Last edited 27 Mar 2022

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Institute of Historic Building Conservation Institute / association Website

Wrought iron, yesterday and today

Wrought iron has not been made since 1974, but some is available and in recent years the blacksmiths of Britain have slowly taken up the ancient ironworking skills again.

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Not for re-use. Australian blacksmith Dave Fleming forging at Chris Topp’s.

Wrought means ‘worked’, which refers to the method of manufacturing the metal by working repeatedly under a hammer. In the past the work of the blacksmith became known as ‘wrought ironwork’, a name that has persisted for the art form even though the metal in use may not be wrought iron. Today the common material of the blacksmith is mild steel, a cheap industrial product lacking many of the virtues of its ancestor.

Wrought iron is a two-component metal consisting of iron and a glass-like slag. The slags are in effect an impurity, the iron and the slag being in physical association, as contrasted to the chemical alloy relationship that generally exists between the constituents of other metals. Wrought iron is the only ferrous metal that contains siliceous slag. It is to this slag that wrought iron owes the properties, which are of interest to the conservator and the blacksmith.

There are essentially two types of wrought iron: ‘charcoal iron’, made in a charcoal fire and used from the iron age to the end of the 18th century, and ‘puddled iron’, made from cast iron in an indirect coal-fired furnace, and used since the dawn of the modern industrial era.

Historically wrought iron has been worked by blacksmiths, using traditional techniques in both forging and construction, to make high-end decorative wrought ironwork. Today the term wrought iron is becoming debased and misinterpreted, to cover all ornamental ironwork, including cast iron and mild steel, as well as incorporating modern construction techniques. The difference in quality and value is enormous. Whereas it would be unthinkable to repair historic stonework with concrete or cast stone and Portland cement, it is common for historic wrought iron to be repaired using mild steel and electric welding.

Wrought iron has been used in building from the earliest days of civilisation, wrought-iron door furniture being common in Roman times. The structural use of iron dates from the middle ages, when bars of wrought iron would be used occasionally to tie masonry arches and domes. This use of wrought iron in tension guaranteed its use throughout the ascendancy of cast iron in the canal and railway ages, as cast iron is strong only in compression. The ill-fated first Tay Bridge was of cast iron beams tied with wrought iron.

The demand for higher dynamic loads in bridges and warehouse buildings, and the ever-greater spans of train sheds towards the end of the 19th century, led the designers of buildings to acquire the technology developed to build ships of iron, and create beams of riveted wrought-iron rolled sections. By the turn of the century this had led to buildings completely framed in wrought iron, and later steel girder sections, and cast iron was once again relegated to an ornamental role.

Our main concern with wrought iron, however, will be in its application to gates and railings, frequently given an ornamental treatment by the blacksmith. There are wrought-iron railings in Westminster Abbey from the 13th century which, in essence, display all the characteristics, which we have come to know as ‘wrought ironwork’. Although often lacking modern refinements such as symmetry and sweetness of line, the great age of British ironwork, known as the English style, began at the end of the 17th century.

A French fashion for the baroque style in gates and railings swept the country houses of Britain following the import of craftsman by William and Mary, and the greater part of our national stock of good ironwork dates from the early years of the 18th century. After the rise of cast iron as an ornamental medium, wrought iron tended often to take a secondary role, owing to its comparative expense, each piece being made by hand, while castings could be repeated infinitely once the patterns had been made. Technically, however, the craftsmen of the age of machines technically bettered their forebears, as indeed they must while making mechanical components. The ornamental blacksmith work of the 19th century displays a perfection of manufacture not seen before or since.

After the introduction of mild steel, cheap because of its ability to be mass produced, wrought iron, and the craft skills associated with it, gradually disappeared in accordance with the general decline of craft standards in the 20th century. The last ironworks ceased production in 1974. From 1982 Chris Topp & Company and the Real Wrought Iron Company have made available a limited supply of puddled wrought iron. The subsequent years have brought a steadily increasing demand, as the blacksmiths of Britain have slowly taken up the ancient skills again.

To the naked eye there is no visible difference between forged bars of wrought iron and mild steel (its modern day equivalent). However, wrought iron is not distinguished by its looks but by its working properties and its resistance to corrosion. Wrought iron is softer to forge, as it is workable at a higher heat than mild steel, making it more malleable under the hammer. It is wrought iron’s superior weathering properties that are most notable.

Like all ferrous metals, wrought iron does corrode in certain circumstances, but evidence of its durability and long life expectancy is common in our towns and countryside. Victorian town house railings have wrought iron 200 years old. Vast ornamental gateways to 18th century great houses have wrought iron 300 years old. Stone cramps on Tudor bridges have wrought iron 500 years old. The simple fact that so much ornamental wrought ironwork survives, often with little or no maintenance, speaks volumes for the material.

Problems with the rapid corrosion of mild steel in comparison with wrought iron were well known by the beginning of the 20th century. This prompted Matthew Verity to investigate in the USA. He concluded that the carbon in steel is responsible for its corrodibility, leading to the theory that removing the carbon removes the problem of corrosion. His efforts to produce pure iron resulted, not in a metal that was proven to have any resistance to corrosion, but which was very tough and malleable, and found favour in the emerging markets for mass-produced cold pressings: ARMCO iron.

No convincing evidence was ever produced to back up claims of the non-corrodibility of pure iron, and pure-iron manufacturers themselves make no claims to its corrosion resistance. These same manufacturers offer other materials known as ‘weathering steel’ in Britain, or ‘Corten steel’ in the USA, for which they do make such claims. One of the chief uses for pure iron in today’s industry is as sacrificial anodes to protect steelwork tanks and ships, as it has been found to corrode preferentially to mild steel.

Verity’s examination of puddled wrought iron and mild steel was a chemical analysis, which appears to have overlooked the presence of the slags. During the manufacturing process, wrought iron does not become molten, as do more highly refined metals, so impurities are included in the matrix of the iron rather than being separated and disposed of. At these high temperatures the impurities are turned to glass, and are commonly known as ‘slag’, consisting of carbides and silicates which give wrought iron its fibrous structure. Approximately 250,000 siliceous fibres appear in each cross-sectional square inch of high-quality wrought iron.

The slag present in the structure of wrought iron inhibits corrosion in a number of ways:

  1. Slags themselves are non-corrodible and serve as an effective mechanical barrier against the progress of corrosion.
  2. The structure of the iron gives rise to a very rough (microscopically speaking) surface texture, which interlocks with the oxide layer, be it rust or mill scale, preventing it from flaking off the surface. The oxides act as a protective coating, preventing further corrosion.
  3. Electrically speaking, where the slag appears on the surface it acts an insulator between the areas of reactive iron, retarding electrolytic action.

When wrought iron is heated in the fire to a high temperature, the slag melts and covers the surface of the iron in a rather similar manner to flux. This glassy layer retards oxidation to the extent that the iron can be heated rather more than purer metal without burning. For this reason wrought iron is more malleable to forge than any other metal. More important, this is why it is so beautiful to fire weld, the slag acting as a flux. Smiths often comment on a property of wrought iron, which renders it softer to forge than even pure iron. This is brought about by the slags, which melt within the iron at forging temperatures and act as an internal lubricant, reducing internal friction and hence resistance to distortion under the hammer.

Given its limited availability and high price compared to mild steel, wrought iron is not always the first or right choice for all metalwork projects. Wrought iron is ideally suited for external and traditionally forged work. Although a number of blacksmiths and commissioners utilise these properties in new work, its prime use today is in the restoration and conservation of historic ironwork.

Modern conservation practice insists on the replacement of materials with like materials. As wrought iron is available for the repair and replication of wrought ironwork, it is not appropriate to use mild steel or pure iron. It would, for example, be considered wrong to repair historic stonework with concrete or cast stone and a similar principle applies to wrought iron. It is generally accepted that mild steel used on external work should be zinc coated by galvanising or hot spraying. The intricate forms and water traps of traditional decorative ‘wrought ironwork’ are notorious hotspots for corrosion. As neither of these zinc treatments is permissible nor effective with ancient work, the use of mild steel is effectively ruled out.

Wrought iron is unlike cast, in that it is not brittle, and will bend rather than break. For this reason, wrought ironwork is frequently far more delicate, although years of paint can obscure this. Cast iron is most frequently identified by its repetitive nature and forms, which could be carved into a wooden pattern, but not made by hammer and anvil.

Telling wrought iron from mild steel is often more difficult for the layman, as both will bend, and not break. Work in mild steel is often readily identified by the lower standards of workmanship often used. Look for evidence of electric welding. Mild steel is often given away by more active corrosion, which tends to run out of the joints and stain paintwork and stonework. This is seldom the case with wrought iron.

Wrought iron may also be dated approximately by its texture. Until the very end of the 18th century, sections of wrought iron were derived by forging billets by hand or waterpower. This resulted in a more-or-less uneven surface texture, and very sharp corners. A foreshortened view of a bar displays well the irregularities of the surface. Rolled bars, on the other hand, produced from the beginning of the 19th century, are perfectly smooth, and the corners can display a small radius. Nineteenth-century wrought iron is known as ‘puddled iron’.

Here are two tests for wrought iron:

  1. Nick bend test The sample is nicked by a cold chisel or sawing to approximately half depth and doubled back cold to show the fracture. Wrought iron will exhibit a ‘green stick’ fracture, showing the grain, whereas steel will exhibit a smooth fracture plane. Polish and examine for grain. The sample is polished in a plane parallel to the length of the bar, and the exposed bright surface examined for signs of a grain caused by linear slag inclusions.
  2. Spark test The sample is brought to an engineer’s grindstone and the resulting sparks examined for colour and nature. Typically a puddled wrought iron will exhibit a more-or-less dead reddish spark, whereas steel will have more-or-less bursting white sparks caused by the inclusion of carbon alloyed with the constituent iron. Charcoal irons, however, may be confused with steel in this test as they frequently contain large amounts of carbon. Pure iron, while containing no carbon, can be identified by the absence of grain in the nick bend test.

This article originally appeared in Context 139, published by the Institute of Historic Building Conservation (IHBC) in May 2015. It was written by Chris Topp, a director of Topp & Co.

--Institute of Historic Building Conservation

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