Context 139 - May 2015

C O N T E X T 1 3 9 : M A Y 2 0 1 5 13 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 newwork, its prime use today is in the restoration and conservation of historic ironwork. Modern conservation practice insists on the replace- ment 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 begin- ning 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: 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. 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. Chris Topp is a director of Topp & Co. Far left: A nick bend test for wrought iron The grain etched in wrought iron Far left: A magnified image of a cross-section of wrought iron A nick bend on mild steel showing the absence of grain