Mould (casting)

“ It is often difficult, and always expensive, to purchase superior specimens, of which, however, exact models may be obtained by casting, without the slightest injury to the originals. The mould is made in the following manner:—Take a strip of paper, a quarter or third of an inch wide; roll it twice tight around the rim of the coin, or gem, of which a cast is intended to be taken, and fasten the end with very stiff gum-water, which will hold it instantly. Rub a very little oil, with a camels-hair pencil, over the coin, in order to prevent the plaster from sticking ”

“ From the old method, in making calculations, of using counters, which were thus “thrown” up into a heap, is probably derived the meaning of “cast” for the “casting up” of figures in an account. Further, the word is found for a mould for the casting of metals, and more particularly for the copy of an original statue or relief taken from a mould; similarly, of fossils, for the mineral filling of the empty mould left by the organism. ”

“ When the newly introduced plaster is set the mould is carefully knocked away with chisels, and a true cast appears beneath, giving an entire fac-simile of the original model. Some skill is required in making moulds, in order to provide for projecting parts and under-cuttings; practice alone can teach the artist how to deal with those difficulties when they occur. The above general instructions sufficiently explain the ordinary processes of moulding and casting in plaster. ”

“ These are subsequently replaced piece by piece in the encircling frame, and a core made within it, leaving a space of 1/4 in. or thereabouts into which the metal is poured. The advantage of this process is that the artist’s model is not destroyed as in the cire-perdue, and if a “waster” results, a second mould can be taken. A large statue occupies from one to three months in the moulding.
The extreme tenuity of objects which are hammered, drawn or rolled cannot for obvious reasons be attained by casting. ”

“ Marwood, of casting some wares," remarked Theo.
The older man smiled.
"You have a good memory, my boy," he said. "I did mention casting. It is an independent process, and shapes of great variety can be fashioned as well as ornamented in this way. By the casting method, too, we are certain of getting articles of uniform thickness. We can even make very thin objects by this means. But the process is destructive of moulds, and therefore has its flaws. The success of the plan is entirely dependent on the mould's absorption of the moisture in the clay ”

“ The com bined matrix and mould are then adjusted to the type-casting machine, which manufactures types at the rate of from 25 to about 120 per minute, according to the body. The metal is kept fluid by a little furnace underneath and is injected into the mould by a pump, the spout of which is in front of the metal pot. ”

“ For statuary, and “undercut” work generally, an elastic mould—of glue and treacle (80 : 20 parts) —may be used; the mould, when set, is waterproofed by immersion in a solution of potassium bichromate followed by exposure to sunlight, or in some other way. The best results, however, are obtained by taking a wax cast from the elastic mould, and then from this a plaster mould, which may be waterproofed with wax, black-leaded, and used as cathode. ”

“ The mold is as shown, and the upper side of the metal is at all times exposed to the air. This makes it necessary to have all core prints on the under side, as this side will be the one in view when the casting is finished, and the upper side, as the casting lies in the mold, will be the inside or unexposed side. In case of curved work, reinforcing strips of sheet brass should be placed in the mold and imbedded in the casting, as shown in the sketch by the dotted lines.
A little practice will enable anyone to produce very neat cored castings. ”

“ Where worn or splintered parts of a wood-carving, or fissures in a stone, or deep undercutting which is not essential to the design, occur, it is often of advantage to fill them up with clay or paper, to which a smooth surface can be given, so that the mould will come away free from them when it is dry; and careful notes and measurements will often enable one to restore the contour to a mould which has suffered some pressure in transport. ”

“ The process of moulding vases is the one most in use nowadays, for the simple reason that when once the required mould has been made the production of any number of vases is a rapid and easy task. But though commercially favored, this method is looked down upon by the artistic potter as being purely mechanical, and there is no doubt that a moulded vase has all the characteristics of machine work.
The material used for moulds nowadays is plaster. The clay can either be poured into a mould in slip form or pressed into a mould while soft and plastic. ”

“ It is likewise advantageous in the moulding of certain pieces of sculpture that are not very delicate, and, finally, it is very useful when we have to do with a damaged mould, which, instead of being repaired with plaster, can be fixed with well ground wet sand covered with a strip of muslin. ”

“ When the iron struts and clamps have been removed, and the plaster holding the parts of the mould in position cut away, the mould itself easily comes off. The workmen are guided in this operation by the yellow colour of the mould. All the plaster of that tint has to be carefully cut away until the cast itself is reached. Portions, however, still remain in the hollows and undercuts, and these have to be deftly worked out with the proper tools. ”

“ Plain, or unenriched, mouldings are formed with a running mould of zinc cut to the required profile. Enrichments may be Mouldings. added after the main outline moulding is set, and are cast in moulds made of gelatine or plaster of paris. For a cornice moulding two running rules are usual, one on the wall, the other on the ceiling, upon which the mould is worked to and fro by one workman, while another man roughly lays on the plaster to the shape of the moulding. ”

“ Those who know and appreciate the subtleties of modelling call for a cast made from a mould taken directly from the original and prefer an early cast from that mould rather than a late one, so easily are the shades of surface destroyed. Imagine, then, what a lifeless thing a cast becomes when its surface is covered by layer after layer of calcimine! ”

“ Sometimes a pattern called a templet is used for cutting a block to any particular shape, and when the work is moulded, the templet is called a mould. Moulds are commonly made of sheet zinc, carefully cut to the profile of the mouldings with shears and files. ”

“ Having chosen a good mould for your bowl, brush it inside with ground flint tied in a cotton cloth, so that the clay will not stick to it. Now beat out a piece of well-worked clay on a board, with the flat of the hand, until it is perhaps two inches more in diameter than the bottom of the mould, and half an inch thick. It should be rolled smooth with the rolling-pin. Lay it in the mould, pressing it firmly against the bottom and sides. It may not be out of place here to say that no tool but a wooden one should be used in working in moulds, as metal tools are liable to injure the plaster. ”

“ The mould in some places is black, fat, and thick laid; in others looser, lighter and thin. The foundation of the mould is also various; sometimes clay, then gravel and rocky stones, and sometimes marl. The middle of the necks, or ridges between the rivers, is generally poor, being either a light sand, or a white or red clay, with a thin mould. ”

“ The mould is made and lowered into one of these pits, the pit is filled up, the sand being rammed as hard as possible all around it. When all is ready, the top of the mould, with the cross by which it is to be lifted and lowered surmounting it, presents the appearance represented on the right hand lower corner of the engraving below. ”

“ The shaft or great body of the mould is a hollow cylinder, finely polished in the inside, and open at both extremities. The top of the mould is a small metallic cup, having a moulding within-side, and a hole to admit the wick. The two parts are soldered together, and when united, as will readily be imagined, have the shape of a moulded candle. A third piece, called the foot, is sometimes added; it is a kind of small funnel, through which the liquid tallow runs into the mould, and, being screwed to the opposite extremity of the shaft, is removable at pleasure. ”

“ One end of the mould has a narrow gutter leading from the top to halfway down the mould. This was probably used for the insertion of a piece of iron prior to the metal being run in, thus permitting the easy withdrawal of the block of tin when cool from the mould. This stone also contains a small bevelled ingot or sample mould, 4 inches long, 2 inches wide, and 11⁄4 inches deep. ”

“ After the removal and cooling of the casting pan, the plates were freed from the plaster and the surplus metal cut off. Only one cast could be made, as the mold was usually destroyed in removing the cast. The stereotype was then sent to the finishing department, where the face was cleaned and examined for defective letters, then trimmed on the sides and planed off uniformly on the back to the desired thickness, in the same manner as a stereotype is treated today. A defective letter could be mortised out of the plate and a good type inserted in its place. ”

“ How to Make a Waste Mould.—When a soft or fleshy object is to be cast, one which will yield, and draw out of the mould regardless of undercuts, a very quick and satisfactory process (provided a second copy of the cast will never be wanted) is to make what is called a waste mould. This, with a fleshy subject, is a short cut to a perfect cast, and often saves hours of valuable time. In obtaining casts of mammal heads, legs, or other parts, or casts of fishes and reptiles, it is the method par excellence. Let us learn the principles of it by making a waste mould and cast of a human hand. ”

“ A mold can be made in two pieces of a round object like a ball and if each piece is exactly one-half, it will draw, because there is no point under which the plaster will hold. Any hollows or projections will form "under cuts" necessitating making the mold in a number of pieces that it may relieve or be lifted off the cast. Molds of heads from which to cast paper forms are often wanted and are easily made. With the skinned head of a fox, let us say, on the table, the lower part is embedded in fine sand or clay about on a line with the mouth. ”

“ A mould would be made by pressing the finger into some plastic material, such as fine modelling clay or hot sealing wax, and then, by pouring a warm solution of gelatine into the mould, and allowing it to cool and solidify, a cast would be produced which would yield very perfect finger-prints. ”

“ WHEN THE PLASTER MOLD IS READY TO REMOVE, a table knife slightly inserted between partitions and carefully pryed will cause the parts to separate. A most important part of casting molds and the using of the plaster molds is the oiling. ”

“ Setting up the mould with plaster of Paris.
When the model has been completed, the next thing to be done is to make a mould on it. For this purpose the moulders are called in. Some sculptors do their own moulding; but if they are busy men it does not pay them to do so, the moulding of a large group being a long job. It is, moreover, a strictly mechanical operation, although one requiring much manipulative skill and a fine touch. The first thing the moulder does, when the model is handed over to him, is carefully to consider the way in which he is going to accomplish his task. ”

“ A mould is familiar to all in domestic and industrial matters, but there are two sides to the metaphorical conception. A plastic object may be moulded by the hand of man as in his ruder, but more laborious days, or it may be pressed into an artificial mould that he has made by means of his hands and tools. ”

“ Plaster Mold.—Nearly all fine grades of metals can be cast in plaster molds, provided only a few pieces of the castings are wanted. Dental plaster should be used, with about one-half of short asbestos. Mix the two well together, and when the mold is complete let it dry in a warm place for several days, or until all the moisture is excluded. If the mold is of considerable thickness it will answer the purpose better. When ready for casting, the plaster mold should be warmed, and smoked over a gas light; then the metal should be poured in, in as cool a state as it will run. ”

“ When the wax is all out give the mould yet another but very moderate firing, in order to get rid of any moisture that may be left in the mould. Then you may give it a regular good firing, first casing the mould in a coat of bricks set one above the other, & at about a three fingers’ distance from the mould; this firing should [119] be of soft wood, such as elder, lime, beech or twigs. Any green wood, or the wood of the oak, is to be avoided, and use no charcoal whatever, because all these fuse the clay and make it become like glass. ”

“ These moulds are of various sizes, from a very big one a quarter-inch square, used for fancy walking-sticks—a mere catchpenny, and purchased only by tourists as mementoes—to the little round ones used for putting into pencil-cases and which are called ‘lead-points.’ The intermediate sizes are known as Carpenters, Drawing, Pocket-book, and Programme. A workman receives the thin strip of blacklead as it is slowly forced through the mould, and at intervals breaks it off, carefully placing it on a board between pieces of wood. ”