But it is not these which we mean by the term, cast steel, but rather those steel products which get their final form by being “cast” from a fluid condition into a mold. These are what are rapidly coming to be understood when the term “cast steel” is used. Satisfactory metal for steel castings may be made in any of three or four types of furnaces, but, as was suggested before, the making of molds for castings is a fine art, as is the preparation of the metal which is to go into them. Further, the making of that special class of castings which are to withstand water, steam or air pressure is a very different thing from the making of steel castings for other purposes, and this is too often forgotten. For the former are necessary particularly close-grained castings, free from flaws or spongy spots. Under the great pressures applied such defects would certainly allow leakage. [Illustration: FLANGES AND FITTINGS OF CAST STEEL ] Whatever the method of production of steel for castings the metal is poured into molds to receive its final shape. Because of the intensely high heat of the steel only sands of great refractoriness (resistance to heat) can be used as material for the mold. White silica sand is such a material and is generally used, mixed with enough clay and molasses-water to give it “bond.” While molds for some steel castings are made in “green” (i.e., undried or unbaked) sand, baked molds are preferred for fine finish and surest results. After the making of the molds in the usual way they are sprayed with very finely powdered white sand or quartz mixed with a little molasses-water. They are then thoroughly dried in an oven. Cast steel shrinks during cooling even more than malleable iron and the pattern and mold must be made to allow for this. Upon the freezing of the surfaces of the casting with consequent attainment of rigidity, the interiors, which freeze last, may have cavities unless means for avoiding them is provided. For this purpose heavier pieces, which later can be cut off, are cast upon such parts of the casting as tend to have “shrink holes.” These may be likened to receptacles filled with fluid metal, which being larger than the parts of the castings which they “feed,” hold excess metal in fluid condition until the casting itself has become solid throughout. Such are usually called “risers” or, in Europe, “lost heads,” and the molten metal in them flows down into the interior of the casting and fills the shrink holes which are forming. Not only must the risers be large enough that the metal in them is the last to solidify but they must be built high enough above the casting that sufficient pressure is exerted on the steel entering the shrinking parts to make its entry sure. [Illustration: GRAIN OF STEEL CASTINGS AS THEY COME FROM THE MOLD ] [Illustration: GRAIN OF STEEL CASTINGS AFTER ANNEALING ] (Magnification 60 diameters)