MEDIUM CARBON TOOL STEEL (.86 PER CENT C) AS CAST (Magnification 400 diameters) ] Carbon is the element the presence of which confers upon iron the ability to harden when _cooled suddenly_ from a cherry-red heat, as by quenching in water or oil. If the steel contains less than four-tenths of one per cent of carbon it has little or no hardening power under this treatment; but steel with six-tenths of one per cent or more of the element, has the wonderful property of being slightly malleable when in the annealed state, but extremely hard and brittle after this sudden cooling—leads a dual life, so to speak. [Illustration: LOW CARBON TOOL STEEL (.50 PER CENT C) HARDENED (Magnification 100 diameters) ] At any time, hardened steel may be returned to its former condition of softness by the well known process of annealing, wherein it is reheated to the same cherry-red heat and _slowly cooled_. At the will of the blacksmith or metal worker alternate hardening and softening may be repeated a great many times without apparent deterioration. Various degrees of hardness also, may be obtained according to (1), the percentage of carbon in the steel, and (2), the completeness and suddenness of the cooling. As considerable brittleness and internal strain in the metal necessarily follow hardening, the hardness is usually “tempered” or “let down” by a careful reheating to a much lower temperature, usually 425 to 550 degrees Fahrenheit. From this temperature a second quenching “fastens” the temper at whatever of the original hardness the steel retains at the temperature chosen by the smith for the second quenching. Much of the brittleness is in this way relieved. The smith calls it “toughening” the steel. Tools so treated are much less liable to break. The steels that will harden (we will call them “carbon tool steels”), range ordinarily from the .60 per cent carbon variety, used for hammers, cold chisels, etc., to those containing 1.50 per cent of carbon which are selected for razors, scalpels, and other tools requiring high temper. Each one of these many grades is susceptible of a wide variety of temper in the hands of a capable man, who must select his steel and give to it the most desirable temper for the work for which the tool is designed. [Illustration: MILD STEEL PIPE (.10 PER CENT C) (Magnification 70 diameters) ] Blacksmiths and other tool makers become extremely proficient in judging steels and the proper temperature at which each should be hardened and “drawn” (tempered). They judge temperatures solely by the color of the steel when heated. Every five or ten degree change imparts a slightly different shade as the steel grows hotter in the forge fire or cooler when about to quench.