Annealing is one of the steps of firing glass. Kilns are programmed to slowly cool (anneal) glass to eliminate any stress after it is formed. Glass pieces may crack or shatter when subject to small temperature change or mechanical shock and annealing glass is critical to its durability. If glass is not annealed, it will retain many of the internal stresses caused by firing it and significantly decrease the overall strength of the glass. Olympic kilns equipped with an electronic controller can be programmed to heat the glass to the desired form and once achieved, the controller is programmed to decrease temperature to a set annealing temperature to strengthen or harden the piece for long life.
A crucible kiln is built to maintain very high temperatures for a sustain period of time to melt glass. As the glass melts it is gathered and used in blowing glass.