Of the several methods of annealing steel, the most common, when but a few pieces are to be treated, is to heat the metal red hot and then bury it in ashes, powdered charcoal, or lime. If the pieces are small, it is advisable to heat a heavy piece of iron red hot and bury it in the ashes or other material, leaving it until the articles to be annealed are heated to the desired temperature. The heavy piece is then removed and the articles are placed in hot ashes. If the small pieces of steel are buried in cold ashes or lime, or if the material is damp, the metal may be chilled and thus rendered hard. Large pieces are not easily affected; but for the process of annealing to be satisfactory, the metal must be dry and warm.
As steel is sensitive to heat, it is necessary to exercise extreme care when annealing or hardening it. If it is heated above a certain temperature, the grain of the metal is opened and made coarse. If the piece is broken, the appearance of the fracture is granular. Such steel is weak and for this reason steel should not be overheated.