My favorite material in the world next to water based clay:
I love waste molds. I always make a pottery plaster waste mold as part of my process. I carve and work the inside the empty mold, and then make a casting in pottery plaster. The plaster casting then gets reworked, and when complete I make a silicone rubber mold on the plaster casting. From the silicone mold I cast the finished piece.
I feel some things work better carved in a hard material. Going through the extra step of making a plaster casting gives me the opportunity to see the piece with a fresh perspective and to use carving techniques to refine forms in a way clay just won't allow. Pottery plaster is soft and forgiving, meaning work goes quickly and tools last longer.