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.
It is a singular fact that melted gold, silver, copper, and iron, if poured hot into a mould, will take an impression of all the details of the pattern from which the mould was made, only if the mould is made of sand. Zinc can be moulded in copper moulds, and that is the principal cause of the low price of spelter or zinc statuettes, known in the trade as imitation or French bronze. The real bronze is an alloy of copper, zinc, and tin, the 2 latter metals forming a very small part of the combination, the object of which is the production of a metal harder than the pure copper would be, and consequently more capable of standing the action of time, and also less brittle and soft than zinc alone would be. Let us follow a statuette through the different processes under which it has to pass from the time it leaves the hands of the artist who has modelled it to that when it reaches the shop where it is to be sold.
The original statuette is generally finished in plaster. The manufacturer's first overation is to have it cut in such pieces as will best suit the moulder, the mounter, and the chaser, for very few statuettes are cast all in one piece. Arms and legs are generally put on after the body is finished. The next operation is to reproduce the different parts of the figure in metal. For this the moulder takes it in hand to prepare the mould. He begins by selecting a rectangular iron frame, technically termed a flask, large enough for the figure to lie in easily. To this frame, which is 2 to 6 in. deep, another similar frame can be fastened by bolts and eyes arranged on the outside of it, so that several of these frames superposed form a sort of box. The workman places the plaster statuette, which is now his "pattern," on a bed of soft moulding-sand inside the first iron frame. The sand used for mould making is of a peculiar nature, its principal quality being due to the presence of magnesia. One locality is celebrated for affording the best sand - that is Fontenay-aux-Roses, a few miles from Paris, in France. This sand, when slightly damp, sticks together very easily, and is well fitted to take the impression of the pattern.
Once the pattern is embedded in the sand, the workman takes a small lump of sand, which he presses against the sides of the figure, covering a certain portion of it. Next to this piece he presses another, using a small wooden mallet to ensure the perfect adhesion of the sand to the pattern. Each one of these pieces of sand is trimmed off, and a light layer of potato-flour is dusted both over the pattern and the different parts of the mould, to prevent them from adhering together. In course of time, the entire part of the pattern left above the first bed of sand, on which it has been placed, will be covered with these pieces of sand, which are beaten hard enough to keep together. Loose sand is now thrown over this elementary brickwork of sand, if it may be so called, and a second iron frame is bolted to the first one to hold the sand together, which, when beaten down, will form a case holding the elementary sand pieces of the mould in place. The workman now turns his mould over, removes the loose sand which formed the original bed of the pattern, and replaces it by beaten pieces, just as he had done on the upper side.
It can now easily be conceived that if the mould is opened the plaster pattern can be removed, and that if all the pieces of sand are replaced as they were, there will be a hollow space inside the mould, which will be exactly the space previously occupied by the pattern. If we pour melted metal into this space, it will fill it exactly, and consequently, when solidified by cooling, reproduce exactly the plaster pattern. For small pieces, this will answer very well; but large pieces must be hollow. If they were east solid, the metal in cooling, would contract, and the surface would present cracks and holes difficult to fill. To make a casting hollow it is necessary to suspend inside the mould an inner mould or " core," leaving between it and the inner surface of the first mould a regular space, which is that which will be filled by the metal when it is poured in. This core is made of sand, and suspended in the mould by cross wires or iron rods, according to the importance of the piece. A method often used in preparing a mould, named by the French cire perdue, will help to illustrate this. The artist first takes a rough clay image of the figure he wants to produce.
This will bo the core of the mould; he covers it with a coating of modelling-wax of equal thickness, and on this wax he finishes the modelling of his figure. The moulder now makes his sand mould over the wax, and, when it is completed by baking the mould in a suitable furnace, the wax runs out, leaving exactly the space to be filled up by the metal. The celebrated statue of Perseus, by Benvenuto Cellini, was cast in this way, and the method is very frequently employed by the Japanese and Chinese. Sometimes flowers, animals, or baskets are embedded in the mould, and, after the baking, the ashes to which they have been reduced are either washed or blown out to make room for the metal. This can easily be done through the jets or passages left for the metal to enter the mould, and through the vent-holes provided for the escape of air and gases.
When the mould has cooled, it is broken to remove the casting it contains; and here is the reason why real bronze is so much more expensive than the spelter imitation. For each bronze a new sand mould must be made, while the zinc or spelter can be poured in metallic moulds, which will last for ever. In this way the pieces are produced with but little more labour than that required to manufacture leaden bullets. These pieces, of course, do not receive the same expensive finish as the real bronze. When the casting is taken out of the mould, it goes to the mounter, who trims it off, files the base "true" prepares the sockets which are to receive the arms or other pieces to be mounted, and hands the piece to the chaser. The work of this artisan consists in removing from the surface of the metal such inequalities as the sand mould may have left, and in finishing the surface of the metal as best suits the piece. The amount of work a skilful chaser can lay out on a piece is unlimited. In some cases the very texture of the skin is reproduced on the surface of the metal. This mode of chasing, called in French chaire, and in English "skin-finish," is, of course, only found on work of the best class. Sometimes pieces are finished with slight cross-touches, similar to the cross-hatching of engraving.
This style of finish, which is much esteemed by connoisseurs, is named " cross-riffled," or riboute. After the chaser has finished his work, the piece returns to the mounter, who definitively secures the elements of the piece in their places.