When iron is fused in smelting, it gets its first carbon, the amount depending upon its temperature, and upon the manganese, silicon and other substances present in the furnace. This amount increases with the temperature and is influenced in different ways by the other substances contained, but is rarely less than 1.8 per cent. When the metal solidifies, its carbon may assume either an invisible form, called combined carbon, in which case the metal shows a white fracture; or it may be visible in the fracture, giving a mottled or grey color, and known as uncombined carbon or graphite. The condition assumed by the carbon as the metal solidifies depends (1) upon the rate of cooling, and (2) still more on the kind and quantity of the other substances contained by the metal.

* The International Association for Testing Materials has, for specific reasons, based upon investigations of Messrs. Carpenter & Keeling, recommended that the line be drawn between steel and cast iron at 2.20 per cent of carbon.

As with other solvents, iron forms a non-crystalline mass if cooled rapidly, and none of the carbon is precipitated, thus showing the white fracture of iron, but slow cooling allows formation of metal crystals, and, if the carbon is above 3%, some of it separates as uncombined carbon.

Carbon renders iron and steel hard, less ductile and more fusible, directly according to the amount contained, and it is well to understand that a small alteration in the amount of combined carbon has very marked effect upon the metal, while a moderate alteration in the amount of uncombined carbon has very little effect.