Iron Casting

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 the molds.

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.