There are many purposes for which the alloy should have extreme hardness and the great resistance to wear which accompanies such hardness. The wearing faces of gears, brake shoes, rolls, and car wheels, for instance, must be hard. For such products, white cast iron, the extremely hard condition of the alloy, just referred to, is utilized. Such castings are usually produced with a white cast iron face, but with a gray iron interior, gray iron being less brittle and less likely to break under shock or strain. A car wheel, for instance, has approximately an inch in depth of white iron on the surface which lies next to the rail on which it runs. [Illustration: CHILLED CAST IRON The white edge resulted from the more rapid cooling against an iron chill, as did the white rim of the wheel shown above. ] Such are known as “chilled castings.” Molds for them are usually made of sand, with pieces of iron (called “chills”) imbedded where white iron is to be produced. The molten iron next to the sand surfaces cools in the usual way and is gray and soft, while that which lies next to the “chill” is white and extremely hard. The “depth” of the chilled layer can be increased or diminished according to the thickness of the iron “chill” used, its temperature, and by the composition and temperature of the molten cast iron with which the mold is poured. The sulphur and total carbon of the molten cast iron also have considerable influence on the depth of “chill.” [Illustration: TWO-PART MOLDING FLASKS ] There is a cast iron alloy which is familiarly known as “semi-steel.” It is simply a high grade and stronger “gray” iron and must be classed as a cast iron, as our table on page 180 shows. While it could undoubtedly be made from materials which are commonly used for cast iron, it is practically always produced by charging with these a certain amount of steel scrap to bring about the lower silicon, phosphorus and total carbon desired. [Illustration: A HOLLOW CYLINDER The casting which we are about to make. ] Because steel has a higher tensile strength than has cast iron, many have inferred that it was the steel addition which made semi-steel stronger than the ordinary soft cast iron alloy. The rather unfortunate name, “semi-steel,” apparently was given because of the steel used and the intermediate strength which the resulting product possessed.