During the manufacture of this alloy all but a small amount of carbon is eliminated by burning it out, as happens with wrought iron. But the steel is _molten_ or _fluid_ when finished and the slag which has been formed floats on top and is also eliminated, which does not occur with wrought iron, which is thick and pasty at its finishing temperature. [Illustration: No. 74. GRAY CAST IRON. THE CROOKED BLACK LINES ARE GRAPHITE FLAKES ] Therefore, steel contains no graphite and no slag but has only the small percentage of carbon which was purposely put back to give to it its valuable properties. [Illustration: No. 92d. SEMI-STEEL, A STRONGER GRAY CAST IRON ] Photomicrograph No. 22c is typical for steel which contains .50 per cent (½ per cent) of carbon. The irregular patches containing the carbon are much larger and more frequent. It will be seen, therefore, that through metallography the various iron alloys, to a considerable extent, may be analyzed as well as classified. Photomicrographs Nos. 74 and 92d represent soft and stronger grades of ordinary _gray cast iron_ respectively. It will be noted at once that both much resemble pig iron in structure, as of course they should, for simple remelting in the cupola does not effect much modification in composition or structure. Occasionally castings are made from molten iron direct from the blast furnace, but such practice is not very satisfactory and is little done. It forms probably the only exception to the statement made above that pig iron has no useful purpose in the commercial world except as something to be transformed by some refining process into another material. [Illustration: No. 132. GRAY CAST IRON 200 YEARS OLD ] [Illustration: No. 109. MALLEABLE CAST IRON BEFORE ANNEALING ] The remelting for the well-known cast iron is usually of such selected brands of pig iron and cast scrap as will produce cast iron best adapted to the purposes intended. The resulting alloy still contains 3 per cent or 3½ per cent of carbon. No. 132 is interesting. It is a photomicrograph of a piece of cast iron which is approximately 200 years old. From the standpoint of the metallurgist it is the same as other photomicrographs of cast iron, the difference in appearance resulting probably from casting conditions, etc. _Malleable cast iron_ is made much as is gray cast iron except that it is brought to such a composition that sections of castings made from the melt show a white fracture when broken. Castings of gray iron of course show a gray fracture. The former are extremely hard and are as brittle as glass until they have gone through a careful long anneal or heat softening process. By maintaining them at cherry-red heat away from air for sixty hours or more and cooling slowly, they become “malleable”; i.e., not brittle at all but capable of considerable distortion without cracking. [Illustration: No. 35. MALLEABLE CAST IRON ANNEALED ]