Making a Mold
A mold of most fossil bones is made of two or more parts. When the design of the mold is being planned at the beginning of the molding process, a division or “partline” is made at the place on the bone that will provide the most efficient mold, taking into account the safety of the fossil, Since only one side of the specimen will be molded at a time, the side of the fossil to be molded at a time, the side of the fossil to be molded last is encased in plasticine to protect it. The plasticine is then set into a flat plate or flange of plaster (Fig. 6a).
A row of stainless steel rods and plastic “locators” with faceted sides are set into the flange of the plaster partline (Fig. 6b). The locators insure that the two halves of the mold will register precisely together, minimizing the possibility of misalignment when a cast is made. Rods are placed on the partline to produce holes which extend through the entire mold. Later, when the fossil is removed and a cast is made from the completed mold, screws are inserted into these holes. Tightening the screws permits a controlled amount of pressure to be applied all around the mold during the casting process.
Once the plaster partline is completed and rods and locators are in place, three coats of molding rubber are brushed over the entire surface of the bone and flange. (In order to insure as much detail as possible, no surface coating is applied to the fossil bone.) After the rubber has cured, a hard plaster backing (the “keeper shell”) is poured. The first section of the mold is then complete and the temporary plasticine and plaster flange is broken away from the bone. The second side of the mold is made in an identical manner.
Making a Pattern
Pattern making begins with the preparation of the mold: a wall made of dental wax is constructed around the face to prevent the epoxy and plaster used in making the pattern from leaking (Fig. 6b). The mold face is then covered with a 1/4-inch-thick layer of epoxy. Once this epoxy layer begins to cure, a layer of gypsum cement about 1 inch thick is poured over the epoxy to form an outer reinforcement. Thus the pattern is composed of a layer of epoxy, which captures the detail of the surface of the bone, backed with gypsum cement.
Making a New Mold
When a new mold is distorted or torn as a result of pattern making or in the production of casts, a new mold is made from the pattern. In this relatively simple process, silicone rubber is brushed on the pattern. After curing, the new flexible mold is removed. The locators and steel pins that were designed into the original mold make it possible for these new generations of molds to align exactly when taken from the pattern.