What is wrought iron?
Wrought iron is a low-carbon, tough iron containing fine fibres of slag, which can be hot-forged and was the main structural iron before cheap steel became available. It contains very little carbon (usually less than 0.1 per cent). Unlike cast iron, it is not brittle but ductile, and it can be forge-welded. In Dutch it is called smeedijzer.
How it was made
- Bloomery. Iron ore was heated with charcoal until a spongy lump of iron, the bloom, formed. Hammering it while red hot drove out slag and impurities.
- Puddling. From 1784 (Henry Cort’s process), pig iron from the blast furnace was stirred in a puddling furnace so the carbon burned off. This made wrought iron on an industrial scale.
By the end of the nineteenth century mild steel had largely replaced it, and genuine wrought iron is now hardly produced.
Application
In older buildings you find wrought iron as wall ties and pattress plates, tie rods, window bars, railings, hinges and step irons. The “wrought iron” gate sold today is almost always made of mild steel.
- Recognise it. Wrought iron often corrodes in fibres or layers along its length because of the slag inclusions. Cast iron shows a granular fracture and is usually recognisable by its moulded shapes.
- Protect it against corrosion. Remove rust back to sound metal, apply a rust-inhibiting primer and finish with a suitable top coat.
- Repair it properly. On listed buildings, have historic ironwork repaired by a specialist blacksmith and check first with the local planning authority whether consent is needed.
| Wrought iron | Often corrodes in fibres or layers along its length, because of the slag inclusions |
|---|---|
| Cast iron | Shows a granular fracture; usually recognisable by its moulded shapes |
Common mistake: replacing a rusty wall tie or railing entirely when it could have been repaired. Corroding iron expands and can split masonry; timely derusting and conservation prevent that.
Related terms
- Corrosion
- Structural steel sections
- Stainless steel
- Security bars
- Step irons
- Blast furnace slag and steel slag
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