It makes a big mess!
Pouring a thin layer of plaster with a little red iron oxide. A thicker layer of white plaster goes on next. After the waste mold is dry, I scoop out the clay, soap the inside and fill the negative space with new plaster.
Using a chisel and a mallet, I break open the waste mold to expose the plaster prototype. I prefer making a mold from a plaster prototype because I can sand and buff it much better than clay. Its also easier to spot imperfections and fix them!