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, and dowels, are also good as regards the after use of the saw and plane for corrections and alterations.
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 tapering. 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 violence tears down the sand, unless the patterns are much knocked 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 on the entry of the metal into the mould. Either the angle of the model should be filled with wood, wax or putty, or the sharp edges of the sand should be chamfered off with the knife or trowel. Sharp internal angles are very injudicious in respect also to the strength of castings, as they seem to denote where they 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.
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 very small are less 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 sand; next, they should be warmed suffcieiently to melt bees'-wax, which is then rubbed all over them, and in great part removed, and then polished with a hard brush when cold. Wax is also used by the founder for stopping up any little holes in the wooden patterns; whiting is likewise employed, as a (pucker but less careful expedient; and very rough patterns 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 pattern-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 hut 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 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 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 the moulds made of fresh sand 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;