The remelting for casting is done in a cupola furnace, somewhat
similar to the blast furnace, but smaller. The pig iron, limestone,
and coke are placed together and a blast of air, this time cold,
is forced through the mass. When the iron is thoroughly fluid, it
is run into ladles, from which it is poured into sand molds, the
interiors of which are the shapes of the objects to be cast. The
melted iron expands and is forced into every part of the space.
As the castings cool they shrink, so that they are smaller than
Imperfections sometimes occur in castings. Blow holes or small
cavities are caused by dissolved gases, which form bubbles of gas
that cannot escape. Jarring the mold after the iron is poured in
sometimes produces imperfect shapes. That is sometimes the reason
why the wheels of cast iron egg beaters or other pieces of machinery
fail to work smoothly.