Hot-working processes used to form a glass object—glassblowing for example—must be followed by a very gradual cooling period called annealing. See what happens when an object is not properly annealed and learn why it breaks.
The process of slowly cooling a completed object in an auxiliary part of the glass furnace, or in a separate furnace. This is an integral part of glassmaking because if a hot glass object is allowed to cool too quickly, it will be highly strained by the time it reaches room temperature; indeed, it may break, either as it cools or at some later date. Highly strained glasses break easily if subjected to mechanical or thermal shock.