The process by which metallic, and other mineral productions, are converted from a brittle to a comparatively tough quality, presumed to be caused by a new arrangement of their constituent particles. In a considerable number of bodies that will bear ignition, it is found that sudden cooling renders them hard and brittle, while, on the contrary, if they are allowed to cool very gradually, they become softened or annealed. We have, however, noticed several alloys of copper (brass in particular) in which sudden cooling has the reverse effect, that of annealing it. The pi*ocess of annealing requires some address and experience to perforin it in the best manner; and varies in the degree of heat applied, as well as in the period of cooling, according to the nature of the metal or other substance operated upon. In the annealing of steel and iron, the metal is heated to a low redness, and suffered to be gradually reduced in its temperature, covered up, on a hearth. Ovens are constructed for this purpose, wherein the pieces of metal, according to their massiveness, and the quality it is desired they should possess, are placed and retained at a low heat for days, and sometimes weeks together.
The annealing of glass is performed precisely in the same manner.