Annealing
The process of removing brittleness from castings, glass, and other substances by heating them to a specified temperature in suitable ovens or furnaces for a certain period, and then allowing them to slowly become cool again, prolonging the time in accordance with the nature of the article under treatment. Glass is heated to almost smelting heat to admit of a uniform arrangement of its molecules before it is in a suitable condition for grinding and polishing--from six hours to two weeks being required for this complete operation, according to thickness; heavy plate requires the longest time, the brittleness to which heavy steel-dies are subject is removed by placing them in water, heating to boiling, and then gradually cooled. Lead, tin, and zinc are annealed in the same manner. Heavy castings in bronze or cast iron may be partially annealed by slow cooling in hot cinders.
Annealing is evidently the inverse process of tempering, the latter operation acting to fix the molecular condition of steel by a rapid (not a slow) change of temperature. See TEMPERING; MALLEABLE CAST-IRON; CAR-WHEEL. Animal Casts in Metal.--See INSECT CASTS IN METAL.