Having prepared the wax, and satisfied himself that it is of the right quality, the artist proceeds to construct his model. He could make this at once in wax, and then cast it, but in event of a failure his work would be lost. As this is undesirable, he usually proceeds exactly as if the work were to be sent to a foundry, and having prepared his plaster model, he cuts it in pieces and moulds it in plaster, making safe moulds of each part. From these moulds the hollow wax models are produced. Of course, it is far better for the artist himself to re-touch these than that they should be so treated by any other person.
There are various ways of producing these wax models. Some artists cast them by pouring the melted wax into the mould, in which a core has been previously placed; others line out the mould by squeezing into it sheets of wax that have been prepared so that they easily take the required impress. Others, again, brush the liquid wax into the various parts of the mould, and having thus satisfied themselves that the surface is everywhere covered with a thin layer, they put the mould together, and fill it with melted wax, which, after a few seconds, is emptied out, leaving, however, an equal layer adhering to the former coat. This operation may be repeated as often as desirable; each time increases the thickness of the wax, and consequently of the bronze. Making the wax casts sounds a simple matter, and yet it is almost the most difficult thing connected with the waste wax process.
In Italy, where this system is much practised, nearly every formatore can get out good casts, but here it is different, and the sculptor will probably have to train his moulder. It is merely knack and practice, and any intelligent moulder will soon get into the way of it.
Extreme sharpness of impression is not so very important, though that of course counts for something; but it is generally preferable to work the waxes entirely over, and instead of merely retouching, to finish the work in the wax. It is the inside of the wax cast that is very important, because it should have a proper thickness of wax in every part, no more and no less. It is safe, however, to have rather too heavy than too light a cast, as there is no vitreous skin formed by a thick casting, as is usually the case when the piece mould system is employed.
The various parts which compose the figure having been prepared in wax, it is the part of the artist to join them together, and finish the work in wax exactly as he wishes that it should appear in bronze. The wax, being hollow, must be moulded or cored on the inside.
If the work is a very small one, say 18 in. high, you may finish the wax first, and then put a few wires through it at various points, which serve to keep the core in place; you can then pour the liquid core into the wax model, which will be strong enough to resist the pressure during the few moments required for the material to solidify.
If, however, the work is much larger, the wax will not have strength to support its own weight, much less the internal pressure of the core. In such a case, it is requisite that the core be cast before removing the wax model from its mould. The plaster safe mould must in this case be so constructed that it can receive and hold in place the irons which support the core and retain it in position. For this purpose, holes must 5 be made through the walls of the safe mould to permit the irons to project. They must afterwards be held by the walls of the waste mould in which the' bronze is to be cast.
The safe mould then, being lined to a proper thickness with wax, and having the core irons in position, is filled up with the liquid composition used for coring. The usual mixture is plaster and brickdust; some use other ingredients, - indeed over 2 dozen have all been used for this purpose. The proportions are very various, some recommending 1/3 brickdust and 2/3 plaster - and others 3/4 brickdust to 1/4 plaster. The truth is that these materials vary so much in their character that those proportions which are quite satisfactory in one locality are worthless in another. It is best to use as little plaster as will suffice to bind the materials together.
The core, being surrounded by liquid bronze, is under conditions exactly opposite to those of the mould which contains the metal. The action of the metal is exerted to compress the core and to burst the mould. Moreover, its greatest bursting power is exerted whilst it is in a state of perfect fluidity; on setting, it contracts, relieving the pressure on the mould, and powerfully compressing the core. If the core i» too hard and unyielding, it is certain to crack the bronze. It should be what is technically called " puffy," that is to say, somewhat easily compressible; on the other hand, unless it has some ventilation to the outer air, it should not be too porous, or it will generate gas too rapidly by contact with the hot metal. In cores of considerable size it is always safest - indeed, necessary - to provide some means of escape for these gases by venting the core.
When such a vent or " lantern" is used, the core can be made more spongy, and consequently more readily yielding to the compression of the cooling metal. It is, however, a fact that, while modern practice amply justifies the use of a core-vent or lantern, the largest works cast by this method of which we have any historical record were all success* fully cast without any such precaution, nor is there any mention of such a practice before the latter half of the present century.
The wax model with the core inside can be kept for an indefinite time, and thus the artist has ample opportunity to work on the wax, and bring it to as high a degree of perfection as his talents will permit. When, however, he desires to cast it, his first care must be the arrangement of the jets and rents. The jets are tubes destined to distribute the metal throughout the cavity of the mould, and the vents are other tubes which afford an exit to the air violently expanded and displaced- by the molten bronze. The jets are comparatively unimportant, for the bronze, following the laws of fluids, will certainly fill every part of the mould, provided the air offers no obstacle to its progress.