Supposing that an ornament, represented in section in fig. 176, has been modelled in relief, either in clay or wax upon a Hat board, from which a thin casting in brass is wanted without the tablet, the process is called reversing, and is to be accomplished in any of three ways.
First, an empty flask is placed upon the board, 176, and rammed full of sand; it assumes the appearance of 177; the second part of the flask is attached to 177, and filled to make the part 178, which is called the back-mould; some clay is then rolled out to the intended thickness of the casting, with a cylindrical roller running on two slips of wood or on two wires, and a narrow band of this clay is placed on 178, around the figure, that it may separate 177 and 178, exactly to the required distance, ready for receiving the metal.
By the second mode, 177 is first made, then 178, and from the latter 179 is moulded, which is a counterpart of 177. A thin sheet of clay is then pressed all over 179, into every cavity, and cut off flush with the plane surface of the mould, by which it assumes the appearance denoted by the double line in 179. After this 178 is destroyed, and made over again in 179, but so much smaller than before as the thickness of the clay lining; when the new back-mould, 178, is placed in contact with 177, it leaves the required space for the intended casting. This mode is only preferable to the first, when many parts of the work are nearly perpendicular; in which case, if the first mode be adopted, a portion of the back-mould, 177, must be pared away at the perpendicular parts, and if incautiously performed there will be a risk of irregularity of thickness, or even of holes in the casting.
The third mode, is to take a casting of 176 in plaster of Paris; when this is thoroughly dry it is oiled, and poured full of a cement of wax, grease, and red-ochre, which is poured out again when partially set, leaving a thin crust behind (as in the pewter handle). A second, a third, or more layers of wax are thus added until the whole is sufficiently thick, when the wax shell is extracted, and then moulded from in the ordinary manner: the first brass casting is finished and chased to serve as the permanent pattern. The management of the wax requires practice.
In constructing such moulds additional care is given to every part of the work; for example, the sand is sifted much finer, the parting is made with fine charcoal dust, and the facing with charcoal and rottenstone mixed together in about equal parts, the mixture being of a slaty colour; sometimes the loam stone, which is found in the pits where clay for making tiles is dug, is used instead of rottenstone. The moulds are well dried in an oven, or over the mouth of the furnace, end the faces are after-wards smoked over a dull fire of cork shavings; this deposits a very fine layer of soot over the face of the mould, which greatly assists the running of the natal; when this additional care is taken the works are known as fine-castings.
In easting figures, such as busts, animals, and ornaments consisting of branches and foliage, considerably more skill is required; the originals are generally solid, but the moulds necessarily divide into very many parts. (See note A A, p. 974 Appendix, Vol. II.) Most persons will have had the opportunity of judging of the complexity of these moulds, from similar works in plaster of Paris, which are frequently purchased by artists and the virtuous before the seams of the mould are removed
A glance at these plaster-casts, at the complex and undercut form of many of these ornamental works, and at the explanatory diagram on page 318, will convey some notion of the method to be pursued as well as of the trouble attending them. It is shown for example, by the diagram just referred to, that all figured works approaching to the circular or elliptical section, require that the mould should be divided into at least three parts, except under most favourable circumstances. In the human figure and quadrupeds, the four limbs and the trunk require at least three parts each, and often many more; it will be easily conceived therefore that such moulded works require considerable skill and patience.
Piece after piece of the mould is successively produced, just as in making the core, fig. 175, p. 339, every piece embracing only so much of the figure, as in no part to require any core to overhang the line in which it is withdrawn. The side of the mould in which the figure is partly embedded is first dusted with charcoal, and then the first core is very carefully rammed into the nook, and pared down to the new line of division; the green or wet sand core is then dusted, and the second core is made, and afterwards dusted, when the moulder proceeds with the third core and so on; every one being carefully adapted to its neighbour and withdrawn, to see that all is right) before the succeeding core is proceeded with. The relative positions of the cores amongst themselves arc readily recognised and maintained by the irregularity of their forms, as in a child's dissected map, or by making a notch or two here and there, which are faithfully copied in the succeeding piece. It is frequently necessary to thrust two or more broken needles through the green cores into the neighbouring parts to connect them together, in imitation of the pins in the flasks.
All the parts of the mould are dried in the oven, and the facings are smoked over a cork fire as before explained; the perfection of the casting is augmented by pouring whilst the mould is still slightly warm, as otherwise on cooling it has an increased affinity for damp; but the mould when hot is more or less filled with aqueous vapour which is equally prejudicial.
"When a figure, such as a bust, is required to be cast hollow from a solid model, it is first moulded exactly as above. The core is now produced as follows: at the foot of the bust a large space, nearly equal in length and bulk to the bust, is cut away in the sand, to serve for fixing the core in the mould, or for the balance, as it is called, as the core cannot be propped up at both ends. The entire hollow, that is for the bust and the balance, is filled with a composition of about one part of plaster of Paris and two of sand or fine brick-dust, mixed with a little water and poured in fluid, a few wires being placed amidst the same for additional support.
The mould is now taken to pieces to extract the core, which is then dried, thoroughly burned, and allowed to cool slowly (which the founder calls annealing, from a similar method being employed in annealing or softening the metals and glass): the core is then returned to the mould, to see that it has not become distorted. If needful the fitting around the balance is made good to suit the reduced magnitude of the core, which latter is then so far pared away as to leave room for the thickness of metal; this is frequently regulated by boring holes at many parts of the core with a stop-drill, having a collar to prevent its penetrating beyond the determined depth; the surface of the core is now pared down to the bottoms of the holes, as uniformly as possible. When the mould has been faced, dried and smoked, the whole is put together for pouring, for which purpose the figure is inverted and filled from the pedestal.
Equestrian and other figures are sometimes cast in two, three, or more pieces, and joined together by solder, screws, or wires; but in all such works the aim of the founder is to leave little or nothing for the finisher of chaser to do.
Some objects which arc cither exceedingly complex in their form, or soft and flexible in their substance, and which do not thenfore admit of being moulded in sand, in the ordinary manner of figure casting, may be moulded for a single copy, provided the originals consist of substances which may be either readily melted or burned into ashes.
A canty is made in the sand of the moulding-trough, a little larger and longer than the object, or else a wooden box of appropriate size is procured, in the midst of which the wax model may be placed; to the end of the model is added a piece to represent the runner, which will be required for introducing the metal. The composition of one-third plaster of Paris and two-thirds brick-dust, mixed with water, the same as for the core of the bust, is then poured in, entirely to surround the model. The mould is first slowly dried, it is then inverted and made warm to allow the wax to run out, after which it is annealed, or burned to redness, and lastly, when cooled, it is buried in sand and filled with metal. The method necessarily throws the chance of success upon a single trial as the model is destroyed.
Should the face of the casting be required to be particularly smooth, a small quantity of brick-dust is washed, (in the manner practised with emery, and to be explained,) and mixed with very fine plaster; a coat of this is brushed over the model, which excludes air-bubbles, the model is quickly placed in its cavity, and the coarser mixture is poured in as before.*
The above method exactly corroborates a mode long since described as being suitable to casting copies of small animals or insects, parts of vegetables and similar objects; these are to be fixed in the center of a small box, by means of a few threads attached to any convenient parts, one or two wires being added to make air-holes, and ingates for the metal. A small quantity of river silt or mud, which had been carefully washed, was first thrown in and spread around the object by swinging the box about; and when partly dry, successive but coarser coats were thrown in, so as ultimately to fill up the box. When it had become thoroughly dry, the wires were first removed from the earthy mould; it was then burned to reduce the object to ashes, and when every particle of the model had been blown out, it was ready to be filled with metal.*
* This mode was practised in outing the feather of the equeatrian statue of George III erected in Pell Mall, London.