This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
2. Cast Iron is the term given to that quality of iron which exists when the melted metal is poured into a suitable receptacle, or mold, and allowed to cool and solidify. Cast iron is hard, brittle, and somewhat crystalline in its composition - that is, it shows on its fractured surfaces an appearance resembling an aggregation of small crystals, instead of a fibrous fracture like wrought iron. It is used, architecturally, for columns and newel posts, and, under certain conditions, for lintels or beams; but, owing to its lack of elasticity, it is not well adapted to conditions of transverse strain. It is also formed into slabs for the purpose of stair treads, pavements, etc., and some forms of ornament are cast and attached to purely structural details.
3. Molds are the receptacles into which the molten metal is poured to give it some definite form consistent with the purpose the cast iron is to serve in the building. The molds are made by impressing in sand a cavity or a number of cavities which correspond, in shape and relation of parts, with the contour of the object to be cast. The process of so impressing the sand is called molding, and is effected by means of a wood or plaster model of the object, called a pattern, which is so embedded in the sand that it will leave its impress when withdrawn.
4. Patterns are generally made of wood, though sometimes of plaster, and occasionally partly of each. Where there are to be a great many castings of one kind, the pattern is sometimes made of bronze or iron, in order to stand the wear, but even in such cases the bronze or iron pattern would be cast in a mold originally impressed with a wood or plaster model. Whether a pattern shall be of wood or plaster depends entirely on the character of the work. Plain straight work, such as molded panels, stair strings, columns, etc., is generally made of wood; but ornamented moldings, decorated panels, complicated capitals, etc. are first modeled in clay or wax, from which a cast is taken in plaster.
All patterns must have an allowance made for shrinkage - that is, the contraction and consequent decrease in size which every casting undergoes in the process of cooling. In ordinary work this amounts to about 1 inch per foot in every direction, so that the patternmaker in working from full-size drawings must use a shrinkage rule to lay out his work. The shrinkage rule is usually a little over 2 feet in length and the divisions on it corresponding to 1 foot are in reality 1 foot and 1/8 inch, and the subdivisions are proportionately excessive. Consequently, the pattern for a cast-iron bar 1 foot long, 3 inches wide, and 1 1/2 inches thick, would measure 1 ft 0 1/8 in. long, 3 1/32 inches wide, and 1 3/6 3/4 inches thick.
In order that they may be freely withdrawn from the mold, patterns are worked with a draft, or taper. For example, if the object to be cast is a solid bar 1 ft. 3 in. long and 3 inches square in section, as at (a), Fig. 1, it would be found in lifting the pattern from the mold that the sand had a tendency to cling to the sides and thus destroy the mold. But if the sides of the pattern are slightly beveled, as at (b), this tendency is overcome, and the sides of the mold are left clean and sharp; therefore, in all cases where it is possible, all surfaces that occupy a vertical position in the mold should have an allowance for draft. The exact amount of this allowance cannot be given, as in some cases it is desirable that there should be as little as possible, while in others it may be quite considerable without in any way impairing either the purpose or the appearance of the part.
5. Casting is the operation of pouring the molten iron through the inlets - or gates, as they are called - into the sand mold from which the pattern has been withdrawn. All the above described details culminate in the casting. The making of the pattern, with its allowances for shrinkage and draft, the preparation of the mold with its inlets and gates, and all the details attendant thereon, are for the sole purpose of securing a perfect casting, and all conditions and contingencies must be considered before the metal is poured.
6. Suppose, for example, a newel post, such as is shown in Fig. 2 (d), is to be cast with ornamental panels on each of the four sides. The first step is to prepare the model; and, as all sides are alike, one complete side is therefore modeled in clay, from which plaster casts are taken and combined to form the complete pattern shown in Fig. 2 (d). The simplest method of making this pattern is in halves, with the joint at the corners. One-half of the pattern is then laid on a board, as shown at a, Fig. 3; and one-half of the molding box c is laid over the board b, as shown. The sand is then rammed tightly all around and over the pattern, and the mold turned upside down. The board b is now removed, and the other half of the pattern is laid in place. The upper part of the molding box is then placed over the lower part, and filled with sand around the upper half of the pattern, as before. The top part of the mold is now lifted off and the pattern withdrawn, leaving one-half of the mold, as shown at (b), Fig. 4; but, as the casting is to be hollow, a core will be necessary to form the cavity. This core consists of a block or bar of sand, the size and shape of the hollow portion, or inside of the casting, as shown at d, Fig. 4. The core is supported at each end in recesses g formed for the purpose by projections on the pattern called core prints. When the core d is in place, the recess f shows the thickness the metal will be when the newel is cast.