The properties of cast and wrought iron
Cast and wrought iron have distinctly different properties that have dictated the ways that they have been used.
Cast and wrought iron can be identified by their intrinsic properties and by the way they have been used. An important distinction is carbon content, which is greater than 2% in cast iron but almost nil in wrought iron, which is virtually chemically pure iron. Cast iron is brittle and is strong in compression but weak in tension. Wrought iron is fibrous in texture, highly ductile and strong in both tension and compression.
The types of component that were made from cast and wrought iron were determined by the methods of manufacture and shaping as well as by the physical properties of the materials.
Because it is cast, (i.e. shaped by pouring the hot liquid metal into a mould and as a consequence of its high liquidity in the molten state, cast iron is well-suited to the production of components, both structural and decorative, of intricate shape. It can be subject to blemishes and flaws, hence the warning made by a 19th century ironwork contractor: "It is not the masses of metal that constitute strength, but the judicious proportions and forms of the casting". Wrought iron is rolled in the final stage of production so it usually occurs in simple shapes of constant section - plates, tees, angles, bars etc. Its manufacture is carried out in small batches - up to 50 kg. at a time. This means that individual components are relatively small. It can be riveted to make up large elements such as beams, trusses or girders. Large structures in wrought iron are, therefore, characteristically fabricated by riveting together many small components.
The production of wrought iron in Britain ceased in 1973, so what is marketed today under that name is either old material recycled or mild steel masquerading as iron.
