Wrought iron is a tough but malleable form of iron. It was once widely used in everything from horseshoes to railways, but while it has largely been replaced by steel it remains in use mainly for ornamental ironwork.
The name wrought iron refers to the way in which this kind of iron is worked or wrought into shape, rather than being poured into a mould like cast iron. Wrought iron is a relatively pure iron alloy, with a much lower carbon content than cast iron. However, it has a small amount of slag added to it during the manufacturing process. Slag is the waste left over when pure iron is extracted from its ore. It is a mixture of silicon, sulphur and metal oxides that gives wrought iron some unique properties.
The main difference between wrought iron and the other commonly used type of iron, cast iron, it the way that it is shaped into the desired form. Wrought iron is worked with tools, while cast iron is melted and poured into a cast or mould. The composition of the iron and the way it is produced is also different, which results in slightly different properties. Cast iron is less malleable and more brittle than wrought iron. It cannot be hammered or forged into shape, but its lower boiling point makes it easy to use with moulds.
Steel shares some of the properties of wrought iron, as both are malleable forms of iron. However, modern steel is a lot stronger, so it has replaced wrought iron in many applications, such as construction. Some “ironwork” items that resemble traditional wrought iron are today manufactured from steel instead.
One characteristic that makes wrought iron stand apart from cast iron and steel is that it has a unique wood grain appearance when it is broken open. The fibrous appearance comes from the filaments of iron silicate mixed in with the iron, which are also responsible for giving wrought iron the valuable properties of ductility, high tensile strength and resistance to corrosion.
Wrought iron has been used for thousands of years, although the methods of manufacturing it have changed. The process essentially involves heating iron and removing slag in order to achieve the correct composition. The wrought iron can then be reheated and hammered, rolled, or otherwise worked into various forms.
Wrought iron was originally manufactured as charcoal iron, which was smelted directly from the iron ore in a furnace known as a bloomery. The ore was heated and separated from the slag, but it was not allowed to melt as this would allow carbon to dissolve into it, forming pig iron. The resulting wrought iron retained enough slag to be malleable, ductile and strong. However, the quality did vary, depending on the type of iron ore that was being used and how much slag was left in it at the end of the process. It could have very different levels of strength and corrosion resistance.
An alternative route for producing wrought iron was developed in the early 13th century. The Osmond process enabled wrought iron to be produced from pig iron, which is a brittle form of iron with a high carbon content that is extracted from the ore by smelting. Droplets of melted pig iron were caught on a staff and spun to encourage the carbon to oxidise in the air. However, charcoal iron continued to be produced too.
Wrought iron production changed during the 15th century, when the blast furnace began to be used to extract iron from its ore. Since the blast furnace could produce large quantities of pig iron, it became more common to use this rather than the ore when making wrought iron. Finery forges were used to remove carbon from the pig iron by oxidisation. The pig iron was melted and then hammered out. The Walloon forge was the main kind of finery used in the UK, but slightly different systems were also in use.
As the Industrial Revolution began, ironmasters made several attempts to design a process for making large amounts of wrought iron more efficiently, without charcoal. The most successful process was puddling, which was invented in 1784. Puddled iron is produced from refined iron in a special kind of reverberatory furnace, where the metal is kept separate from the fuel to prevent contamination. The iron is stirred and exposed to a current of air, as well as to oxidising agents like iron oxide in the hearth. This removes carbon and impurities, causing globs of wrought iron to form on top of the puddle of molten iron. These puddle balls are removed and processed by shingling and rolling to remove remaining impurities.
Puddling provided the wrought iron that was in widespread use throughout the late 18th and 19th centuries, with more than 4 million tonnes of wrought iron being produced every year in the UK alone by the 1870s. Puddled iron was used in buildings, railways, ironclad ships, and even for the construction of the Eiffel Tower.
However, in the 1860s the Bessemer process was created. Initially the steel it produced was brittle, but by 1890 it was creating enough high quality steel to replace wrought iron for most purposes. Although the Aston process was invented in 1925 to produce wrought iron quickly by cooling molten steel alongside slag, it could not compete. Wrought iron was weaker than steel, more expensive to produce, and required much more effort to shape and use.
Wrought iron isn’t now produced on the same scale as it once was, but it is still in use. It can be produced by the traditional methods, which are still demonstrated at living history sites such as Blists Hill. Wrought iron is also often obtained by recycling scrap iron.
In the past, a wide range of items were made of wrought iron. It was commonly used for making nails, horseshoes, nuts and bolts, rivets, rails and railway couplings, pipes to carry steam and water, as well as for ornamental ironwork. Today, wrought iron is mainly used for ornamental purposes. It is often used to make metal gates, iron railings, garden furniture, driveway gates and other decorative ironwork for outdoor display. Decorative items such as bedsteads, candle holders, curtain rods and wine racks are still sometimes made from wrought iron. It is also used in conservation work to preserve or replace older wrought iron work.
Wrought iron can be a good choice for these kinds of ironworks because: