By Joseph A. McGeough
Iron technology was derived from the known art of reducing copper and bronze. The principal requirement was a furnace capable of maintaining a reducing atmosphere—i.e., one in which a high temperature could be maintained from a good draft of air. The furnace had to be tall enough to allow the iron to drop from the smelting zone and form a slaggy lump, usually called a bloom.
After aluminium, iron is the most abundant metal, constituting about 5 percent of Earth’s crust. Copper is in short supply, having a presence of only 0.01 percent. Iron ore suitable for simple smelting was widely distributed in the form of surface deposits that could be scraped up without elaborate mining procedures.
The limitations imposed by the dearth of metals in the Bronze Age were now lifted; new tools and implements became possible, and their numbers could increase until even the poorer classes would have access to metal tools. The iron of antiquity was wrought iron, a malleable and weldable material whose toughness was enhanced by forging. Brittle cast iron, versatile and widely used in modern industry, was unknown to the ancients, and it would have been of no value for their edged tools and implements. The earliest history of smelted iron is obscure, with the first scanty evidence of man-made iron dating from about 2500 BCE in the Middle East. A thousand years later the abundance of ores led to the displacement of copper and bronze by iron in the Hittite empire.
During most of its history, iron was not recovered in a molten state but was reduced to a spongy aggregate of iron and slag formed at a temperature well below the melting point of pure iron (1,535 °C, or 2,795 °F). This plastic metallic sponge was consolidated by hammering to squeeze out slag and weld the iron particles into a compact and ductile mass; thus it was called wrought iron, essentially pure iron with remnants of unexpelled slag coating the iron particles. Wrought iron contains so little carbon that it does not harden usefully when cooled rapidly (quenched). When iron containing 0.4 to 1.25 percent carbon is heated to 950 °C (1,740 °F) and then plunged into water or oil, it is hardened.
By about 1200 BCE, when iron had become important in the Middle East, humans had learned how to create on wrought iron a steel surface, or case, that could be hardened by heating and quenching. This case was produced by the prolonged heating of wrought iron packed in a deep bed of glowing charcoal. The procedure worked because a surface of red-hot carbonless iron readily absorbs carbon from the carbon monoxide generated in the enveloping charcoal fire.
Knowledge of casting gathered from working with smelted copper and bronze did not apply to a metal whose shape could be changed only by hammering. Moreover, the malleability of iron is less than that of copper for the same temperatures, which means that the smith has to work harder to change the shape of the metal. Stone hammers gave way to hafted bronze hammers, iron itself coming into use later. The first anvils—for copper and bronze—were convenient flat stones; they were followed by increasingly larger cast-bronze models that in turn were superseded by rudimentary forms of the modern type, in which several pieces of iron are welded together. The earliest iron artefacts are of ruder appearance than the bronze articles that came before them.
A valuable property of wrought iron is the ease with which two or more pieces may be united by hammering while the metal is at a high temperature. Even at the production stage, small pieces of spongy iron were united into larger blooms. Hammer welding had been practiced before by goldsmiths and, in spite of the difficulties due to gassing, was even used for joining copper to make, for example, tape by welding together strips cut from plate. Welding became an essential production procedure. When iron tools had reached the end of a useful life, they could be reused by welding the scrap into a blank and starting over, a process akin to the melting of copper and bronze scrap to cast new tools.
Iron ordinarily has twice the flexibility of bronze and is much tougher, for a bar of iron can be bent back upon itself without fracturing, whereas a bronze bar (such as a sword blade) breaks after only a light bend (bronze blades repaired by casting new metals into the fractured sections are known). Bronze, in other words, is brittle when compared to iron, although copper is not. As the tin content of bronze rises, hardness increases, but ductility is lost. Most of the malleability is missing from cold bronze with 5 percent tin, and ductility becomes practically nil at a 20 percent tin content. The cutting edge of a hammered bronze tool is superior to that of a similarly treated iron tool, and it is corrosion resistant.
In the Early Iron Age, when the metal was still in scarce supply, local armament makers were the chief consumers of the new metal. Agricultural tools, needed for clearing forests and for cultivation, were the next iron tools to develop. Axes, picks, and hoes also were needed. Iron was smelted in the Middle East before 2500 BCE, but the Iron Age proper was 1,000 or more years in maturing. Its full development came with the discovery of hardening by carburization (addition of carbon) and heat treating, which led to superior edged tools of great toughness.