wires being fixed, take some soft modelling wax, plasti cine or clay, and roll it into slabs. Use a round bar for this, after the fashion of the domestic rolling-pin. The thickness of the slabs must be rather less than an eighth of an inch. Cut these slabs into convenient pieces and cover the inside of the mould with them, pressing the wax into the various parts without thinning the pieces. If you make it thin at any place, the metal which will later on replace the wax, will be thin too,—so try to make the wax layer of an even thickness, though you need not trouble to press it into every crevice or fold. Remember that you are settling now only the size and shape of the hollow inside the cast, so any parts which you wish to make solid in the metal you may fill up with wax now. But if you leave a fold in the drapery, say half an inch thick, and near it is a part only one-eighth of an inch in thickness, you will probably get a cracked cast. The thickness should be pretty regular throughout. You have to see that you spread the layer of wax to such a thickness that the metal which replaces it is strong enough. You must leave the wires in, crossing the hollow space, inside the mould and wax, in various directions. See that when the various parts of the mould are put together the layer of wax joins neatly. Replace the mother mould and tie all together. Leave an opening at the centre of the base, or at some other part of the mould where it will not seriously damage the modelling, through which you may pour into the mould the material for the core. 

Before mixing this material it is well to consider what happens when the metal, which is poured into the space now occupied by the wax, is cooling. It must contract on to the core very powerfully. Now if the core is hard and the metal thin, the latter will be likely to crack. Some moulders use the same material for the core as for the mould. But to avoid any danger it is well to make the core of a slightly yielding substance. Such a material is made from one part plaster of Paris, one part brickdust, two parts sawdust and a little alum, borax or soda. The sawdust will be burnt out when the mould is fired, and will leave the core porous. In mixing these materials put the saw dust and brickdust in the water first and get them soaked before you add the plaster. Beat the mixture up thoroughly with a spoon, and pour it into the hollow space inside the wax in the mould. When it has set, say, in ten minutes' time, open the mother mould. Lift out each piece of the inner mould, and put it in its place in the mother mould. Remove the core and all the wax. Next make the pour and the vents as described below. When these are completed, mould and core are ready for firing. 

Some moulders make the core in another manner. They first soap the inside of the mould just as they would were they making an ordinary plaster copy. Then after fixing the wires as described above, but omitting the wax lining, they fill the whole of the mould with the material for the core. They therefore on opening the mould find a replica of the figure they are to cast. This figure they pare down all over, taking off just the amount of material which they wish to replace by metal. The iron wires will, of course, keep the core in the centre of the hollow inside the mould during the subsequent operations. The drying of the mould is discussed on page 228. 

When a thin, flatfish casting has to be made, a relief, for instance, it is usual to arrange the work as shown in section in Fig. 288. A is the mould for the front of the work. B is that for its back. C is the cast itself. D is a large hollow at the top of the mould, filled at the bottom by the plug, E. F F F F are the inlets or " gates " for the metal, which flows from the hollow, D, when the plug is removed. G G G are vents to allow the air to escape. You will notice that they come from the top of each undulation in the shape of the upper surface of the cast—wherever, in fact, the air might be imprisoned by the molten metal. The inlets, F, run towards the lower parts of the undulations in the form. The air-vents should be as large or larger than the inlets. When the mould has been thoroughly dried, the molten metal is poured into the hollow, D. The plug is then removed and the metal flows through the various inlets to every part of the mould at once. With a thin casting this is essential, for if there were but one inlet the metal would be likely to set before it reached the extremities of the mould. You would therefore get a faulty casting. 

Piece moulds are also made in loam or casting sand. The method employed, however, differs considerably from that for compo-work described above. It is well to have a large wooden tray on which to work. On this place the loam you are to use. Mix a little water with it, and work it about with a piece of board measuring, say, 8 by 3J inches. Put the heap of loam on the far side of the tray and scrape it towards you a little at a time with the board. Be careful 
not to add too much water. The loam, when mixed, should just hold together when it has been gripped tightly in the hand. It will hold together if made much wetter, but you requireitasdryas possible—ifonlyitwillbind. Nexttake a pair of flasks or casting boxes, Fig. 289. These are iron boxes without tops or bottoms. They are made in pairs; register " pins " on the rim of one box, going into holes or " eyes " in the rim of the other. Lay one flask with its register marks upwards on a piece of board or a small tray. Set both down in the large tray, and with a thin piece of board used as a shovel, fill the flask. Press the loam together lightly, and then lay your model or " pattern," which has previously been brushed over with French chalk, in a space you scrape for it. About half of the model should project above the top of the flask. Press the sand against the lower half of the model and level it off neatly all round, leaving the surface of the loam smooth and firm. Now dust over the model and the loam in the flask with parting dust. This is to prevent the loam which is afterwards put on from adhering to the other part of the mould. Pea-flour or lycopodium is usedfor this purpose. You have now to make of loam a sufficient number of pieces to cover the exposed half of the model; each piece to be of such a form that it may be withdrawn without binding in any undercut place. 

Take a pinch or a handful, as the case may be, of the loam and press it against the model, building it into a neat little mass of the shape required. Each piece of the mould is to be dusted over, when completed, with the parting dust. A fork made from a couple of stout needles stuck in a piece of wood, is a convenient tool for lifting pieces of the mould without doing them any damage. See that the back of each piece is smooth before you finish with it. When the upper half of the model is, in this way, covered, you must make the cope, or case, or mother mould, which is to keep all the pieces in position. Put parting dust over all the pieces as they lie, and then put the second flask in position on the first. Fill it up to the top with loam, pressing it round the pieces and against the sides of the flask. When it is quite tightly packed and full, level off the top of the loam with a stick. You may nowturn the flasks over and scrape out the loam from the first. Lift this box off and remove all the loam which was in it. The parting dust will separate it definitely from that which belongs to the other box. Put fresh parting dust over thework,andproceedtocovertheexposedhalf ofthe model with pieces as you did on the other side. When these are finished you make the cope or case for this side also. You may now separate the two flasks, lift out the pieces one by one and carefully put them in their places. Next bend and fix the irons which are to support the core. Dust over the inside of the mould with parting dust, and fill the cavity withloam,pressingitwellagainstthe irons. Youhavenow a replica of the figure which is to be cast. Pare it down so that a space may be left between it and the surface of the mould the exact thickness you wish the metal to be. The pour and vents should now be made, and it may be well to consider for a moment the position of the former. 

There are two principles on which the metal may be poured,—the descending and the ascending. In the former case the molten metal enters the mould at its highest point and runs between the core and the face of the mould to the bottom; then gradually filling up all the space provided for it, it finally reaches the top again. On the other hand when metal is poured on the ascending principle the pour is taken down to the lowest part of the mould before it enters the cavity, the metal thus filling the mould from the bottom and gradually rising to the top. If you think as to what happens inside a mould when the metal is poured in you will see at once that the ascending method is likely to be the better of the two; and experience proves that it is. When the metal enters the mould the air inside becomes very greatly heated and expanded. Now, it must all get out somehow, and vents are provided to carry it off. But on its way to the vents, if the casting is arranged on the descending principle, it must meet the stream of molten metal, which comes trickling over the core and face of the mould. Some of the air will be caught by the molten metal and carried along with it. Expanding very violently, it will bubble out through the molten metal, causing a great disturbance and perhaps injuring the surface of the mould. If, however, the metal enters the mould from the bottom all the air and the vents are above it. The air can then get out as fast as it likes without any danger of trapping or other interference. Also any scum or dross will float on the surface of the metal and be carried to the vents instead of being caught by the descending metal and carried perhaps to the face of the mould. Metal poured on the descending principle passes twice over the face of the mould ; and has thus more opportunity of damaging it. But on the other plan, it enters the cavity from the bottom and flows gently up from the bottom without splashing or other disturbance. For these reasons it is well to pour always on the ascending principle. The pour or inlet is very often branched, so that the metal may reach all the principal parts of the mould at once. These branches are known as " jets " or " gates." 

To return to our work: make the pour by scraping a passage from that end or side of the flask which is to be uppermost, to the lowest part of the cavity. It is generally possible to make this passage run in the loam along the crack which separates the two flasks. Branches may run from this passage to other parts of the cavity where necessary. Then make vents wherever there is any chance that the air may get trapped when the metal rises in the mould. The series of vents should be kept quite separate from the pour and gates. The mould should now be fired as described on page 228. 

A wooden pattern is generally well varnished to keep the damp from getting at it. In casting from such a pattern dust it over with French chalk to keep the loam or sand frora sticking. To remove a wooden pattern from the mould, stick a point firmly into it and give it a few taps sidewise with a pillar-file. This will free it from the mould, and you will be able to lift it out. Use French chalk and charcoal to face up. Some founders smoke the mould withapitchtorchandthen replace thepatternfora moment, to ensure a good surface.