Cast iron has become a synonym for durability — a cast iron stomach or alibi. As metals go, however, it is actually something of a Brunswick stew. While the pig iron that forms its basis is being melted, scrap iron and steel are often thrown into the mix.
When the process is complete and impurities such as sulfur are subtracted, the end product is actually only 95% iron. The rest of the chemical makeup is primarily carbon and silicon, in various percentages. Anything over 2% silicon is known as "gray cast iron," while a lesser amount gives birth to "white cast iron." The "white" form is not actually white, but gets its name from a reaction with the increased carbon that creates small white deposits of cementite on an otherwise dark surface.
Despite its metaphorical reputation, cast iron is more brittle than pure iron and steel, and melts at a lower temperature. This is not a bad thing as far as industrial applications are concerned, because it makes the metal more malleable and thus quite versatile. Its first application in 17th-century China was as cannonballs and shot. Today, it is used in pipes, machine parts, automobile components and, perhaps most commonly, skillets.
The cast iron skillet that served a cook's great-grandmother so well has enjoyed something of a comeback in recent years, as some people have become wary of Teflon® coatings as being possibly unhealthy. With this revival, though, has come renewed awareness that using this metal for cooking often requires considerable vigilance.
Unless it is properly "seasoned," cast iron can often cause cooking food such as eggs to stick to the surface. Unlike non-stick cookware, cast iron actually bonds with fats and oils to modify its surface — thus, the longer a frying pan made from this metal is used, the more user-friendly it becomes. Many professional chefs are fans of the metal, which they praise for its heat retention and evenly radiating surface.
Cast iron bridges are generally relics of the 18th and 19th centuries, although some survive. The use of this metal was also considered a breakthrough in the early 20th century building construction trade because of its weight-bearing capacity, but it moved to the sidelines when new forms of steel were introduced to take its place. After the World Trade center towers collapsed on 11 September 2001, a large cast iron cross was dug out of the wreckage, still intact.