Cast iron is an alloy of iron and carbon. It is usually made of pig iron melted with scrap iron, undesirable impurities being removed. Cast iron can't be shaped by beating or carving and must be cast into shape. It can be used to produce fine decorative detail. Cast iron which shows white along fractured surfaces is called white cast iron. If the fractured surfaces are grey, the metal is called grey cast iron.
Cast iron is very brittle. The relatively high carbon content (two to five per cent) means that cast iron hardens with a crystalline structure that breaks easily. It can crack if dropped. Machining this metal is difficult if not impossible, as it can shatter in the process. It cannot be wrought (beaten into different forms) even when heated.
This brittleness means that cast iron can't be used to make knives, axes or other tools that need to retain a sharp edge. In white cast iron, the carbon has combined chemically with the iron, forming iron carbide. In grey cast iron, the carbon is separate from the iron itself, forming minute flakes that are distributed all through the metal. These flakes make grey cast iron more brittle than white cast iron.
Tensile strength is the measure of how much pulling strain a material can withstand. The tensile strength of cast iron is low. White cast iron has a higher tensile strength than grey cast iron.
The compression strength of a material is how much pressure it can take before it fails. The compression strength of cast iron is high, making it very useful for construction. The advent of cast iron as a construction material enabled the architects of the time to design buildings that were much taller than had previously been possible. Cast iron columns and beams can be used to hold up stone or concrete structures.
The corrosion resistance of cast iron is very low. When first manufactured it has a protective film on the surface, which initially increases its resistance to corrosion. This film cannot be relied upon in the long term, however. When exposed to air and moisture cast iron rapidly oxidises (reacts with oxygen in the air), producing the familiar red-brown iron oxide known as rust. Cast iron must be painted or given a protective coating to protect it against rusting. Protective coatings may include waxes or plating with less reactive metals.