When a metal is heated to unequal degrees throughout its mass, or the parts of the mass are cooled at different rates, as is common with large forgings or large castings, or when hammering, rolling, or other work is done on cold metal to change its shape, an arrangement of crystals is forced upon the metal such that some parts of the mass are under stress. For example, a large forging or casting may cool quickly on the surface and, by contracting, pull the hotter parts of the interior into a shape which they would not assume naturally as they cool. Excessive hammering or rolling makes cold metals brittle. In steels, internal stresses and brittleness are most marked when the per cent of carbon is greatest.

Internal stresses, which may be dangerous in many cases, and brittleness in metals worked cold, are relieved by annealing. This consists of heating the mass to a red heat and allowing it to cool very slowly in the case of steel, or plunging into water in the case of copper and some of the copper alloys. Annealing brings metals to their softest and most ductile states.

Steel should be heated to the absorption point for best results and finest grain in annealing, though this is not always done possibly because its importance is not well understood. Steels purposely hardened would lose their hardness if annealed at a high heat, though they may be partially annealed by moderate heating in oil or molten lead, which will relieve the greatest stresses.

Slivers have been known to break and fly with considerable force from extremely hard projectiles while they were awaiting annealing, due to internal stresses.

Annealing is usually done in furnaces not unlike reheating furnaces, though the degree of heat is not so great in annealing as in reheating for working. Coal or wood fires can be better regulated to maintain the low temperatures needed in annealing. To prevent oxidation in annealing, particularly with thin or delicate pieces, the material is placed in a muffle furnace which transmits heat through a brick partition to it, thus preventing contact with the flame, or, better, the pieces are placed in cast-iron boxes, the lids of which are luted with clay, and these boxes with their contents are placed in an ordinary furnace.

Steel forgings and castings demand slow cooling for proper annealing, and it is a common practice to heat them for two or three days in a furnace, then seal up all furnace openings with clay and allow several days for the furnace and contents to cool gradually.

All annealing furnaces must be fitted with pyrometers to gage the degree of heat in obtaining the best results.