CHAPTER XXVI 

CASTING {continued) The " Cire perdue " or waste wax process—Composition of the wax—The wax model—The core—The pour, gates and vents—Materials for the final mould-Binding with wire-Cellini-Melting out the wax-Firing the mould—Furnaces—Fusing the metal—Fusible alloys—Pouring— Finishing casts. 

Casting by the " cire perdue," "cera perduta " or waste wax process has been practised from very early times. The method, roughly, is this. The model to be cast in metal is first made in wax. A mould, in one piece, is formed all round it. The wax is melted out and replaced by molten metal. When this has cooled the mould is destroyed and the metal casting exposed. For all except quite small works (which are cast solid) a core must be prepared as described in the last chapter; the wax model being made of the exact thickness required for the metal.

The first thing to consider is the wax. This must be suitable for modelling in, and its composition must be such that it can be melted or burnt out from the mould without leaving any residue. Plasticine therefore cannot be used. A mixture of beeswax and Venice turpentine may be employed, though many founders add resin, pitch, tallow, or other ingredients. Paraffin wax, as used for candles is quite good, but resin should be added to toughen it. Almost any mixture which will fulfil the two conditions mentioned above may be employed. Wax being semi­transparent, however, it is well to add some colouring matter to make it easier to see the modelling. Any dye may be used which will entirely burn away when the mould is fired. You should try a piece of the coloured wax before you employ it for any work. Put it in a clean, covered 
crucible and burn it all away; if it leaves no residue it is safe to use. It should of course be made harder in warm weather, for a very sticky surface is difficult to work on; and however you prepare the wax model you are bound to work on it to some extent. 

The wax being ready, the next thing to do is to prepare in it a cast from your model. For a large work this would be made in several pieces. Even in a small one, say of a man on horseback, it might be prepared in several parts which would be afterwards joined together. For this wax cast you will require a piece mould, made in plaster as described in the last chapter, or a gelatine mould. A quite small work may be cast solid, soall youhave to do yet is to fill the mould with melted wax and wait for it to set. 

For cored work, however, you must proceed on a different plan. Inside the mould you have to spread an even layer of wax of the exact thickness you wish the metal cast to be. You may wish the wax, and therefore the metal, to be thicker in some parts than in others, see page 213. You must therefore spread a thicker layer of wax in those places. 

There are several different ways in which you may prepare the wax model, much depending on the shape of the work. One plan is that in which the wax is poured into the mould round the core. Cellini used to work in this way sometimes. Hefirstgreasedthemouldwithbaconfat. Thereis,however, a small chance that the wax may not penetrate to every part of the mould, especially if the latter is not made fairly warm first. For the complete system of vents, which will be arranged later, is not provided for in the plaster mould; and the wax may be chilled by contact with the mould before it has penetrated everywhere; or the air may be trapped in places and make the wax faulty. Another plan is therefore generally adopted. In this, molten wax is painted into the two halves of the mould to make sure that the whole surface is properly covered. Then the two halves are put together and the whole mould filled with melted wax. After a few seconds this is emptied out, leaving another layer of wax over that first painted in. This operation is repeated again and again until there is a sufficientlythicklayerofwaxinsidethemould. Themould should thenbeput intoa cool place, sothat the waxmaybe properly set before it is removed. It may then be kept cool in a bowl of water. If the work measures but a few inches in height and is not an awkward shape it will be safe to handle without a core. But for larger work it is well to make the core before the wax is removed from the mould. Holes may be cut through mould and wax to take the iron wires which are to keep the core in position when, later on, the wax itself is removed. If the wires had been fixed before the wax cast was made they would have been coated with wax and would not have held the core firmly. The wires may be arranged to end in a line with the seams of the mould. A sufficient number should always be provided to hold the core safely in whichever direction the mould may be turned. 

Mix up the material for the core as directed on page 214, and pour it inside the wax in the mould. When the core has set remove the mould. If you had removed the wax from the mould first it might have been distorted by its own weight; and the pressure of the core might injure the wax if the latter had not the support of the mould when the former was setting. The irons which penetrate the core will be seen projecting from the wax in all directions. You must now remove all the seam marks and give the final touches to the modelling. Another plan is that in which the irons to hold the core are fixed after the final touching up of the wax, holes being drilled through the wax into the core, and the wires pressed into them. The outer extremi ties of the wires are firmly gripped later by the new mould which is to be formed round the work. But of course, if the work is large, irons may be necessary from the first to support the weight of the core. 

When you have finished tidying up the modelling you must fix the pour, the gates, the vents, and any lugs required 
underneath the work by which it may be fastened uponits base when completed. These lugs are just lumps of metal into which screws may fasten. They are all made from the same wax as the model. Make a number of rods or threads of wax ready for use. Then decide where youwill have the pour and gates. Remember that each gate or vent which you make will be replaced later on by a rod ofmetalfixedtothecast. Theserodswillhavetobe sawn off, and the surface of the metal chased up again; so do not fix them in places Avhere they will damage the modelling much. Atthe sametimeyoumusthavea sufficientnumber of gates to allow the metal to get to every part of the work before it sets. 

Let us suppose that you are to cast a small standing figure with wings, the tips of which point downwards, clear of the legs; the figure resting on a small wax base : the metal to be poured on the ascending principle. For convenience in working it will be well to prepare the mould with the figure standing on its feet, and afterwards to turn it upside down to pour the metal in. The three highest points of the figure as it stands, are the head and the tops of the wings. It will be well to let the metal enter the mould at these three places; so take thickish rods of wax, warm their ends and stick them on. Take care to put them in places where they will not injure the modelling much. Bend the rods of wax well clear of the figure and unite the three into one. This rod must extend to below the wax base of the figure as it stands : it is the pour. Now you must fix the vents. One or more will be required at the tip of each wing, others at the hands, and at the lowest parts of the wax base. If you follow in imagination the course of the metal from the head and the top of the wings towards the feet, you will be able to see where the air might get trapped. To every such place attach a thread of wax of suitable thickness. The mould will be turned upside down for the metal to be poured in, so that the pour and vents, which now come down below the feet of the figure, will be uppermost then. There will be a great deal of heated air to escape, so put plenty of vents. All these rods and threads of wax, which represent the vents, should be bent well clear of the figure and reach lower than the wax base. They may combine here to form one or more large vents. The vents must be kept quite separate from the pour and gates. When all these have been fixed you are readjr for the final mould. 

Every founder has his own special material for moulds. They generally have finely ground loam as their basis, with a proportion of some non-fusible substance to make them stand fire well, and some other substance to bind all together. There is no reason, however, why the plaster and brickdust compo given in the last chapter should not be employed. However, for a loam mould, take some loam and grind it extremely fine in a mortar with paraffin. Grind also as finely as possible some burnt fireclay (old crucibles, if you have any), ganister, emery or almost any other fire-resisting substance. Mix these two powders together and, using them as a paint, go carefully over the wax model with a brush. Many founders add some binding substance to this loam mixture—white of egg or a solution of cow or horse dung. For the second coating add fine sawdust and finely shredded asbestos to the mixture. The use of sawdust as an ingredient in furnace casings, moulds, etc., is largely due to the work of Mr. Thomas Fletcher of Warrington, the well-known maker of gas-heating appliances. The sawdust is, of course, burnt out later on, leaving a very strong, light and porous mould. The fibres of asbestos tie the other materials together. The Japanese use boiled paper instead of asbestos fibre. Their paper is made from the inner bark of the mul berry tree. It is the only long-fibred paper known,—that made from rags is much shorter in the grain. The outer surface of each coat should be left rough, so that the next may hold well to it. Each coating should go over all the surface of the model, the vents, the gates and the pour. The materials for the outer coatings need not be so finely ground. Let each coating get fairly diy before the next is painted on. 

When the spaces between the model and the vents and pour have been filled up, and the mould has grown strong enough to allow it, wind binding wire round and round the whole mould. This will do much to strengthen it against the expansive power of the metal. Then add a few more coats to the mould. The thickness for the mould may be f inch for a mould a foot high, and thicker in proportion for larger work. Before the metal is poured in, the mould will be packed round with sand, as described later. 

A number of different receipts have come down to us from the old founders, and one may get good hints from them. Benvenuto Cellini, describing how he cast the Nymph of Fontainebleau, says : "I pounded up some ox bone, or rather the burnt core of ox horns. It is like a sponge, ignites easily, and is the best bone you can get anywhere. With this I beat up half a similar quantity of gesso of tripoli " (we should use plaster of Paris), " and a fourth part of iron filings, and mixed the three things well together with a moist solution of dung of horses or kine, which I first passed through a sieve Avith fresh water, till the latter took the colour of the dung." He gave the wax model three coatings each of the thickness of the baek of a table knife, letting the mould dry between each coating. Then he gave it several coatings of clay, or loam, in which rags had been left to rot for some months. Another mould he made of loam well dried and sifted, mixed with rotten rags and a little cow's dung. These he beat well together. Then he took " tripoli such as jewellers use to polish their gems with," powdered it up very finely and painted it over the model. After this he used the loam. 

To cast a figure Cellini proceeded in the following manner. He first made a piece mould in plaster. Inside this he spread a layer of wax of the exact thickness he wished the metal to be. (It might be thicker in parts, where required for strength.) When he had covered the inside of the mould in this manner, he made an iron framework to support the core. The extremities of this framework rested in recesses cut in the piece mould. The core was then built up round the irons. The material used was clay, or loam, in which rags had been left to rot for some months. Two-thirds clay to one-third rags was the proportion employed. When the core filled the entire space within the wax, Cellini wound thin iron wire round it from head to foot, and baked it. The core was given a final coating with a mixture of powdered bone, brickdust and loam. It was then fired again. The layer of wax was now removed, and the surface of the mould greased with bacon fat. An opening was made for the pour, and a number of vents—to prevent any trapping of the air. The core was placed inside the mould and well melted wax poured in. The mould was opened after a day or two, and the final touches given to the modelling of the wax figure. The pour, gates and vents were now made in wax. -


When casting his figure of Perseus, Cellini arranged the pour so that it ran down at the back of the figure to both heels, with many gates. He kept the figure in the vertical position all the while. In fixing his vents, therefore, he was careful to lead them all downwards; he was thus able to melt the wax out without reversing the mould. The vents were afterwards connected to vertical pipes, which rose to the level of the top of the pour. In the case of his Perseus, to make sure that the core should not shift in the mould, and to provide spaces through which the core might be afterwards removed, Cellini cut away the wax at a number of places in the flanks, shoulders and legs. At these points, therefore, the outer mould would be in contact with the core, and support it firmly. The outer mould was made as described above, and bound round with hoop iron. The wax was melted out in a gentle fire. The whole mould was then thoroughly fired. It was afterwards put into the casting pit—a deep hole dug in the floor, opposite the mouth of the furnace. The vents were carried up to the floor level by earthen pipes, and the pit filled up with tightly rammed earth. A walled channel was built from the furnace to the pour, and a fire kept alight in the channel till it was baked thoroughly dry. 

Cellini made both core and mould sometimes from a mixture of plaster, burnt bone and pounded brick. This, he points out, is a much quicker method of making a core than that above described. But unless you are sure of the plaster it is an uncertain one. For poor plaster, or plaster which has been kept too long, will not set firmly. He very properly advises you to try your materials first. He used one part of plaster to an equal portion of pounded bone and brick. The core was made by pouring compo (round the irons) inside the wax lining of the piece mould; the core was then bound round with iron wire and given a final coat of the compo. Then it was fired. The final wax cast was next made as described above, and the compo mould formed round it. When this had reached a thickness of about 1 1/2 inches it was strengthened with iron bands, given a final coat of compo and fired. 

The mould having been formed round the wax model in one of the ways above described, the wax must be melted out. If the work is small the mould may be turned upside down, and heated in an oven until all the wax has run out. For a larger work a rough kiln must be built round the mould and a fire kept up till all the wax has been removed. The mould is then allowed to cool, and any drain holes made for the escape of the wax, which will not be required afterwards for vents, are carefully stopped up with the same material as that of which the mould is made. The fire is then lighted again. And, in any case, whether the mould be large or small, of loam or compo, the firing must be continued till the whole mould is a good cherry-red colour. In this manner the mould is hardened, fusible materials are burnt out of it, and every trace of moisture expelled. It should be hot enough to set fire to a piece of paper or tarred string let down into the pour. For small castings the metal may be poured while the mould is red hot. But in large ones it is first allowed to cool down, for when there is so much metal there is no likelihood that it will get chilled; and there would be danger of melting the face of the mould if that were red hot when the metal entered. 

The mould having been fired, it is usual to bury it in a box or pit, and to tightly pack it round with earth,—which should not be damp. If the vents are not entirely within the mass of the mould they must be carried up to the top of the box, or to the ground leArel, as the case may be. Remem ber that the molten metal will follow the air through them and it may rise as high as the level of the metal in the pour, so the pipes used should be quite dry, and any joints care fully looked to. A hollow or basin should have been formed at the head of the pour, for convenience in getting the metal into it. Some founders make a large enough hollow to hold all the metal required for the work. They fix a fireclay or plumbago plug to the pour, and withdraw it when the basin has been filled. The molten metal then cannot carry any air bubbles down with it; nor can any dross enter the mould, for it will naturally float on the top of the molten metal. The plug must be tall enough to project above the top of the hollow, to enable it to be lifted out with the tongs when the basin is full. 

For small castings, weighing anything up to 12 lb, one of Fletcher's injector furnaces is the most convenient. A gas supply pipe of about § inch internal diameter and a No. 5 foot blower are necessary. These furnaces are simple cylindrical casings of specially prepared fireclay, in which the crucible stands. The jet enters the furnace through an opening near the bottom and the products of combustion escape through a small hole in the lid. For larger work than this a draught furnace of the ordinary foundry pattern is generally employed; although gas furnaces of much larger size are sometimes to be met with. The foundry furnace is arranged like Fig. 292. It has at the bottom an iron grating through which the air comes, and the ashes fall. The crucible is supported on a stand which rests on this grating, a handful of ashes having been previously thrown on it to keep the crucible from sticking. The entrance to the chimney is at the side or back of the furnace, a couple of inches above the top of the crucible. It is provided with a damper, which can entirely close it. A lid covers the top of the furnace. The crucible, ready filled, and covered by its lid, is placed on the stand inside the furnace. A few shovelfuls of red-hot coke are thrown round it, and the remainder of the space inside the furnace, up to the level of the top of the crucible, is filled up with broken coke. The lid of the furnace is replaced and the damper drawn. A series of these furnaces is built along a wall of the foundry; several being heated at once when a considerable amount of metal is required for one cast. 

Of much greater power is the reverberatory furnace, Fig. 290, which is used when a very large amount of metal is required,—in casting a statue, for example. No crucibles are employed in this furnace, the metal resting on the floor of the furnace itself. The floor slopes down to the outlet hole, which is closed with a plug till the metal is required. The fireplace is at one end of the furnace and the chimney at the other. The flames pass from the fire place over a low wall and strike against the roof of the furnace. This, which has a flattened dome shape, drives them down on to the metal before they reach the chimney. A bricked channel, thoroughly dried, must run from the mouth of the furnace to the pour in the mould, where it ests in the casting pit. 

The metal must be our next consideration. Platinum cannot be melted with the ordinary gas blowpipe or in the furnace; a supply of pure oxygen, as in the oxyhydrogen blowpipe, being necessary to fuse it. Gold and its alloys cast well, as do fine and standard silver, see Chapter II. The metal or alloy is generally put in a fireclay crucible with a little powdered borax on top. Pieces of charcoal should be put in also. Powdered charcoal is likely to get into the cast and injiire it. More metal can be added when that put in first settles down in the crucible. Add a tiny piece of zinc to the gold or silver before pouring. Copper does not cast well, so it is generally alloyed. The addition of a little tin, zinc and lead to the copper, produces an excellent alloy. When copper is alloyed with a small proportion of tin the result is known as bronze : if alloyed with zinc it is known as brass. But there are so many alloys in which copper predominates that many people call them all brasses. The ingredients of a few well-known alloys are to be found on page 303. Bronze composed of copper with a small per centageoftin doesnot flow sowell asa similaralloyto which a little zinc or lead has been added; so the bronze used by founders frequently has 3 or 6 per cent, of both tin and zinc and perhaps 1 per cent, of lead. It making these alloys the copper should be melted in a crucible under a layer of charcoal. Pearlash, cream of tartar or even common salt is used as a flux. They are all better than borax. The tin, or tin and zinc should be warmed to near their melting point and then added to the copper. The mixture should be thoroughly stirred: many founders do this with a stick of green wood. Casts are frequently made in lead, zinc or in one of the fusible alloys; these casts being afterwards chased up and used as patterns for casting a number of replicas from. Lead or zinc casts can be made in plaster moulds, thoroughly dried. The surface of the mould is sometimes smoked with a pitch torch to obtain a finer surface. 

Thirteen partslead,3partszincand6partsbismuthmake a useful alloy for small castings. Very sharp casts may be obtained with it, as there is considerable expansion of the mass on cooling. Type-metal acts in a similar manner and gives good casts. 

Spon gives a list of alloys having very low melting points, as follows :— 
"(1) 8 parts bismuth, 5 lead, 3 tin. Meltingpoint202°F. (94-5° C). 
(2) 2 parts bismuth, 5 lead, 3 tin. Melts in boiling water. 
(3) 5 bismuth, 3 lead, 2 tin. Melts at 197° F. (92° C). 
(4) 15 bismuth, 8 lead, 4  tin, 3 cadmium. Known as Wood's patent, has a brilliant metallic lustre, does not tarnish readily and melts between 150° and 160° F. (65|° to 71° C). 
(5) 5 volumes each of bismuth, lead and tin, with 4 of cadmium, form an alloy which is quite liquid at 150° F. (651° C). 
(6) 4 volumes each of bismuth, lead and tin with 3 of cadmium. Fuses at 153£° F. (67|-° C). 
(7) 2 volumes each of bismuth, lead and tin, with 1 of cadmium, or 1 volume of each of the 4 metals fuses at 155|° F. (68|° C). 
(8) 1 part tin, 1 lead, 2 bismuth. Melts at 200° F. (93-5 C). 
(9) 15 parts bismuth, 8 lead, 4 tin, 2 cadmium. Melts below 140° F. (60° C). 
N.B.—All alloys containing cadmium are liable to under go rapid oxidisation in contact with water." 

Although lead and tin alloys are very generally melted in an open iron ladle it is wasteful to do so unless the surface of the molten metal is covered by some substance which will exclude the air. Linseed or sweet oil will do quite well. Otherwise it is better to use a deep, narrow vessel, for molten lead oxidises very rapidly in contact with air, so the less surface exposed the better. The dross which forms on the surface should be removed Math an iron spoon and the metal poured immediately afterwards. 

Copper alloys should be stirred well before the crucible is removed from the furnace. Some founders do this with a piece of charcoal gripped in the tongs, others stir with a green stick. The crucible tongs have curved jaws which securely grip the crucible so that it may be lifted safely. The molten metal is now skimmed to remove any dross or floating charcoal, and tipped into the basin-shaped hollow at the head of the pour. If there is no such holloAv provided, it is well to keep back with a strip of iron any dross, flux or charcoal Avhich may yet float on the metal. The metai should be poured in very steadily, that is to say, in a continuous stream Avithout any stoppages. The metal should run in quietly, without bubbling or Avelling up, and gradually rise in the vents. Continue to pour steadily until the metal ceases to run down. Leave a good mass, or " head," of metal in the pour. Its weight will force the metal below it into all the holloAArs of the mould, if the vents are working properly. If all has gone quietly the head of metal will go a little hollow in the centre as the metal cools. If hoAArever, the metal bubbles and splutters when you pour, either the gates and vents have not been properly constructed or the mould is damp. Li either case the cast will probably be damaged. 

The mould should be broken down as soon as possible after the metal has set. But it must be clone carefully, for the heated metal is easily injured. With a hack saw cut off all the gates and vents, for they are now represented by rods and threads of metal attached to the cast. Saw them off as close to the work as possible. If the core is to be removed, rake it out through any aAvailable opening. In casting large bells it is usual to rake out the core before the metal has had time to cool down, for the shrinkage of the bell on the core might otherwise cause cracks in the work. 

The irons which supported the core should be cut out. The whole work is noAV scrubbed clean; and afterwards, if small, boiled out in pickle. Casts made in the fusible alloys above mentioned cannot, however, be pickled in hot solutions. The holes made by the irons are iioav to be plugged up. They should be tapped, and pieces of metal screwed into them. All rough parts are now gone over with files and riffles, and finally with chasing tools. After tap ping the lugs for the screws which are to hold the cast on to its base if required, the work is ready for colouring.