Phosphorus, Sulphur and Copper are the most noted impurities which occur in steel. The first two are practically always present in greater or smaller amounts as the case may be. The best processes of tool steel manufacture are capable of producing steels with no copper. While Aluminum is not generally classed as an impurity, it nevertheless sometimes shows up in the finished product when its presence is not desired, and therefore, might be considered an impurity. Combinations of iron with some or all of the above elements in the form of slags and oxides are other well known impurities. From the forgoing pages it must be evident that producing a steel with exactly the correct chemical content is only =one= step towards securing a satisfactory product. However, it might be well if we were to briefly sum up a few of the more important features of our discussion on this interesting subject. HEAT TREATMENT. The heat treatment of tool steels is of the utmost importance. Tool makers of the old school proved their ability to accomplish certain desired results in the art of heat treatment without really fully understanding exactly how or why they were able to do so. Today, however, progressive manufacturers are using the results of research and such thorough scientific investigation that the process has become far more complicated and complex, and the results obtained are correspondingly more remarkable. Chemically perfect steel may be easily and completely ruined during the process of melting, cogging, rolling, hammering, annealing, heat treating and tempering. It is the business of the steel manufacturer to carefully guard his product up through the process of annealing, but it usually falls to the tool maker to undertake the delicate operations of heat treatment and tempering. HARDENING. The application of heat alone to steel can very materially affect the condition of the structure of the metal, either with or without simultaneous mechanical treatment. Depending upon the degree of heat, the rate of heating and cooling and the duration of such treatment, this application may be decidedly beneficial or harmful as the case may be.