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.
A window such as shown in Fig. 8 is sometimes so wide that it requires a mullion, and the design of this mullion must be consistent with the rest of the building, no matter what its construction may be. There are three ways of constructing the mullion: (1) with an upright I beam, and cast-iron facing, as shown in plan at (a);
(2) with a cast-iron box on which the ornament is cast, as at (b); and (3) a modification of the two preceding, which consists of a T-shaped casting with the ornament on the face, as at (c). The advantage of the first method is that the ornament may be applied without regard to the structural support, while in the other two, sufficient allowance must be made in the thickness of the metal to proportion the support to the load.
In the first example (a), the cast-iron facing is screwed or bolted to the I beam. The decoration may be a simple panel as shown, or a highly ornamented arabesque, according to the requirements of the design, for so long as the sides are plain, the casting of the mullion presents no particular difficulty, being but a plain panel, the pattern for which would be of wood about 1/2 inch thick with the sides slightly beveled or drafted to allow it to be drawn freely from the mold.
The length of the mullion is 8 feet; therefore, the pattern should be 8 ft. 1 in. in length, to allow for shrinkage.
If the design of the mullion be ornamental, the pattern may be entirely of plaster, or the body of wood and the ornament modeled in clay. If there are to be a number of these mullions in the facade, only one model is made; and, by covering this with plaster of Paris, a mold is obtained from which any number of wax duplicates can be cast of the original, which are then mounted on the body of the pattern for the mullion.
In the second case (b), the pattern should be made in a similar manner to (a), but, in addition, a core box is necessary, in which the sand core is made to be placed in the mold so that the desired thickness of metal is obtained. The sides of this mullion are checked or rebated to receive the window frames, while in (a) and (c) the frame passes back of the reveal or side.
The third case (c) should only be used where the superimposed load is small; its chief advantage lies in the fact that it does away with the use of the I beam without requiring a core for the casting, saving thereby both in the cost of manufacture and in the setting.