The first color observed after rubbing a bright spot on a piece of quenched steel is a light straw, denoting the greatest degree of hardness possible for that particular piece of steel. As the heat travels into the quenched end, the hardness decreases, as denoted by the change of colors in the order of light, medium and dark straw, light and dark purple, dark and light blue. These colors are temperature indicators, and as the reduction m hardness of the steel is dependent upon the amount of annealing which the hardened end gets from the heat in the other end, these colors correspond with degrees of relative hardness in regular order. A color, purple for example, does not indicate the same degree of hardness in all grades of carbon steels, as the hardness in different carbon steels at a given temperature depends upon the contained carbon.