Cast iron is a family of iron-carbon alloys with carbon content typically between 2.0% and 4.0%, well above the 0.02% to 2.0% range that defines steel. The high carbon content depresses the melting point to roughly 1150 to 1350 degrees Celsius, about 300 degrees lower than pure iron, making it practical to melt and pour into moulds using technologies available since ancient China, where cast iron production dates to at least the 5th century BCE.
Grey vs Ductile Iron
Grey cast iron, the most common grade, contains carbon primarily as graphite flakes within a pearlite or ferrite matrix. These flakes, visible under a microscope as dark, interconnected networks, give grey iron its characteristic properties: excellent compressive strength around 700 to 1,000 MPa, good machinability because the graphite acts as a chip breaker, superb vibration damping capacity roughly 10 times that of steel, but essentially zero ductility. A grey iron casting subjected to tensile stress will fracture with essentially no plastic deformation. The fracture surface appears grey owing to exposed graphite, hence the name.
Ductile cast iron, also called spheroidal graphite iron or nodular iron, was invented in 1943 by Keith Millis at the International Nickel Company. Adding a small amount of magnesium or cerium just before pouring causes graphite to precipitate as spheres rather than flakes. These nodules interrupt the metal matrix far less than flakes do, giving ductile iron tensile strengths of 400 to 800 MPa with elongation typically 2% to 18%. The material combines the casting advantages of grey iron with mechanical properties approaching those of steel, at roughly 70% of the cost.
Cookware Seasoning Chemistry
Cast iron cookware requires seasoning: a layer of polymerised oil baked onto the iron surface. The chemical process involves heating a thin oil film, traditionally flaxseed oil or vegetable shortening, above its smoke point. At these temperatures, unsaturated fatty acids undergo radical polymerisation, cross-linking into a hard, hydrophobic, non-stick coating chemically bonded to the iron oxide surface. Multiple thin layers produce a more durable seasoning than a single thick application. The black colour comes from carbonisation of the organic compounds during heating.
Because cast iron is reactive, acidic foods such as tomatoes, wine-based sauces, and citrus can strip seasoning if simmered for extended periods. Well-maintained seasoned cast iron develops a patina that improves with use, which is why century-old skillets remain in daily service.
Pipes and Radiators
Cast iron dominated drainage and soil pipe applications from the late 19th century through the 1960s. Unlike modern PVC, cast iron pipes are non-combustible and provide excellent sound attenuation, reducing the noise of flowing water to a murmur. In high-rise construction, service-weight cast iron soil pipe with hub-and-spigot joints remains specified for its fire resistance and acoustic performance, though at higher material and labour cost than plastic alternatives.
Cast iron radiators represent one of the most durable heating technologies ever produced. Victorian-era column radiators, still manufactured today as reproduction items, weigh 30 to 50 kilograms per section and can last indefinitely if periodically flushed and repainted. Their high thermal mass produces gentle, even heat release long after the boiler cycles off, making them compatible with modern condensing boilers and heat pump systems alike.