The process of annealing is to heat a material (normally metal or glass) and allow it to cool slowly, in order to remove internal stresses and toughen it.
With glass, annealing is typically a process of slowly cooling the glass to eliminate weakness and make it less breakable after it is formed. This is sometimes done in a temperature-controlled kiln known as a Lehr.
Glass which has not been annealed can crack or shatter when subjected to a temperature change or shock.
In metallurgy and materials science, annealing can help to soften or refine the structure of the material and improve the 'cold working' properties of it.
Watch a short film from Corning Museum of Glass on annealing:
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