SECT. 11.-PLASTER OF PARIS AND SAND MOULDS.

Other examples of metallic moulds might be given, but there are far more frequent cases in which one single casting is alone required; or else the number is so small, or the pieces themselves are so large or peculiar, that the construction of metal moulds would be found almost or quite impracticable, even without reference to an equally fatal barrier, the expense.

In making these single copies in the metals of considerable fusibility, plaster of Paris is sometimes employed; thus, after the printer has arranged the loose types into a page, and the requisite corrections have been made, a stereotype, or solid type, is taken of the whole as a thin sheet of metal, which serves to be printed from almost as well as the original letters; and its small cost enables the printer to retain it for future use, after the types themselves have served perhaps for a hundred similar regenerations, and are ultimately worn out.

The stereotype-founder takes a copy of the entire mass of type in plaster of Paris; this is dried in an oven, and placed face downwards within a cast iron mould, like a covered box, open at the four top-corners. The mould and plaster-cast are heated to the fusing temperature of the type-metal, and gradually lowered into a pan or bath of the same by means of a crane; the hot fluid metal runs in at the corners of the mould, and raises the inverted plaster, which latter would rise entirely to the surface but for the restraint of the cover of the mould.

Type-metal is about eleven times as heavy as water; and if the mould be immersed four inches below the surface, it is subjected to a pressure equal to that of a column of water forty-four inches high, or of above two pounds upon each square inch.

The necessity for this arrangement is shown when a few ounces of type-metal are poured from a ladle on the face of the plaster; the metal looks like a dump, almost without any mark of the letters, whereas the stereotype-cast is nearly as sharp as the original type. The immersion fulfils the same end as the jerk of the hand-caster, or of the pump occasionally employed; and the long continuance of the mould in the fluid metal allows ample time for the air to escape in bubbles to the surface; after which the mould is raised and cooled in a vessel of water, and the plaster is mostly destroyed in its removal.

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SAND MOULDS, FLASKS, OUTLINE OF MOULDING.

Plaster of Paris, although it may be, and frequently is used for the fusible metals, such as lead, tin, and pewter, cannot be employed alone for iron, copper, brass, and many other metals, the intense melting heats of which would calcine the material, and cause it to crumble; even the soft metals should not be very hot, or they will make the plaster of Paris blister off in flakes or dust. We must therefore seek a substitute better capable of enduring the heat, and likewise susceptible of receiving definite forms; for which purpose damp sand, with a small natural or subsequent admixture of clay or loam, is found to be perfectly adapted.

The moulding-sand cannot however be used without external support, and which is given by shallow iron frames without tops or bottoms, called flasks, represented in figs. 160 and 161. The bottom part, 4, 5, is supposed to be rammed full of sand, and to stand upon a flat board, 6. The model of the plain flat bar which is to be cast, is now laid on the surface of the sand, that of the round bar is embedded half way in the same, and the mould is dusted with dry parting sand.

The top part of the flask 2, 3, is shown still empty, and in the act of being attached to 4, 5 by its pins, which enter corresponding holes in the latter, easily but without shake: 2, 3 is also rammed full of sand, and covered with a top board, 1, not represented to avoid confusion. The mould is now opened, the models are removed, and channels are scooped out from the ends of the cavities left by the models, to the hollows or pouring-holes

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at the end of the flask; the parts are all replaced in the order 1 to 6, represented in fig. 160, and the whole are fixed together, so as to assume the condition of fig. 161.

The flask is now placed almost perpendicularly beside the pouring-trough, and the metal is poured into it from the crucible as shown in fig. 138, p. 306; but the flask, if small, is put on the surface of the pouring or spill-trough, and propped up with a short bar.

This brief sketch of the entire process of moulding and casting in sand moulds, will be now followed by some remarks in greater detail: first, on the patterns of the objects to be cast; secondly, on the conditions required in the sand; and thirdly, the process of moulding simple and solid bodies. The succeeding section will be devoted to moulding cored works, and figures, after which a few lines will be given upon the subject of filling the moulds.

SECT. III.-PATTERNS, MOULDS, AND MOULDING SIMPLE OBJECTS.

The perfection of castings depends much on the skill of the pattern-maker, who should thoroughly understand the practice of the moulder, or he is liable to make the patterns in such a manner that they cannot be used, or at any rate be well used.

Straight-grained deal, pine, and mahogany, are the best woods for making patterns, as they stand the best; screws should be used in preference to nails, as alterations are then more easily made in the models, and glue joints, such as dovetails, tenons, dowels, &c., are also good as regards the after use of the saw and plane.

Foundry patterns should be always made a little taper in the parts which enter most deeply into the sand, in order to assist their removal from the same, when their purposes will not be materially interfered with by such taperness. The pattern

maker, therefore, works most of the thicknesses, and the sides or edges, both internal and external, a little out of parallel, or square, perhaps as much as about one-sixteenth to one-eighth of an inch in the foot, sometimes much more.

When foundry patterns are exactly parallel, the friction of the sand against their sides is so great when they penetrate deeply, that it requires considerable force to extract them; and which tears down the sand, unless the patterns are much knocked •

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REMARKS ON FOUNDRY PATTERNS.

about in the mould, to enlarge the space around them. This rough usage frequently injures the patterns, and causes the castings to become irregularly larger than intended, and also defective in point of shape, from the mischief sustained by the moulds; all which evils are much lessened when the patterns are made consistently taper and very smooth.

It must be distinctly and constantly borne in mind, that although patterns require all the methods, care, and skill, of good joinery or cabinet-making, they must not, like such works, be made quite square and parallel, for the reasons stated. Sharp, internal angles should in general be also avoided, as they leave a sharp edge or arris in the sand, which is liable to be broken down in the removal of the pattern; or to be washed down in the entry of the metal into the mould. Either the angle of the model should be filled with wood, wax, putty, &c., or the sharp edge of the sand be chamfered off with the knife or trowel. Sharp internal angles are injudicious, in respect also to strength, as they seem to denote where the castings will be likely to break; and more resemble carpentry than good metallic construction.

Before the patterns reach the founder's hands, all the glue that may have been used in their construction should be carefully scraped off, or it will adhere to and pull down the sand. The best way is to paint or varnish wooden patterns, so as to prevent them from absorbing moisture, as they will then hang to the sand much less, and will retain their forms much better. Whether painted or not, they deliver more freely from the mould when they are well brushed with black-lead, like a stove-grate.

In patterns made in the lathe, exactly the same conditions are required; the parts which enter deeply into the sand should be neither exactly cylindrical nor plane surfaces, but either a little coned, or rounding, as the case may be; and the internal angles should not be turned exactly to their ultimate form, but rather filled in, or rounded, to save the breaking down of the sharp edges of the mould.

Foundry patterns are also made in metal; these are very excellent, as they are permanent; and when small, they are not apt to be blown away by the bellows used for removing the loose sand and dust from the moulds. To preserve iron patterns from rusting, and to make them deliver more easily, they should be allowed to get slightly rusty, by lying one night on the damp

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sand; next, they should be warmed sufficiently to melt bees'-wax, which is then rubbed all over them, and in great part removed; they are polished with a hard brush when cold. Wax is also used by the founder for stopping up any little holes in the wooden patterns; whitening is also employed, as a quicker but less careful expedient; and when they are very rough, they are seared with a hot iron. The good workman, however, leaves no necessity for these corrections, and the perfection of the pattern is well repaid by the superior character of the castings.

Metal patterns frequently require to have holes tapped into them, for receiving screwed wires, by way of handles for lifting them out of the sand; and in like manner, large wooden patterns should have screwed metal plates let into them for the same purpose, or the founder is compelled to drive pointed wires into them, to serve as handles, which is an injurious practice.

The flasks or casting-boxes for containing the sand, are made of various sizes, each side is about 2 to 3 inches deep; they are poured at the edge when placed nearly vertical, but for large brass works the practice of the iron-founder is generally followed, who mostly pours his work horizontally, through a hole in the top, as will be explained. The pins of the flask should fit easily but without shake, or the two halves will shift about and cause a disagreement or slip in the casting. The tools used in making the moulds are very few and simple; namely, a sieve, shovel, rammer, strike, mallet, a knife, and two or three loosening wires and little trowels, which it is unnecessary here to describe.

The principal materials for making foundry moulds are very fine sand and loam; they are found mixed in various proportions, so that the respective quantities proper for different uses cannot be well defined: but it is always judicious to employ the least quantity of loam that will suffice*. These materials are seldom used in their new or recent states for brass castings, although more so for iron, and such moulds are always dried as will be explained.

The ordinary moulds are made of the old damp sand, and they are generally poured immediately or whilst they are green;

The brass-founders' sand and loam used in the metropolis are principally obtained from Hampstead, the iron founders' sand from Lewisham likewise.