Annealing
Annealing is the slow, controlled cooling of glass that relieves the stresses built up while it was hot. If glass is cooled too fast, the outside sets before the inside, and the finished piece may crack hours, days or even years later.
In a kiln, the glass is held for a time at a temperature just below the point at which it starts to soften, then cooled slowly through the range in which stress could develop. For common soda-lime glass the annealing point is in the region of 510 to 540°C, although it varies with the make of glass, and the correct hold and cooling rate depend on its thickness.
Thicker pieces need much longer annealing than thin ones. A firing schedule always includes an anneal stage, and a good result in fusing or casting depends on it.