In making large castings shrink heads are put on the cope. These are in the form of gates and large pouring basins. When the mould is filled, the iron rises in these basins and forms a reserve, From this reserve metal is drawn to keep the mould filled as the metal shrinks away from its sides in cooling. It is necessary that the basin or riser shall be connected with the main body of the casting by a narrow neck, which may easily be broken away, without injury to the minting As this neck will solidify before the heavier parts in the mould or the body of metal in the basin above, feeding is necessary. Feeding is accomplished by working an iron rod up and down through the neck while the metal is hardening. The method of doing this is shown in Fig. 69. In this figure a is the riser, b the mould; the rod is worked up and down in the neck connecting the two. When this is not done the upper portion of the casting is apt to be spongy and unsound.
Risers are also used for pressure feeding. To accomplish this comparatively small gates for filling the mould are used. These gates solidify as soon as the mould is filled. The risers which are placed on the heavy portion of the mould are then filled with metal. These have a head or depth of metal considerably greater than that of the original gate. By having two such risers and pouring the metal into them alternately, that in the mould is kept in circulation for the longest time possible.