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.
The core is the substance with which is filled the hollow left in the mould after the liquid wax is poured out of it; if the bust were cast in bronze without a core, it would come out solid and weighing 10 or 15 times heavier than is necessary, and the casting itself would be faulty, owing to the great shrinkage produced by such a mass of molten metal, which would also have the effect of vitrifying the earths forming the mould. The core is, in fact, indispensable in the reproduction of artistic bronzes. The core in use at the Brussels Compagnie des Bronzes is formed of a mixture consisting of 2 parts of fine plaster of Paris, and 3 parts of a pulverized earth composed of quartz sand, thin argillaceous clay with traces of iron oxide, carbonate of lime, magnesia, and potash, mixed together with pure water, forming a liquid paste which is called "potin," and which, like plaster of Paris, hardens very rapidly.
Having calculated the capacity of the hollow left by the wax, a quantity of "potin," sufficient to fill it, is prepared and poured into the hollow, leaving enough of the mixture to form a pedestal projecting about 4 in. from the bottom of the bust The core, having been thus poured into the hollow, is left to harden.
Before proceeding further it is necessary to describe the means by which an escape is provided for the air or gases of the core, which, if not set free, might destroy twist, or otherwise injure the bronze.
This is effected by what is called, in the language of the foundry, a "lanthorn" or chimney, by which the core of every work in bronze must communicate with the external air. The core being composed of porous matter, it is easy to understand that when the molten metal enters the channel prepared for it, the core being completely isolated and superheated, the gas within it is violently dilated, and would force a passage through the fused metal if a vent were not prepared for it. If, owing to an accident or faulty arrangement, the lanthorn should not act, the bronze figure containing the core would be inevitably bulged and distorted, and would have other defects which would considerably diminish the value of the work.
In the case of the bust already described, when the piece-mould is emptied of the liquid wax that has been poured into it, and just as the "potin" which is to form the core is about to be poured in, a round stick, about 5/8 in. in diameter, having a pin or iron point at the end, after being well oiled, must be fixed into the centre of the hollow of the bust, so that the pin should project through the wax of the top of the head. The stick must be held in this position while the "potin" is poured in round the stick, and when the "potin" begins to harden, which it will do in a few minutes, the stick is twisted out, leaving, of course, a hollow the size of the stick traversing the bust from the base to the head. After the artist-sculptor has retouched the wax bust, the mark left by the point of the stick is sought, and sufficient wax is removed round it to permit of a small iron tube of the same diameter as the hole left by the stick being forced 2 or 3 in. deep into the head, leaving, however, a portion projecting from the head and beyond the block-mould when it is formed over the wax bust.
Any crack that may appear between the tube and the hole is carefully closed, and the wax is retouched where the tube projects from the head. If the tube were not forced sufficiently into the head, or if the joint were not properly closed, the molten bronze would find a passage and fill up the chimney left for the escape of air from the core - an accident which would give rise to effects like those above referred to. In complicated pieces the proper formation of the lanthorn is of the greatest importance; it is often difficult to arrange, and requires considerable experience to make and place it properly. The precise proportions of the earths of which the "potin" is composed is the only part of the process concerning which any reserve is shown.
The mould is then placed on the table, the cords are unfastened, the clay closing the joints of the 2 copes is removed, and by inserting a wedge between the 2 copes the upper cope is carefully lifted off. The workman then removes one by one all the little pieces forming the mould, exposing the corresponding parts of the bust in wax. When all the pieces are removed from the front, the bust is placed upright on its base of "potin" and the cope covering the back is then removed in the same way, together with the pieces forming the mould. These pieces are then carefully returned to the cope each in its place, and the mould when put together again is ready to be used for another was bust when required.
The bust now appears in wax reproducing exactly the original bust in clay, with the exception of the seams from the joints of the mould, which are then removed by the artist-sculptor himself. Although wax is neither as easy nor as pleasant a material to work in as modelling-clay, a very short time suffices to enable the sculptor to manipulate it with facility, and an opportuuity is afforded him of giving the finishing touches to his work with still greater delicacy than in clay.
It is at this period that the beard and curls of the hair which were removed before making the mould, and which have been separately reproduced in wax by the same process, are fixed in their respective positions by iron points which are driven through the wax into the solid core and hold the pieces firmly in their places; the artist then going over the joints with a modelling tool renders them invisible.