Wrought iron is an iron alloy with a very low carbon content (less than 0.05%) in contrast to that of cast iron (2.1% to 4.5%), or 0.25% for low-carbon "mild" steel. Wrought iron is manufactured by heating and melting high-carbon cast iron in an open charcoal or coke hearth or furnace in a process known as puddling. The high temperatures cause the excess carbon to oxidise, the iron being stirred or puddled during the process in order to achieve this. As the carbon content reduces, the melting point of the iron increases, ultimately to a level which is higher than can be achieved by the hearth, hence the wrought iron is never fully molten and many impurities remain. The primary advantage of wrought iron over cast iron is its malleability – where cast iron is too brittle to bend or shape without breaking, wrought iron is highly malleable, and much easier to bend. Wrought iron is a semi-fused mass of iron with fibrous slag inclusions (up to 2% by weight), which give it a wood-like "grain" that is visible when it is etched, rusted, or bent to failure. Wrought iron is tough, malleable, ductile, corrosion resistant, and easily forge welded, but is more difficult to weld electrically. 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. It was given the name wrought because it was hammered, rolled, or otherwise worked while hot enough to expel molten slag. The modern functional equivalent of wrought iron is mild steel, also called low-carbon steel. Neither wrought iron nor mild steel contain enough carbon to be hardened by heating and quenching. The properties of wrought iron vary, depending upon the type of iron used and the variability inherent in the relatively crude and labour intensive manufacturing process. It is generally relatively pure iron with a very low carbon content plus a small amount of mostly silicate slag, which forms fibreous or laminar inclusions, caused by the hot rolling process used to form it into long bars or rods. Because these silicate inclusions separate layers of iron and form planes of weakness, wrought iron is anisotropic, its strength varying depending on its orientation. Wrought iron may typically be composed of around 99.4% iron by mass. The presence of slag can be beneficial for blacksmithing operations, such as forge welding, since the silicate inclusions act as a flux and give the material its unique, fibrous structure. The silicate filaments in the slag also protect the iron from corrosion and may diminish the effect of fatigue caused by shock and vibration. Historically, a modest amount of wrought iron was refined 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, being in high demand for ironclad warships and railway use. However, as advances in ferrous metallurgy improved the quality of mild steel, and as the Bessemer process and the Siemens–Martin process made steel much cheaper to produce, the use of wrought iron declined. Many items, before they came to be made of mild steel, were produced from wrought iron, including rivets, nails, wire, chains, rails, railway couplings, water and steam pipes, nuts, bolts, horseshoes, handrails, wagon tires, straps for timber roof trusses, and ornamental ironwork, among many other things. Wrought iron is no longer produced on a commercial scale. Many products described as wrought iron, such as guard rails, garden furniture, and gates are made of mild steel. They are described as "wrought iron" only because they have been made to resemble objects which in the past were wrought (worked) by hand by a blacksmith (although many decorative iron objects, including fences and gates, were often cast rather than wrought).
Terminology The word "wrought" is an archaic past participle of the verb "to work", and so "wrought iron" literally means "worked iron". Wrought iron is a general term for the commodity, but is also used more specifically for finished iron goods, as manufactured by a blacksmith. It was used in that narrower sense in British Customs records, such manufactured iron was subject to a higher rate of duty than what might be called "unwrought" iron. Cast iron, unlike wrought iron, is brittle and cannot be worked either hot or cold. In the 17th, 18th, and 19th centuries, wrought iron went by a wide variety of terms according to its form, origin, or quality. While the bloomery process produced wrought iron directly from ore, cast iron or pig iron were the starting materials used in the finery forge and puddling furnace. Pig iron and cast iron have higher carbon content than wrought iron, but have a lower melting point than iron or steel. Cast and especially pig iron have excess slag which must be at least partially removed to produce quality wrought iron. At foundries it was common to blend scrap wrought iron with cast iron to improve the physical properties of castings. For several years after the introduction of Bessemer and open hearth steel, there were different opinions as to what differentiated iron from steel; some believed it was the chemical composition and others that it was whether the iron heated sufficiently to melt and "fuse". Fusion eventually became generally accepted as relatively more important than composition below a given low carbon concentration. Another difference is that steel can be hardened by heat treating. Historically, wrought iron was known as "commercially pure iron"; however, it no longer qualifies because current standards for commercially pure iron require a carbon content of less than 0.008 wt%.
Types and shapes Bar iron is a generic term sometimes used to distinguish it from cast iron. It is the equivalent of an ingot of cast metal, in a convenient form for handling, storage, shipping and further working into a finished product. The bars were the usual product of the finery forge, but not necessarily made by that process:
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