The term used to describe any of the hot or cold methods of hardening metal. It is frequently followed by tempering. Some metals, like brass, can only be hardened by a cold working operation, while a heat treatment is used for steel. Mild steel cannot be hardened, except in instances when it can be case-hardened, converting the metal skin into a harder steel. Carbon steel, containing various amounts of carbon, is affected by the temperature to which it is raised, the time it remains at this temperature, the rate at which it is cooled, and the atmosphere in which the work is carried out. Specialist manufacturers of steel tools and parts use carefully controlled methods of heating and cooling the steel, while clockmakers in small workshops acquire considerable skill in judging the correct temperature of the steel before plunging it into water, or oil, or otherwise cooling it to ensure the correct hardness.