When the sculpture is completely finalized, a mold must be made of the original. Excellent surface replication of the original can be achieved with a latex or silicon rubber that is formulated to be able to pick up all the minute details in a sculpture.
The rubber mixture is applied directly to the surface of the clay, using a brush to paint it onto the surface. Special attention must be be given to avoid air bubbles that may be trapped within the rubber mixture.
All of the details from the original clay are now picked up within the rubber material that has been painted on. A mold generally consists of three to five coats of rubber (applied over the course of several days).
Once the final coat of rubber has dried, a firm outer shell is made to help retain the shape of the more flexible rubber mold for pouring the wax replica. This shell is usually made out of plaster, or fiberglass and is called the “mother mold”.
When all of this is complete and it is all dry, the outer shell “mother mold" is removed, and the rubber is then cut away from the clay, on what is called a parting line (simply the division line for the mold). The sculpture has now gone from a positive form to a negative form.
If the sculpture is large or complex, it will usually have to be divided into smaller pieces, with each piece needing its own individual mold. The pieces will be rejoined later, after the sculpture is cast in bronze, to make the complete sculpture whole again.