Forging Wrought Iron

This post is first in a series about my experience recreating early medieval iron artifacts.

I recently received a box of wrought iron bars from a demolished Victorian house. The house was built in 1870 and the is iron original to the structure, making it about 150 years old. What good is 150 year old scrap iron? I plan to forge this antique iron into reproductions of early medieval artifacts because it has a unique contaminate that modern processed steel lacks: slag.

What is wrought iron? The term “wrought iron” is used today as a catch-all for any metal that a blacksmith has worked by hand (wrought is from the late Old English / Middle English wroht = worked). If you Google “wrought iron,” you’ll find hundreds of rustic-looking home furnishings with a rough-hammered arts and crafts aesthetic.

Among blacksmiths, however, “wrought iron” has an older, technical meaning that refers to the process through which the metal was made. Wrought iron is iron manufactured with techniques that leave the metal contaminated with silicates, called “slag.” Modern steels do not have this contaminate, thanks to Henry Bessemer who patented the “Bessemer Process” for manufacturing slag-free steel in 1856. Steel barons like Andrew Carnegie used the Bessemer Process to produce millions of tons of inexpensive, high-quality steel alloys that contain few impurities. Our world is built on this industrial steel.

Before Henry Bessemer patented this new furnace, however, iron was made, behaved, and looked different from the modern material that replaced it. Wrought iron was a messier metal with a lot more personality.

Medieval European smiths made iron in small furnaces. These furnaces consisted of a bellows and a short clay chimney (about 1 meter tall). The smelter filled this furnace with a mix of iron ore and charcoal, lit the mixture on fire, and pumped the bellows to bring the furnace to a high heat. As the furnace grew hotter, the iron molecules in the ore reacted with the carbon in the fuel, stripping out excess oxygen and allowing the iron molecules to weld together into a lump of metal at the base of the furnace. This lump (the “bloom”) might weigh anywhere from 5-50kg depending on the furnace size. The bloom contained a mixture of pure iron, steel, and other impurities from the ore and the fuel. The chief impurity was slag.

Slag is an integral part of the premodern bloomery smelting process. Iron ore contains silicates (sand), and these silicates melt at high temperatures to form a dirty glass that settles into the bottom of the furnace and mixes into the iron bloom. “Bloomery” furnaces need slag to work, because the slag acts as a flux that lowers the iron’s melting temperature and helps its molecules weld together into a solid block of metal. Without slag, the iron bloom would crumble apart. Slag is so crucial to the process that some modern smiths will add sand to their ore if it does not contain enough natural silica to begin.

All premodern iron contained slag, sometimes up to 2% of the metal by weight. This slag was mixed into the iron in thin ribbons. These ribbons gave premodern iron a fibrous, wood-grained appearance. You can still identify wrought iron today by its distinctive grainy texture, and by the way it splinters like wood when it’s broken. When polished and etched with acid, wrought iron has a wavy, watery appearance unlike any homogeneous modern alloy.

The slag in wrought iron changes how the material behaves when it’s forged. Slag acts as a flux–a material that lowers iron’s melting point and helps the metal to weld more easily. Wrought iron is consequently very easy to weld using simple hand tools. Smiths forge weld iron by heating two pieces of iron to a white-hot heat and tapping them together with a hammer. The metals melt into one another, producing a durable bond. Any iron or steel can be forge welded, but naturally fluxed wrought iron is particularly easy to join in this manner. Laboratory analysis of premodern artifacts shows that premodern smiths forge welded different alloys of iron and steel together in almost every artifact they made. To modern blacksmiths, accustomed to work with weld-resistant modern alloys, this sometimes seems like excessive labor. For smiths who worked exclusively with slaggy wrought iron, however, forge welding was not only easy—it was necessary to accommodate their material’s physical properties.

Wrought iron has to be forge welded because of the principle disadvantage of its slaggy composition. The ribbons of slag that run through wrought iron bind the metal together, but they also cause it break apart while it is being forged and used. Wrought iron splinters if it’s forged carelessly, and finished artifacts can snap under stress if a line of slag happens to run through a crucial juncture. Wrought iron is best forged at a welding temperature, hot enough so that when the material breaks it can be forge welded back into a solid piece. When I forge wrought iron, I keep the forge several hundred degrees hotter than when I work modern slag-free steels. If I work the metal too cold (for example, when it’s red-hot or orange instead of a hotter yellow or white heat), it fractures under my hammer and the pieces have to be carefully reheated and welded back together.

Each piece of slaggy wrought iron responds differently to forging, as well. Wrought iron is no longer industrially produced, so every piece I work is recycled scrap and has varying concentrations of slag and other contaminates. Consequently, each time I forge this metal into a new shape I have to discover its unique properties and limits on the fly. If I fail, the metal fractures and I have to try to weld its pieces back together before I can continue. Medieval smiths, working with small-batch bloomery iron, shared this uncertain experience. Wrought iron’s inconsistency is almost organic, and indeed many medieval authors described wrought iron using metaphors of living organisms. In contrast to modern steels, wrought iron often acts like it has a mind of its own.

Wrought iron is, consequently, a different material to forge than modern alloys. It has to be worked at a higher heat, is more liable to splinter while it’s being hammered, and lacks the consistent, predictable behavior of modern homogeneous alloys. At the same time, it welds more easily, possesses unique tensile properties due to its grain, and is aesthetically unique in the finished artifacts. For historical reproductions, there really is no substitute for wrought iron.

By the 19th century, large foundries were producing wrought iron on an industrial scale. Large stone furnaces melted tons of ore into a liquid metal that was cast into dozens or more bars (called “pigs”) that could later be cut into smaller pieces to be forged. These industrial furnaces still dot the landscape in rural Appalachia, and you can visit them if you know where to look. The furnace pictured below is a dozen miles from my childhood home. When it was in full operation (between 1843-1857), it consumed 1200 bushels of charcoal every day (making this much charcoal meant clear-cutting about 100 acres forest every season), and produced 3 tons of iron each day.

These industrial furnaces made wrought iron that, like its medieval antecedent, still contained silica slag. Like medieval bloomery iron, industrial wrought iron had a grainy, wood-like appearance, welded easily, and behaved differently from modern, homogeneous alloys.

Henry Bessemer changed all this when he patented a new type of furnace in 1856 that worked faster, cleaner, and more affordably. The Bessemer Process blew oxygen through molten iron ore, removing impurities to produce a clean steel that lacked, for the first time, slag. Over the course of the 19th century, this new slag-free steel replaced wrought iron. Unlike the iron used from prehistory through the 1850s, this modern metal is clean, consistent, and homogeneous. This is an advantage for how we use steel today. Slag-free steel is easy to mill into precision shapes, and its consistency allows us to depend upon it to support our buildings and transportation infrastructure. A premodern blacksmith would have envied us this material’s homogeneous composition and predictable behavior.

Yet for all that, modern steels behave differently when they’re forged. Lacking slag, they also lack the organic, wood-like textures of their premodern antecedents. For replicating historical metalwork, there’s no substitute for real wrought iron.

This poses a challenge for folks like me, because the last factory to manufacture wrought iron closed in the mid twentieth century. Today, the only way to get wrought iron is to smelt new metal from scratch or else to recycle old wrought iron made in the 19th century. Historical smelting has become a thriving niche hobby, practiced by historians of metallurgy, reenactors, and iron enthusiasts. Concurrently, a small but enthusiastic market has emerged for identifying, salvaging, and recycling antique wrought iron from demolished 19th century buildings.

So you can guess how excited I was when I scored a box of antique iron bars salvaged from a demolished Victorian house last month. These bars originally formed part of a window grate. Given their age, odds were good that they were made from wrought iron rather than a modern steel alloy. To test this, I cut one bar in half and broke it in a vice. The iron splintered where it broke, a sure giveaway that it was slaggy, wood-grained wrought iron. I forged and etched a piece to confirm, and the watery, wavy patterns that appeared on the metal’s surface confirmed that it was slag-filled wrought iron. Score!

Over the next several months, I’ll be forging this metal into tools, knives, and other reproductions of early medieval artifacts. Stick around, and I’ll share what I make!

Further Reading

Light, John D. “Observations Concerning the Hand Forging of Wrought Iron.” Materials Characterization 45, no. 4–5 (October 2000): 327–40.

McDonnell, G. “Iron and Its Alloys in the Fifth to Eleventh Centuries AD in England.World Archaeology 20, no. 3 (February 1989): 373–82.

McDonnell, G., E. Blakelock, and S. Rubinson, “The Iron Economy of Wharram Percy – Modelling the AngloSaxon Iron Working Landscape,” in S. Wrathmell, Wharram Percy Archive, Archaeology Data Service (2012).

Pleiner, Radomír. Iron in Archaeology. The European Bloomery Smelters. Praha: Archeologický ústav AVČR, 2000.

Tylecote, R. F., and Brian J. J. Gilmour. The Metallography of Early Ferrous Edge Tools and Edged Weapons. BAR British Series 155. Oxford: B.A.R, 1986.

Welton, Andrew J. “Encounters with Iron: An Archaeometallurgical Reassessment of Early Anglo-Saxon Spearheads and Knives.” Archaeological Journal 173, no. 2 (July 2, 2016): 206–44.