PREPARING FOR CASTING. The equator or half-way mark should be marked on the surface of the object, as one-half of the mold should be made at a time. Instead of one-half, one-third is often used on round objects to permit easier release of the molds. Mold divisions when possible should follow corners of objects.
THE FIRST SECTION of the mold is made by placing a layer of clay along the division lines on the object which have been indicated as division lines for the sections of molds. Sometimes the object can be laid on the table or on glass and the plaster poured around it up to the half-way line as shown in the accompanying plate. In this method a strip of linoleum or metal can be placed around it to make a retaining wall for the plaster.
AFTER THE FIRST SECTION is cast, the model is removed from the plaster and if the plaster has risen past the equator it should be scraped back and indentations or “keys” made in the plaster portions that are not those producing the parts of the model. The model is replaced in its hollow and the entire surface of the mold that is exposed is oiled, including the sides of the metal, linoleum or whatever is used as retaining walls. Plaster is again poured for the second half after which it is permitted to dry before separating the mold sections.
OILING. It must be remembered that the molds must be oiled after each casting as each pouring absorbs the oil. All surfaces coming in contact with the next pouring of plaster must be oiled to cause separation.
POURING. A hole must be cut in the molds to permit the plaster to enter. Air holes leading upward as shown in the engraving are scraped upward out of the molds to permit the escape of the air; otherwise air bubbles may be formed in the plaster causing defects.
TO HOLD MOLD PARTS TOGETHER tie a cord or wire around parts. Notches cut on corners or edges of molds to hold the tieing cord will prevent them from slipping. Molds should fit closely together. If for any reason they do not, the crevices can be filled with modeling wax as a temporary filler. The best results are obtained, however, from perfect molds and it is a saving of time to make a mold over if it is not perfect in the first casting.
RELEASING CAST. After the plaster has set long enough to harden, the molds are opened by gentle prying or tapping with a wooden wedge. Often an obstinate cast is released by placing the mold over a stove or flame for a few seconds. This causes the steam formed in the mold to separate the mold from the cast.
CEMENT BOWLS AND VASES are made by pouring thin cement into the molds and rotating the mold and pouring out the surplus cement. After a few minutes another layer is poured in and the operation repeated. This method is explained more fully in the chapter on Bowls and Vases.
