The “precipitation” of graphite which is necessary for softness is brought about mainly through the influence of silicon, which we before termed the “softener.” Other conditions being equal, the higher the silicon (if not above 4 per cent), the higher will be the graphite and the lower the combined carbon; and vice versa, the lower the silicon the lower will be the graphite and the higher the combined carbon. It is mainly due to the “combined carbon” which is left after precipitation of the graphite that the alloy has greater strength, hardness, and closer grain. So, just as the steels are stronger and harder as the carbon increases (in steel all the carbon is combined), so, other conditions being equal, the strengths and hardnesses of the cast irons, within usual limits, increase as the combined carbon increases. [Illustration: No. 92d. SEMI-STEEL. A CLOSER GRAINED AND YET STRONGER CAST IRON ] [Illustration: No. 33e. MOTTLED CAST IRON So-called because it is a mixture of white and gray iron. ] Just here it is interesting to remember that from the standpoint of metallography _cast irons are simply steels in which there is what we might call an impurity or an adulterant, graphite crystals_. It will be seen at once that could these graphite crystals be removed from the cast irons shown in photomicrographs No. 74, No. 92d, No. 30 and No. 31, we would have alloys quite similar in appearance to the steels shown in photomicrographs No. 3b and No. 22c which appeared on pages 77 and 78.[7] Footnote 7: Magnification 70 diameters. [Illustration: No. 7. WHITE CAST IRON ] So the softer cast irons which are used for valves and fittings, machine parts, radiators, hollowware, etc., have high silicon. Parts that do not have to be machined can be of “harder” iron; i. e., made of iron having lower silicon content. Manipulation of the silicon content is not the only method by which the hardness of cast iron can be influenced. Graphite can “precipitate” (i.e., separate throughout the casting) only if sufficient time is given it to do so. That is, the cooling of the casting after pouring must be sufficiently slow. In a sand mold the iron remains molten for a time, and after solidification it cools slowly enough that the greater portion of the carbon separates as graphite. Therefore, castings of proper composition made in sand molds are soft and machinable. If the iron is poured into a mold the surfaces of which are made of iron, the molten metal upon entering becomes solid almost as soon as it takes the form of the mold, and it cools with great rapidity. Under such conditions the carbon of the alloy is denied the time necessary to change into the graphitic form and the casting has a white fracture and is so hard that it cannot be machined. [Illustration: SECTION OF CHILLED CAR WHEEL Showing white iron rim. ]