Iron Molds.—Iron molds have the same disadvantages as wooden molds in the use of wet mixtures. They can be made to mold more intricate ornaments, and in the matter of durability, are, of course, far superior to wood. Iron molds can be ordered cast to pattern in any well equipped foundry. Many firms making block machines also make standard column, baluster, ball and base, cornice, and base molds of various sizes and patterns. These molds are made in two, three or more sections which can be quickly locked together and taken apart. A column mold, for example, will consist of a mold for the base, another for the shaft, and a third for the capitol, each in collapsible sections. Where the pattern of the shaft changes in its height, two shaft molds are commonly used, one for each pattern. Prices of iron molds are subject to variation, but the following are representative figures: Plain baluster molds 14 to 18 ins. high, $7.50 to $10 each; fluted square balusters, 14 to 18 ins. high, $10, each; ball and base, 10 to 18-in. balls, $15 to $25 each; fluted Grecian column, base, capitol and one shaft molds, $30; Renaissance column, base, capitol and two shaft molds, $45.
Sand Molding.—Molding concrete ornaments in sand is in all respects like molding iron castings in a foundry. Sand molding gives perhaps the handsomest ornament of any kind of molding process, the surface texture and detail of the block being especially fine. It is, however, a more expensive process than molding in wooden or iron molds, since a separate mold must be made for each piece molded. The process was first employed and patented in 1899, by Mr. C. W. Stevens, of Harvey, Ill., and for this reason it is often called the Stevens process. Sand molded ornaments and blocks are made by a number of firms to order to any pattern. The process as employed at the works of the Roman Stone Co., of Toronto, Ont., is as follows: The stone employed for aggregate, is a hard, coarse, crystalline limestone of a light grey color, being practically 97 per cent. calcium carbonate, with a small percentage of iron, aluminia and magnesia. Nothing but carefully selected quarry clippings are used and these are crushed and ground at the factory and carefully screened into three sizes, the largest about the size of a kernel of corn. Daily granulometric tests are made of the crusher output to regulate the amount of each size got from the machines. It has been found that next in importance to properly graded aggregates is the gaging of the amount of water used in the mixture. This is done by an automatically filled tank into which lead both hot and cold water and in which is fixed a thermometer to properly regulate the temperature. In gaging the mix about 20% of water is used, but of course when the cast is made the surplus is immediately drawn off into the sand, where it is retained and serves as a wet blanket to protect the cast and supply it with the proper amount of water during crystallization. Experiments seem to indicate that about 15% by weight gives the greatest amount of strength of mortar at the age of six months, while, giving less strength at shorter time tests than mortar gaged with a smaller percentage of water.
The method of handling the mix and casting is quite simple and almost identical with the practice in iron foundries. The mixture is made in a batch mixer to about the same consistency as molasses, from which it is poured into a mechanical agitator and carried about the foundry by a traveling crane. This agitator is so constructed that it keeps the materials in motion constantly and prevents their segregation. In each cast is inserted the proper reinforcing rods, lifting hooks and tie rods, and the casts are allowed to remain for a proper period in the wet sand after they are poured; they are then taken to the seasoning room which is kept at as constant a temperature as it is practical to maintain. Each cast is marked with the number which determines its location in the building and the date it was cast, and it is then kept in the storage shed a fixed time before shipping.
Records are kept of each cast made and the company is able to get, as in mills rolling structural steel, the exact number and location of all casts made from the same mix. Careful records are always kept of the tests of cement and material, and test cubes are made from each consignment of cement so tested; in this way all danger of defective stone through inferior cement is eliminated. The patterns used in making the molds and the method of molding are quite similar to ordinary iron foundry practice except that the sand used is of special nature. The finish of the stone is generally tooled finish molded in the sand, the different textures of natural stone being produced by the veneering of the pattern with thin strips of wood which are run through a machine producing the different finishes. Each stone is provided with setting hooks cast in the blocks which take the place of the ordinary lewis holes used in cut stone.
Plaster Molds.—Plaster of Paris molds are made from clay, gelatin or other patterns in the usual manner adopted by sculptors. They are particularly adapted to fine line and under cut ornaments. The concrete is poured into the plaster mold and after the cement has become hard, the plaster is broken or chiseled away, leaving the concrete exposed. Two examples of excellent work in intricate concrete ornaments are furnished by the power house for the Sanitary District of Chicago, and by the State Normal School building, at Kearney, Neb. In the power house, the ornamental work consisted of molded courses, cornice work; and particularly of heavy capitals for pilasters. These capitals were very heavy, being 7½ ft. long and of the Ionic design. These were made from plaster molds; made so as to be taken apart or knocked down and to release in this way, perfectly. There were also scrolls, keystones and arches in curved design over all of the 40 windows. None of this ornament was true under cut work. In building the Normal School building, Corinthian capitals, in quarters, halves, corners and full rounds were made in plaster molds. There were some 30 of these capitols. They were made in solid plaster molds; the molds having been cast in gelatine molds, one for each capitol. Into these, the concrete was tamped, made very wet, and after the concrete had hardened, the plaster cast was chiseled away. This was very easily accomplished. These capitols were true Corinthians, having all the floriation and under-cut usually seen in such capitols.
ORNAMENTS MOLDED IN PLACE.—Molding ornaments in place is usually, and generally should be, confined to belt courses, cornices, copings and plain panels. Relief work, like keystones, scrolls or rosettes, can be molded in place if desired, by setting plaster molds in the wooden forms at the proper points. This method is often advantageous in bridge work, where comparatively few ornaments are required, such as keystones.
Fig. 293.—Spandrel Wall Mold for Arch Bridge.
The construction of forms for ornamental work in place is best described by taking specific examples. Figure 293, shows the face form for the arch ring, spandrel wall and cornice or coping course of the Big Muddy River Bridge on the Illinois Central R. R. The section is taken near the crown of the arch. The lagging only is shown; this was, of course, backed with studding. The point to be noted in this form is the avoidance of any approach to under cut work; there are, in fact, very few straight cut details. This brings up a point that must be carefully watched if trouble is to be avoided, namely, the construction of the form work in sections which can be removed without fracturing the ornament. To illustrate by an assumed example, supposing it is required to mold the wall and cornice shown by Fig. 294. It is clear that if the backing studs are in single pieces, notched as shown, the forms cannot be removed without fracturing at least the corner A. If the studs and lagging be constructed in two parts, separated along the line a b, the form is possible of removal if great care is used without damage to the concrete. The construction shown by this sketch does not greatly exaggerate matters. Figure 295 shows a wall form that has been given several times as a presumably good example in which, as will be seen it is impossible to remove the board a, without breaking the concrete even if the narrow face were not broken by the swelling of the lumber before ever it became time to take down the forms.
