Firing of the green ceramic shell is necessary to address the hydroscopic moisture associated with the ceramic. This is the dot H2O water which must be driven off to thermally stabilize the ceramic. Calcining of limestone to lime is a good example of what is involved. The shells are typically fired at 1800F for a minimum of one hour. Also typically, the hot shells are removed from the firing furnace and metal is poured into them while still hot. This can vary depending on the exact selection of the ceramic materials. Fused silica ceramics will transform to a crystalline silica upon cooling and lose the needed expansion characteristics while aluminas can be cooled to room temperature and re-heated for pouring if desired. The firing step will also burn any remaining wax trapped in corners or such of the shell.