Sculpting in plaster is different from sculpting in clay, both in the size and type of figure which can be made, and in the means and pace of working. Unlike clay, plaster may be built over an interior support, or armature. (Most kinds of armatures would cause a sculpture modeled in clay to burst in the firing process.) This makes it possible to sculpt a standing figure in plaster. If the armature is sturdy enough, a sculpture in plaster can be very large indeed.
The working pace is very different with plaster and clay. A bag of clay will stay moist for a long time, and the artist can work more or less continuously. Plaster, however, hardens very quickly and must be mixed, and applied, in small batches. The artist is constantly mixing plaster, working quickly to add it to the sculpture, and then cleaning the rapidly-hardening plaster off all of the tools. (This must be done after each batch.) While it is easy to carve into the clay with tools or even with the fingers, hardened plaster must be cut away with knives or plaster rasps.