Perhaps the best method ever discovered for tempering steel, resulting in hardness, toughness, and elasticity combined, is that followed in hardening the blades of the famous Damascus swords. The furnace in which the blades were heated was constructed with a horizontal slit by which a current of cold air from the outside entered. This slit was always placed on the north side of the furnace, and was provided on the outside with a fiat funnel - shaped attachment by which the wind was concentrated and conducted into the slit. The operation of tempering the blades was only performed on those days of winter when a cold strong north wind prevailed. The sword - blade, when bright red hot, was lifted out of the fire and kept in front of the slit, and by this means was gradually cooled in the draught of air. It acquired the proper degree of temper at the single operation. From this account it may be seen that gradual cooling is to be preferred to sudden cooling. While water has been used in the process of tempering for a long time, it was only at a comparatively recent date that hot water was used in place of cold, having various temperatures for various kinds of steel.

For instance, for springs, the steel must not be made more than red - hot, and tempered in water heated to 150° F. (65 1/2° C). Low spring steel containing 0.002 to 0.004 of carbon, when made red hot and plunged into boiling water, has its tenacity and elasticity increased without its softness being altered. The colder the water, the more coarsely crystalline is the fracture, and by using hot water the fracture becomes more finely granulated in proportion to the temperature. By experiment it has been demonstrated that red - hot steel plunged into water at a temperature of 35° F. (2° C.) will be as brittle as glass, but when plunged into water at 212° F. (100° C), it will be annealed and toughened. Water is not essential in the process of tempering. The only necessary condition is that of temperature, and other bodies than water may be used for contributing the right degree of heat. There are various liquids which can be heated to a higher degree than that named without requiring to be kept in a close boiler; such liquids are oil, melted tallow, wax, pitch, etc. Concentrated solutions of various salts in water make its boiling - point higher. It is substances of this kind which constitute the various tempering pickles.

That the chemical constituents of the matter have nothing to do with its effects, but that it is only a question of temperature, is conclusively proved by the fact that when cooled below the freezing - point of water it by no means produces a fine granulated fracture. Oils and fats do not cool off the metal as rapidly as water. The boiling - point of water is comparatively low, and the latent heat of the steam is not very great. Its evaporation therefore carries off the heat which is in the steel with great rapidity. In using oil or melted fats it is not necessary to heat them very much; oil of 100° F. (38° C.) will be equivalent in its effects to boiling water. Jewellers and watchmakers do not temper their drills in water, as this makes them so hard as to be quite brittle. They use oil instead, or they stick the hot drill into solid sealing - wax, pulling it out quickly and sticking it in a fresh place, and repeating the operation until the drill is too cold to enter. Mercury, which is a good conductor of heat, has been used to make steel very hard; by its conductive properties it cools the steel quickly, while its evaporation aids in carrying off the heat rapidly. (Blacksmith and Wheelwright)