METALLURGY OF CAST IRON.
82, with cylinders cast on end. The feeding head O, which is intended to supply the shrinkage of all below it, is often made so small that it solidifies before the heavy portion at P, and then what metal settles to supply the shrinkage of the lower body of the casting P comes from the thicker or more fluid section at N, and leaves shrink holes at that point. This whole difficulty could be stopped by making the feeding head O larger, as per dotted line R, as then this would be the last to solidify, and when the feeding head O was cut off to give a finished flange a solid body of metal would be found under it, providing the feeding head O had been fed with hot iron by means of a feeder or heavy riser head (not shown) placed on top of the feeding head O as is the common practice.
Blow holes. Having treated the subject of shrink holes, we will say a few words on what are called blow holes. Such holes may often appear to some as shrink
holes, but they generally differ in being found in lighter parts of castings, than where shrink holes are liable to be found, and are generally of a smoother character. Not only are blow holes found on the interior but the exterior as well; in either place, they FIG. 83.—CASTINGS SHOWING BLOW HOLES, are caused by gases