Annealing is a process of slowly cooling hot glass objects after they have been formed, to relieve residual internal stresses introduced during manufacture.
Glass that has not been properly annealed retains thermal stresses caused by quenching, which will indefinitely decrease the strength and reliability of the product.
After annealing is complete the material can be cut to size, drilled, or polished without risk of its internal stresses shattering it.
[3] Stresses acquired at temperatures above the strain point, and not relaxed by annealing, remain in the glass indefinitely and may cause either short-term or much delayed failure.
Stresses resulting from cooling too rapidly below the strain point largely are considered temporary, although they may be sufficient to cause short-term failure.