Foundry artisans repeatedly pour melted wax into the rubber mold to build up layers of wax inside the mold. When the
wax is hard, the mold is removed. This makes another "positive" like the original sculpture, except it's made out of wax instead of clay.
A crucial difference between the original maquette and this wax copy is that the wax version is hollow. Its thickness depends on how
many times wax was added to the rubber mold. Parts of the wax pattern may be cut off and cast separately in order for the ceramic mold to coat the
inside of the wax pattern as well as the outside. If this weren't done, the bronze casting would be solid, making it too heavy and subject
to problems related to the bronze being of different thicknesses in different parts of the sculpture.
Wax sticks called "sprues" are attached to the wax pattern and connected to a temporary base. These sprues make channels inside the
ceramic mold to allow bronze to be poured into all parts of the mold. Each separate piece of the wax model is attached to the base by its own sprue.
Why not just make the sculpture out of wax to begin with? Some simple shapes like jewelry are often made directly out of wax. Ancient bronze
artisans probably did create their models directly in wax. But modeling clay is a lot easier to work and allows the sculptor to make complex shapes.
The rubber mold also allows the foundry to make as many copies of the wax pattern as the artist wants to cast in bronze.