The metallography of wrought iron, cast iron, malleable iron and steel differentiates them to us with considerable accuracy, as is shown by a glance at the accompanying photomicrographs, as photographs taken through the microscope are called. For our immediate purpose of gaining a general knowledge of the relative positions of these products, this method of analysis probably cannot be excelled. After the processes of manufacture of these materials have been taken up one by one in later chapters, the photomicrographs will be even better understood than now, as will the differences of chemical composition and physical properties of the alloys, such as strength, hardness, brittleness, forging quality, etc. The photomicrographs are given at this time to show that the materials are structurally very different and to aid in the general classification. To make them comparative, all have been taken at the same magnification of seventy diameters; i.e., the microscope has made everything shown just seventy times as large as it actually was in the alloy. [Illustration: No. 198. PHOTOMICROGRAPH OF SAND CAST PIG IRON. THE THICK BLACK LINES ARE GRAPHITE FLAKES ] As stated before, the alloy _pig iron_ normally contains from 3 to 5½ per cent of carbon. This was absorbed during the journey through the blast furnace. As long as the iron was molten all of this carbon was in the “combined” form; i.e., in chemical combination with the iron itself. Cold iron, however, cannot retain in the chemically combined form as much carbon as does molten iron, so, during the solidification and cooling of the alloy, more or less of its carbon was precipitated, i.e., thrown out of solution and from chemical combination with the iron, the amount depending mainly upon the speed of the cooling. It appeared then as the “free” carbon (crystalline graphite) which remained distributed throughout the alloy and may be seen as the jet black flakes in photomicrograph No. 198. Every pure metal is supposed to be composed of crystals or grains which would have been of the true cubic form if the severe internal pressure during solidification and cooling had not distorted them. Photomicrograph No. 99b represents quite well a pure metal. It is that of an extremely mild steel made by special methods in the open-hearth steel furnace. It is so highly refined that it can hardly be called steel at all but is often called “_open-hearth iron_” or “_ingot iron_.” It is probably the purest iron on the market in commercial quantities to-day. While in the chemical laboratory iron of considerably greater purity can be made, for a commercial product this is remarkably pure, seldom containing more than ¼ per cent of elements other than the metal, iron. [Illustration: No. 99b. OPEN-HEARTH IRON. PROBABLY THE PUREST COMMERCIAL IRON PRODUCT ]