These are hollow cast aluminium rocks I am making which will eventually become part of a much larger rock ball sculpture I am planning on making (also making some cast glass Rocks).

The crucible and furnace I currently have are only just big enough to fill this mold as can be seen by the very short riser, ideally it would be considerably taller than this to provide plenty of aluminium to be sucked back into the mold as it cools and shrinks and provide adequate back pressure to fill the mold. However its seems to be working fine like this, the mold is completely filling and not getting much in the way of shrinkage (not that it matters in this case as it is not a precision part and in the final product this part of the rock will not be seen anyway). The riser (short cylinder where the air can vent out as the molten aluminium is poured in) and sprue (funnel shaped part where the molten aluminium is poured in) will be cut off and ground flat.