First, the artist creates the sculpture out of clay. Then he pours either rubber or plaster over the sculpture to create a mold. When the clay is pulled out, the hollow area is a copy the original sculpture.
Next, wax is poured into the mold, creating a copy of the sculpture in wax. The artist must perfect any detail that didn’t come out right in the wax cast. Another layer of wax is poured over the cast to create a new hollow copy of the sculpture. Since Degas’ sculpture was made out of wax, his family would have started here when making bronze casts.
This hollow wax shell is covered with a fireproof material called an investment. When the investment has hardened, the wax is melted away. Into the investment, the artist pours the melted bronze. Once the bronze has cooled and hardened, the investment is broken and pulled away leaving the bronze cast.
All that is left to do now is clean up the cast and fix anything that didn’t come out of the invested smoothly. Often this means smoothing out seams and fixing small details such as eyes or fingernails.
I have created an illustration of the process of lost wax casting which you can find below.
This is not something you could do at home. In fact, many artists send the hollow wax mold to a professional rather than melt and pour the bronze themselves.