Wrought iron

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Iron pillar at Delhi, India, containing 98% wrought iron

Wrought iron is an iron alloy with a very low carbon (0.1% to 0.25%) content in contrast to cast iron (2.1% to 4%), and has fibrous inclusions, known as slag up to 2% by weight. It is a semi-fused mass of iron with slag inclusions which gives it a "grain" resembling wood, that is visible when it is etched or bent to the point of failure. Wrought iron is tough, malleable, ductile and easily welded. Before the development of effective methods of steelmaking and the availability of large quantities of steel, wrought iron was the most common form of malleable iron. A modest amount of wrought iron was used as a raw material for refining into steel, which was used mainly to produce swords, cutlery, chisels, axes and other edged tools as well as springs and files. The demand for wrought iron reached its peak in the 1860s with the adaptation of ironclad warships and railways, but then declined as mild steel quality problems such as brittleness were solved and it became inexpensive and widely available.


Wrought iron is no longer produced on a commercial scale. Many products described as wrought iron, such as guard rails, garden furniture[2] and gates, are made of mild steel.[3] They retain that description because in the past they were wrought (worked) by hand.[4]






Historically, wrought iron was known as commercially pure iron;[6][7] however, it no longer qualifies because current standards for commercially pure iron require a carbon content of less than 0.008 wt%.[8][9]

Types and shapes[edit]

Bar iron is a generic term sometimes used to distinguish it from cast iron. It is the equivalent of an ingot in cast metal, being a convenient form for handling, storage, shipping and being worked into a finished product.

. The bars were the usual product of the finery forge, but not necessarily made by that process. They might be square or flat, and flat bars might be narrow or broad.

  • Hoop iron—suitable for the hoops of barrels, made by passing rod iron through flat rolls.
  • Plate iron—sheets of iron suitable for use as boiler plate.
  • Blackplate—sheets of iron, perhaps thinner than plate iron, from the black rolling stage of tinplate production.




Tough iron 
Tough iron, also spelled "tuf", is not brittle and is strong enough to be used for tools.
Blend iron 
Blend iron is made using a mixture of different types of pig iron.
Best iron 
Best iron is iron that, having gone through several stages of piling and rolling, might reach the stage of being regarded as the best quality of iron (in the 19th century).
Marked bar iron 


Wrought iron is a form of commercial iron, containing 0.10% of carbon and less than 0.25% of impurities total of sulfur, phosphorus, silicon and manganese and less than 2% slag by weight.[14][15] Wrought iron is redshort if it contains sulfur in excess quantity. It has sufficient tenacity when cold, but cracks when bent or finished at a red heat. It is therefore useless for welding or forging.


Phosphorus is not necessarily detrimental to iron. Ancient Indian smiths did not add lime to their furnaces. The absence of CaO in the slag, and the deliberate use of wood with high phosphorus content during the smelting, induces a higher P content (> 0.1%, average 0.25%) than in modern iron. Analysis of the Iron Pillar of Delhi gives 0.10% in the slags for .18% in the iron itself, for a total P content of 0.28% in the metal and accounts for much of its corrosion resistance. The presence of phosphorus (without carbon) produces a ductile iron suitable for wire drawing, for piano wire.[16]






Bloomery process[edit]

Main article: Bloomery

Wrought iron was originally produced by a variety of smelting processes, all described today as "bloomeries". Different forms of bloomery were used at different places and times. The bloomery was charged with charcoal and iron ore and then lit. Air was blown in through a tuyere to heat the bloomery to a temperature somewhat below the melting point of iron. In the course of the smelt, slag would melt and run out, and carbon monoxide from the charcoal would reduce the ore to iron, which formed a spongy mass containing iron and also silicate (slag) from the ore, referred to as an iron "bloom", from which the technique got its name. The iron remained in the solid state. If the bloomery were allowed to become hot enough to melt the iron, carbon would dissolve into it and form pig or cast iron, but that was not the intention.[18]


During the Middle Ages, water-power was applied to the process, probably initially for powering bellows, and only later to hammers for forging the blooms. However, while it is certain that water-power was used, the details remain uncertain.[19] That was the culmination of the direct process of ironmaking. It survived in Spain and southern France as Catalan Forges to the mid 19th century, in Austria as the stuckofen to 1775,[20] and near Garstang in England until about 1770;[21] it was still in use with hot blast in New York State in the 1880s.[22]

Osmond process[edit]

Main article: Osmond process

Osmond iron consisted of balls of wrought iron, produced by melting pig iron and catching the droplets on a staff, which was spun in front of a blast of air so as to expose as much of it as possible to the air and oxidise its carbon content.[23] The resultant ball was often forged into bar iron in a hammer mill.

Finery process[edit]

Main article: Finery forge




Potting and stamping[edit]

From the late 1750s, ironmasters began to develop processes for making bar iron without charcoal. There were a number of patented processes for that, which are referred to today as potting and stamping. The earliest were developed by John Wood of Wednesbury and his brother Charles Wood of Low Mill at Egremont, patented in 1763.[25] Another was developed for the Coalbrookdale Company by the Cranage brothers.[26] Another important one was that of John Wright and Joseph Jesson of West Bromwich.[27]

Puddling process[edit]

Schematic drawing of a puddling furnace
Main article: Puddling (metallurgy)



In the fully developed process (of Hall), this metal was placed into the hearth of the puddling furnace where it was melted. The hearth was lined with oxidizing agents such as haematite and iron oxide.[29] The mixture is subjected to a strong current of air and stirred with long bars, called puddling bars or rabbles,[30][31] through working doors.[32] The air, stirring, and "boiling" action of the metal help the oxidizing agents to oxidize the impurities and carbon out of the pig iron to their maximum capability. As the impurities oxidize, they form a molten slag while the retaining iron particles solidifiy into spongy wrought iron balls, called puddle balls.[29]


There is still some slag left in the puddle balls, so while they are still hot they must be shingled[33] to remove the remaining slag and cinder.[29][33] That may be achieved by forging the balls under a power hammer, or by squeezing the bloom in a machine. The material obtained at the end of shingling is known as bloom and it is still red-hot.[33] The blooms are not useful in that form, so they must be rolled into a final product.

Sometimes European ironworks would skip that step completely and roll the puddle balls. The only drawback to that is that the edges of the rough bars are not as well compressed. When the rough bar is reheated, the edges may separate and be lost into the furnace.[33]


Main article: Rolling mill

The bloom is passed through grooved rollers and flat bars were produced. The bars of wrought iron were of poor quality, called muck bars[33][34] or puddle bars.[29] To improve the quality of wrought iron, the bars were cut up, piled and tied together by wires, a process known as faggoting or piling.[33] They were then reheated and rolled again in merchant rolls. The process may be repeated several times to get wrought iron of desired quality. Wrought iron that has been rolled multiple times is called merchant bar or merchant iron.[31][35]

Lancashire process[edit]

Main article: Lancashire hearth

The advantage of puddling was that it used coal, not charcoal as fuel. However, that was of little advantage in Sweden, which lacks coal. Gustaf Ekman observed charcoal fineries at Ulverstone, which were quite different from any in Sweden. After his return to Sweden in the 1830s, he experimented and developed a process similar to puddling but using firewood and charcoal, which was widely adopted in the Bergslagen in the following decades.[36]

Aston process[edit]



Steel began to replace iron for rails as soon as the Bessemer process for making steel was adopted (1865 on). Iron remained dominant for structural applications until the 1880s, because of problems with brittle steel, caused by introduced nitrogen, high carbon, excess phosphorus or excessive temperature during rolling or too rapid rolling.[37] By 1890 steel had largely replaced iron for structural applications.

Sheet iron (Armco 99.97% pure iron) had good properties for use in appliances, including being well-suited for enamelling and welding, and being rust resistant.[38]

In the 1960s, the price of steel production was dropping due to recycling, and even using the Aston process, wrought iron production was a labor-intensive process. It has been estimated that the production of wrought iron costs approximately twice as much as the production of low carbon steel.[3] In the United States the last plant closed in 1969.[3] The last in Great Britain (and the world) was the Atlas Forge of Thomas Walmsley and Sons in Bolton, which closed in 1973. Its equipment, of a type dating from the 1860s, was moved to the Blists Hill site of Ironbridge Gorge Museum for preservation.[39] Some wrought iron is still being produced for heritage restoration purposes, but only by recycling scrap.


The microstructure of wrought iron, showing dark slag inclusions in ferrite



Due to the variations in iron ore origin and iron manufacture, wrought iron can be inferior or superior in corrosion resistance compared to other iron alloys.[3][45][46][47] There are many mechanisms behind that corrosion resistance. Chilton and Evans found that nickel enrichment bands reduce corrosion.[48] They also found that in puddled, forged and piled iron, the working-over of the metal spread out copper, nickel and tin impurities, which produces electrochemical conditions that slow down corrosion.[46] The slag inclusions have been shown to disperse corrosion to an even film, enabling the iron to resist pitting.[3] Another study has shown that slag inclusions are pathways to corrosion.[49] Other studies show that sulfur impurities in the wrought iron decrease corrosion resistance,[47] but phosphorus increase corrosion resistance.[50] Environments with a high concentration of chlorine ions also decreases wrought iron's corrosion resistance.[47]


Table 1: Chemical composition comparison of pig iron, plain carbon steel, and wrought iron[33]
Material Iron Carbon Manganese Sulfur Phosphorus Silicon
All units are percent weight
Table 2: Properties of wrought iron
Property Value
Ultimate tensile strength [psi (MPa)][52] 34,000–54,000 (234–372)
Ultimate compression strength [psi (MPa)][52] 34,000–54,000 (234–372)
Ultimate shear strength [psi (MPa)][52] 28,000–45,000 (193–310)
Yield point [psi (MPa)][52] 23,000–32,000 (159–221)
Modulus of elasticity (in tension) [psi (MPa)][52] 28,000,000 (193,100)
Melting point [°F (°C)][53] 2,800 (1,540)
Specific gravity 7.6–7.9[54]

Amongst its other properties, wrought iron becomes soft at red heat, and can be easily forged and forge welded.[56] It can be used to form temporary magnets, but cannot be magnetized permanently,[57][58] and is ductile, malleable and tough.[33]















See also[edit]


  6. ^ Imhoff, Wallace G. (1917), "Puddle Cinder as a Blast Furnace Iron Ore", Journal of the Cleveland Engineering Society 9 (621.76): 332 .
  27. ^ Morton and Mutton, 725–6.
  34. ^ W. K. V. Gale, Dictionary, 137.
  38. ^ Misa 1995:242
  40. ^ About wrought iron, retrieved 2008-02-22 .
  47. ^ a b c Walker VII, Robert (April 2002), "The Production, Microstructure, and Properties of Wrought Iron", Journal of Chemical Education 79 (4): 443–447, Bibcode:2002JChEd..79..443W, doi:10.1021/ed079p443 .
  48. ^ Chilton & Evens, Journal of the Iron and Steel Institute, 1955
  50. ^ Balasubramaniam, R (2003-01-25), "Delhi iron pillar and its relevance to modern technology", Current Science 84 (2): 162–163 .
  54. ^ Solids and Metals - Specific Gravity, retrieved 2008-02-20 .
  57. ^ American Technical Society (1916), Cyclopedia of Applied Electricity 1, American Technical Society, p. 14, retrieved 2008-02-21 .
  63. ^ http://www.realwroughtiron.com/about_wrought_iron-217.html

Further reading[edit]

  • Bealer, Alex W. (1995), The Art of Blacksmithing, Edison, NJ: Castle Books, pp. 28–45, ISBN 0-7858-0395-5. 