Blueing Of Metals, the process of giving a color to metallic substances by heat Iron when heated becomes first of alight, then of a darker gold color, and finally blue. Steel heated to redness and suddenly cooled is rendered hard and brittle. It is restored to any degree of softness by heating it up to certain temperatures and allowing it to cool slowly. These temperatures are precisely indicated by the color of the film of oxide which forms upon its surface. The first perceptible tint is a light straw color, which is produced by the lowest degree, and indicates the hardest temper; the heat required is from 430° to 450° F.; it is used for lancets, razors, and surgical instruments. At 470° a full yellow is produced; it is the temper fitted for scalpels, penknives, and fine cutlery. The temperature of 490° gives a brown yellow, which is the temper for shears intended for cutting iron. At 510° the first tinge of purple shows itself; this is the temper employed for penknives. The purple hue which appears at 520° is the tint for table and carving knives. A temperature from 530° to 570° produces various shades of blue, such as are used for watch springs, sword blades, saws, and instruments requiring great elasticity.

The different degrees of heat may be exactly regulated by plunging the articles in an oil bath, the temperature of which is ascertained by means of thermometers. Blacksmiths usually temper their cold chisels, drills, and other tools, by chilling them from a red heat by immersion in water; a bright spot is then filed upon the point, which is then heated in the forge until this spot has assumed the desired color.