This process, called the Bessemer process, was improved by others and eventually replaced by the open-hearth process. Steel was known and available in limited quantities in the early 1800s and became commercially available after Henry Bessemer developed an efficient process for producing steel, which he patented in 1856. Steel, which is also an alloy of iron with low carbon content and other elements such as manganese, silicon, sulfur, and phosphorus, eventually replaced cast iron and wrought iron. Both metals were used in early building structures, particularly industrial buildings in England, to replace and span farther than the heaviest timbers available. Wrought iron is a more malleable or workable (hence the name “wrought”) alloy of iron with low carbon content and good tensile and compressive strengths. Wrought iron, cast iron, and, eventually, steel were made by further refining pig iron.Ĭast iron is an alloy of iron, with a high carbon content, cast into molds to create various shapes it has good compressive strength and low tensile strength. Pig iron is basically a cast iron or alloy of iron with high carbon content. This advanced technology produced metal in liquid form at the bottom of the furnace, known as “pig iron.” The term came from the collected metal shapes of molds and bars that resembled a litter of pigs. Coke was a fuel made from bituminous coal that was first heated to reduce water content and impurities – a process called “coking.” The resulting coke burned better and had a higher carbon content. The bloomery was eventually replaced by a blast furnace – blast referring to a blast of air (oxygen) and coke as a fuel instead of charcoal. However, the technology slowly advanced and came to fruition with the industrial revolution, first in England and then in other parts of Europe and the United States. Early development was limited by the availability of fuel and the ability to achieve high temperatures. The introduction of air or oxygen via bellows and pipes, called tuyeres, was added to increase the heat within the furnace. When the carbon combined with pure iron, the process created the first ferrous metals like wrought iron. The fuel typically consisted of charcoaled wood or coal, which had a dual purpose of creating heat and providing carbon. Smelting was done in a furnace known as a bloomery, consisting of a chamber or pit with masonry walls and an exhaust stack. The basic process consisted of smelting mined iron ore (naturally occurring deposits of iron-rich minerals) and forging iron bars from the resulting “bloom” or sponge-like mass of iron, separating the resulting slag and impurities. Iron technology was used and developed predominantly in Europe, China, the Middle East, and India. The industrial revolution of the 18 th and 19 th centuries brought iron making technology to an advanced state where cast iron and then wrought iron could be mass-produced and used, first in transportation and then building projects. All three materials are man-made metals (alloys) whose primary ingredient is iron.

The three primary ferrous metals used in building construction from approximately the 1850s to the 1920s were cast iron, wrought iron, and structural steel. For a structural engineer working on historic renovations and adaptive reuse of pre-war buildings, working knowledge of the history, development, and metallurgy of structural metals is necessary for the engineer to be effective and efficient. Although steel is not considered a particularly remarkable material today, Vaclav Smil’s book, Still the Iron Age, illustrates how important iron and steel have been and continue to be in industrialized societies. Structural steel has been a dominant building material for more than 100 years.

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