Wrought means "worked" in medieval English, a time when blacksmiths worked iron rods into intricate curving designs for fences, furniture, gates and window boxes. Two kinds of wrought iron are used. Charcoal iron was first forged in a charcoal fire in the Iron Age and beaten into shape by hand. It was replaced in the 18th century by smooth rolled bars and named "puddle iron" during the Industrial Revolution, which created coal furnaces.
Wrought iron is made of iron with less than 1 percent carbon and a glass-like slag. It is the only ferrous metal that contains siliceous slag. Wrought iron does not become molten in the manufacturing process, so carbides and silicates are changed into glass and remain in the matrix. These give wrought iron its fibrous structure and anti-corrosive properties. While forged wrought iron and mild steel, which replaced it in the 1800s, appear identical, their working properties and resistance to corrosion are very different. Wrought iron is softer and workable at a higher heat than mild steel, which makes it more mallable when beaten by a hammer. Before the 18th century, wrought iron was forged into billets by water power or hand. The surface of the bars was uneven and corners very sharp. Beginning in the 19th century, cast iron was purified in a refinery and renamed puddle iron, a form of wrought iron. The bars were rolled smooth, which rounded the corners.
When wrought iron is heated, the slag melts and coats the surface. This glassy layer gives it a shiny luster and retards oxidation. Wrought iron can be heated to a higher temperature than other metals, which makes it more malleable and easy to form into elaborate designs. Wrought iron is durable; it has been used in three 100-year-old ship beams and cramps on 500-year-old Tudor bridges still in excellent condition. Wrought iron railings in England's Westminster Abbey are from the 13th century.
Wrought iron is not brittle and will bend, not break, allowing delicate design work. Decorative wrought iron is full of intricate forms and water traps that are notorious areas for corrosion with other metals not protected by slag. Because of its coating of glass slag, wrought iron is virtually maintenance free and its low carbon content makes it relatively easy to weld.
Iron and steel don't contain wrought iron's glass slag, so they corrode rapidly. Although hand-made wrought iron is more expensive to make, it lasts longer and requires less maintenance than mild steel or cast iron. It is a good choice for an outside decorative feature, like a gateway or door frame, and is a favorite material for window boxes or balcony railings. Wrought iron is preferable to modern metals to restore historic sections of wrought iron work.
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