This is a plaster shell or mold constructed around a rubber inner mold or used as a backing for support of flexible materials. Rubber molds are generally stored in the mother mold.
Outer, often rough mold made of the back of a mold. Used to secure the mold during the casting process when it is made of fragile or flexible material.
In sculpture, the outer hard mold that supports the inner mold and casting material during the casting process. Most often made from plaster, it can also be made from lighter weight materials such as resins and molding compounds e.g. Magic Sculp.