The great shrinkage of steel would cause surface or interior cavities in the casting were it not fed by its runners, risers, and feeding heads. The static pressure of the metal in these insures the filling of the mould provided they are large enough in diameter to remain liquid until the casting has solidified. A feeding head is merely an extra riser over a heavy part of the casting, and the fluidity of both feeding heads and risers is prolonged by churning the metal in them with small iron rods. Often a riser or feeding head is 1/2 or 1/3 the weight of the casting, and is as much as ten inches in diameter for large castings.

The removal of large risers, runners and feeding heads requires considerable work, and adds much to the expense of steel castings.

An old and effective method is to saw them off with a cold steel saw, but where a shop has facilities for electric cutting, a much cheaper way is to cut them off by use of the electric arc. One electrode is fastened to the riser and the other, suitably rigged to be handled by a workman, is passed around the riser neck close to the casting. The intense heat of contact melts a groove around the riser, reducing its diameter until it can be knocked off by a sledge. The brilliancv of the arc is such that the workman must wear a metal head shield provided with black glass sight openings, and the work is done within an enclosure to keep others from looking at the arc.

Another method lately developed for such cutting is the oxy-acetylene burner.