We now know that when steel is heated above the critical point, and is then allowed to rapidly cool, a very marked hardness in the metal is produced. The degree of hardness so attained will, in general, vary directly with (1) the percentage of carbon, (2) the rate of cooling, (3) and the temperature above the critical point from which the cooling takes place. When the steel comes from the rolling mill and from the finishing hammers it is in this hardened condition. Therefore, in order to render it soft and ductile enough to cut and work up into certain desired shapes, sizes and tools, it is necessary to subject the steel to the process of annealing. This operation is usually undertaken by the steel producer, under which circumstances he is able to control his product through this delicate procedure, and deliver the same to his customers in the best possible condition for their use. ANNEALING. Annealing has for its object: (1) Completely undoing the effect of hardening, leaving the steel soft and ductile (2) refining the grain, in which case the crystals are allowed to re-arrange and re-adjust themselves, usually growing to a rather large size (3) and removing strains and stresses caused by too rapid cooling. Such cooling strains are particularly likely to exist where the rate of cooling is different in different parts of the bar, but the process of annealing ought to remedy any such condition, leaving the steel soft, ductile and of refined and uniform crystalline structure throughout. The process of annealing is easier to explain than it is to actually put into practice. The steel is first packed in lime, charcoal, fine dry ashes or sand, and then sealed in long air-tight tubes or boxes. The whole receptacle is next slowly brought up to a dull red heat, of about 1500 degrees Fahrenheit. It is very important to heat the material uniformly all the way through, and then hold it in this condition from three to eight hours. Thus, allowing the slipping of one allotropic condition into another. The receptacle must be cooled equally slowly, either allowing the packed steel to cool slowly down with the furnace, or by placing the same in a soaking or cooling pit, which also accomplishes the desired result. After the receptacle has become entirely cooled it is opened and the steel unpacked and removed. The steel is then ready for its final inspection before shipping to the tool maker. TEMPERING. The process of tempering usually has to be undertaken by the tool maker or user after the annealed steel, which he received from the steel mill, has been cut up and shaped into the desired form and size. The main object of tempering steel is to re-harden the material to such an extent that it will cut other metals, retaining its desired shape size and cutting edge, while at the same time it must not possess too much brittleness. The treatment varies materially with different brands of steels.