Ferrous metals and glass are annealed by heating them to high temperatures and cooling them very slowly; copper and silver, however, are best annealed by heating and cooling quickly, then putting in water. Large masses of metal or glass are cooled within the heating furnace; sheets are usually annealed in a continuous-process furnace. They are carried on a moving table through a long chamber in which the temperature is carefully graded from an initial value just below the softening point to that of room temperature at the end. Annealing time, especially of glass, varies widely according to the thickness of the individual piece; window glass, for example, requires several hours; plate glass, several days; and glass mirrors for reflecting telescopes, several months. Annealing is required as an intermediate step in metal-forming processes such as wire drawing or brass stamping in order to restore the ductility of the metal lost because of work hardening during the forming operation.

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