Wrought iron is made by heating iron ore or pig iron to just below its melting point, then repeatedly working the hot metal to squeeze out impurities. The result is nearly pure iron with only about 0.15% carbon, threaded with thin layers of glassy slag that give it a distinctive fibrous grain. Two main processes have been used over the centuries: the ancient bloomery method and the later industrial puddling process.
What Makes Wrought Iron Different
The word “wrought” simply means “worked,” and the name tells you everything about how this metal gets its character. Unlike cast iron, which contains 2% to 4% carbon and is poured into molds, wrought iron is shaped by hammering and rolling while hot. Its carbon content sits around 0.15%, making it soft, malleable, and easy to bend without cracking.
The other defining feature is slag, a glassy mixture of oxides and silicates that makes up 1% to 4% of the finished metal. This slag isn’t chemically bonded to the iron. It sits in thin physical layers throughout the material, creating a laminated, wood-grain-like structure you can actually see when a piece is broken or etched with acid. Those slag fibers are what give wrought iron its reputation for corrosion resistance: they create internal barriers that slow rust from penetrating deep into the metal. They also make wrought iron exceptionally easy to forge weld, since the slag acts as a natural flux when two pieces are hammered together at high heat.
The Bloomery: The Original Method
For thousands of years before industrial furnaces existed, wrought iron was produced in small clay or stone furnaces called bloomeries. The process dates back to pre-Roman times and remained the only way to make iron for most of human history.
A bloomery works by heating iron ore and charcoal together while forcing air through the furnace with bellows. The critical detail is temperature. The furnace reaches roughly 1,200°C to 1,300°C, hot enough to soften the iron and melt the slag but not hot enough to melt the iron itself (wrought iron melts at about 1,482°C). Because the iron never liquefies, it doesn’t absorb much carbon from the charcoal fuel. Instead, it collects at the bottom of the furnace as a spongy, porous lump called a bloom, riddled with trapped slag and charcoal ash.
The bloom then has to be worked. A blacksmith pulls the glowing mass from the furnace and hammers it repeatedly, folding and compressing it. Each blow squeezes out pockets of liquid slag and welds the spongy iron into a denser, more uniform piece. This process might be repeated dozens of times, with the smith reheating and hammering until the metal is clean enough to use. The slag that remains gets stretched into long, thin fibers running through the iron, producing that characteristic layered grain.
The Puddling Process: Industrial Scale
By the late 1700s, demand for iron had outgrown what bloomeries could supply. The puddling process, refined by Henry Cort in 1784, made it possible to convert pig iron (the high-carbon product of a blast furnace) into wrought iron in much larger quantities. It dominated iron production for over a century.
Puddling starts with a reverberatory furnace, a design where the fuel and the metal never touch directly. The heat bounces off a curved roof onto the iron below. A puddler shovels about 250 kg of pig iron into the furnace along with iron oxide (mill scale), then heats and melts it over about 30 minutes.
Once molten, the real work begins. The puddler’s helper stirs the bath with long iron rods to expose the metal to the oxide-rich slag forming on its surface. This first stage, called “clearing,” takes eight to ten minutes and burns off silicon and phosphorus. Then the puddler adds more mill scale to trigger the “boil,” a dramatic phase where carbon in the pig iron reacts with the oxide and escapes as carbon monoxide gas, making the molten bath bubble violently. This is the core of the whole process: it’s what drives the carbon content down from about 4% to a fraction of a percent.
After roughly ten more minutes of strenuous stirring, the metal thickens as its carbon drops and its effective melting point rises. It stops being liquid and turns into a pasty, dough-like mass. The puddler says the metal has “come to nature.” He then separates it into three balls, each one a mix of soft iron and residual slag, and transfers them to a machine called a squeezer. The squeezer compresses each ball under enormous force, expelling most of the remaining liquid slag, in a step known as shingling. The resulting rough bar is then rolled or hammered into its final shape.
A single heat took about an hour of exhausting physical labor in front of an open furnace, and puddlers were among the highest-paid industrial workers of the 19th century.
From Rough Bar to Finished Product
Whether the iron comes from a bloomery or a puddling furnace, it needs further working to become useful. The consolidated metal is reheated and passed through rolling mills or hammered on an anvil, stretching it into bars, rods, plates, or sheets. Each pass through the rolls elongates the trapped slag inclusions, drawing them out into finer and finer threads. The more the iron is worked, the more refined its grain becomes.
Blacksmiths can then take these bars and forge them into gates, railings, tools, chains, and structural elements. Wrought iron is forgiving to work with: it bends without snapping, it can be split and scrolled into decorative shapes, and two pieces will forge weld to each other at bright orange heat without modern welding equipment. Its melting point of 1,482°C is significantly higher than cast iron’s (around 1,175°C to 1,290°C), which means it holds its shape well in fires, one reason it was long favored for structural beams.
Why Genuine Wrought Iron Barely Exists Today
By the early 1900s, mild steel had replaced wrought iron for nearly every application. Steel is cheaper to produce, stronger in tension, and can be made in enormous quantities using processes like the Bessemer converter and open-hearth furnace. The last large-scale wrought iron production ended in the mid-20th century.
Today, only one company in the world, Topp & Co. in the UK, still supplies genuine wrought iron. They don’t smelt new material. Instead, they collect old puddled wrought iron from demolished bridges, ships, and buildings, then re-roll it in a traditional hot-rolling mill. The supply is finite and increasingly rare.
Almost everything sold as “wrought iron” today, from garden furniture to stair railings, is actually mild steel shaped to look like traditional ironwork. Mild steel has a similar low carbon content but lacks the slag inclusions that define real wrought iron. Without those internal glass fibers, mild steel doesn’t have the same corrosion resistance or the same grain that reveals itself when the metal ages. For restoration of historic structures, genuine re-rolled wrought iron remains the only material that matches the original in both composition and behavior.