Cast iron is one of the oldest ferrous metals used in construction and outdoor ornament. Its composition is primarily iron (Fe), carbon (C) and silicon (Si), but it may also contain traces of sulphur (S), manganese (Mn) and phosphorus (P). It has a fairly high carbon content of 2 to 5 percent. Unlike wrought iron, cast iron is hard, brittle, nonmalleable (it can't be wrought or worked, bent, stretched or hammered into shape) and is more fusible than steel. While cast iron can fracture with extreme force, it performs well in compression. Its composition and manufacturing method are vital in determining its characteristics.
Grey, or common, cast iron is the most traditional form, and it is easy to cast but can't be forged or worked (wrought) mechanically in either a hot or cold form. Grey cast iron's carbon content is in the form of flakes distributed throughout the metal, while white cast iron the carbon content is combined chemically as carbide of iron. White cast iron has superior tensile strength and malleability (it's easier to manipulate).
Historically, cast iron has been made by heating iron ore in a blast furnace along with coke and limestone. This process deoxidizes the ore and drives out any impurities, producing molten iron. It is then poured into molds of a desired shape (garden furniture) and allowed to cool and crystallize. The most common traditional form is grey cast iron. Common or grey cast iron is easily cast but it cannot be forged or worked mechanically either hot or cold. In grey cast iron the carbon content is in the form of flakes distributed throughout the metal. In white cast iron the carbon content is combined chemically as carbide of iron.
White cast iron has superior tensile strength and malleability. It is also known as 'malleable' or 'spheroidal graphite' iron. Cast iron is still manufactured by much the same process as it was produced historically. Iron ore is heated in a blast furnace with coke and limestone. This process "deoxidizes" the ore and drives off impurities, producing molten iron. The molten iron is poured into molds of the desired shape and allowed to cool and crystallize. If manufactured right, cast iron develops a protective film or scale on its surface, which makes it more resistant to corrosion than wrought iron or mild steel. Finishes, coating or factory preservative treatments are applied to prevent the cast iron products from rusting (oxidizing) when exposed to humidity. These might include bituminous coating, wax, paint, galvanizing and plating.
Outdoor Uses for Cast Iron
Because it is relatively inexpensive, durable and can be cast easily into various shapes, cast iron is still used for a wide range of structural and decorative purposes. These include:
- Garden furniture: At The White House, many of the white Rococo pieces of outdoor furniture, like benches or small dining sets, are made of cast iron, which fits with the famous house's architecture.
- Historic markers and plaques
- Columns, posts and balusters
- Fences and gates
- Hardware like hinges and latches
- Decorative features
References: U.S. General Services Administration (GSA) gsa.gov
Dictionary of Furniture by Charles Boyce (Owl Books; Henry Holt and Company; 1985)