Wrought iron is a historic iron alloy recognized for its high malleability and distinctive fibrous structure. Historically, it was the most common form of workable iron until modern steel production became widespread. The name reflects its nature, as “wrought” is the past participle of the verb “to work,” referring to the material being mechanically worked while hot. Wrought iron is chemically distinct from brittle cast iron and homogeneous mild steel, possessing superior ductility and corrosion resistance.
The Specific Elements of Wrought Iron
True wrought iron is characterized by its remarkable purity, consisting of roughly 99.4% iron by mass. This high purity classifies the metal as nearly pure iron, or ferrite, which contributes to its softness and ability to be shaped easily. The carbon content is exceedingly low, typically less than 0.1%, which is the primary reason for its high melting point and ductile properties.
The defining feature is the inclusion of silicate slag, which makes up about 1% to 3% of the material by weight. This slag is physically mixed and distributed throughout the iron matrix, not chemically dissolved. The presence of this glassy, non-metallic material gives the metal a laminated, wood-like grain structure. These slag fibers, which are iron silicate, contribute to the material’s ability to resist corrosion and fatigue.
The Historical Manufacturing Process
The composition of wrought iron was primarily achieved through the Puddling Process, developed in the late 18th century to refine pig iron. This process took place inside a reverberatory furnace, where the heat source was kept separate from the molten metal to avoid contamination. Pig iron, which is high in carbon and very brittle, was melted and continuously stirred or “puddled” by a worker.
The stirring exposed the molten iron to an oxidizing atmosphere, causing carbon and other impurities to burn off as gases. As the carbon content dropped, the iron’s melting point rose, causing the metal to solidify into a semi-molten, pasty mass known as a “bloom.” This bloom contained entrapped molten slag, a byproduct of the purification.
The crucial final step involved removing the bloom and subjecting it to intensive hammering, called “shingling” or “fagoting,” followed by rolling. This mechanical working expelled most of the liquid slag and consolidated the iron mass. The remaining slag became stretched into the long, thin, fibrous inclusions that are the signature characteristic of wrought iron.
Wrought Iron Versus Modern Metals
Wrought iron is highly malleable, ductile, and easily forge-welded due to its low carbon content and fibrous slag inclusions. The embedded slag fibers also provide a natural resistance to corrosion by helping to deflect corrosive agents.
Cast iron, by contrast, contains a much higher carbon content, typically ranging from 2.1% to 4.5%, which makes it brittle and hard. While this high carbon content allows cast iron to be easily melted and poured into complex molds, its granular structure lacks the fibrous grain of wrought iron, meaning it cannot be easily bent or worked without breaking.
The modern metal that largely replaced wrought iron is mild steel, an alloy with a controlled, homogeneous carbon content, usually below 0.25%. Unlike wrought iron, mild steel contains virtually no slag inclusions but possesses superior tensile strength, making it ideal for structural applications. While mild steel is the functional equivalent today, it lacks the unique fibrous structure and high corrosion resistance of its historical predecessor.
Current Availability and Applications
Commercial production of true wrought iron largely ceased by the middle of the 20th century, replaced by cost-effective methods for manufacturing mild steel. Genuine wrought iron is no longer produced on a large industrial scale, making it difficult and expensive to source today.
The material’s primary modern use is in the restoration and conservation of historic structures, such as old bridges, gates, and architectural elements where authenticity is required. Due to its superior resistance to atmospheric corrosion, it is sometimes used for specialized applications in harsh environments. Most items marketed today as “wrought iron,” including garden furniture and gates, are actually fabricated from mild steel.