Throughout history iron has been produced from naturally-occurring iron ores. Very small amounts of iron—more accurately, a natural alloy of iron and nickel—have been found as meteorites, and they were hammered out into useful shapes, but the quantities were so small that meteoric iron has never been more than a curiosity. Specimens can be seen in some museums. Most iron ores—there are several varieties—are a dusty reddish-brown rock, though some are darker in hue, almost black or purple. Iron is the fourth most abundant element in the world, and reddish-coloured earth gives a clue to its presence. The red soil of Devon, for example, shows that iron is present, though in fact the few ores there are not rich enough to be worth working.
Iron ores are all, basically, a chemical mixture of iron and oxygen (iron oxide), with small quantities of other elements, and as found in the earth they also have varying amounts of contaminants such as clay, stone, lime and sand mixed with them. Some of the impurities are easily removed; others are more difficult, and many of the important inventions in the history of iron and steel have been connected with the removal of impurities.
Iron ore is an oxide because iron has a strong affinity for oxygen and there is always a supply of oxygen available in the air. If metallic iron is left exposed to the air it will slowly become an oxide again: it will rust. Fortunately for the ironmaker, carbon has an even greater affinity for oxygen than iron. If iron ore is heated strongly in contact with carbon, the oxygen and carbon will unite to form a gas, which burns away, leaving the iron behind. That is the basis of iron ore conversion into iron—reduction or smelting.
One of man’s earliest technical achievements was to make fire, and it could be that when a fire was started—for protection, warmth and cooking— somebody noticed a change in the nature of the stones used to surround and contain the fire. If two of the stones were banged together, they gave off a dull sound and did not crack or splinter: the charcoal (which is a very good and pure form of carbon) of the wood fire, urged perhaps by a strong wind, had reduced to iron some of the stones, which were actually iron ore. It would not be long before somebody had the curiosity to try other likely-looking stones round the fire, then it would only be a matter of time before somebody tried hammering one of the changed stones while it was red hot. He would find that he could beat it out into useful shapes which, when cold, were strong and did not break or bend easily. By hammering the material into, say, a knife or a spearhead, and rubbing the point on a rough stone to sharpen it, our early experimenter could make a much better tool or weapon than he had ever had before. Such speculation is justified in the absence of known facts. At all events, ironmaking had spread to Europe by about 1000 BC from the Middle East, where it apparently began much earlier.
At first, and for many centuries, the equipment used was very simple and the production of iron extremely small. A group of men working for several hours could only make a piece of iron perhaps not much bigger than a man’s fist, and weighing no more than one or two kilograms. But the trade of ironmaking had started, and villages began to get their ironmakers—just as they had their millers, potters and weavers—wherever iron ore could be found. In those parts of the world where there was no iron ore, traders began to take iron goods to exchange for other products and international trade in iron began to spread. Iron was still scarce, however, and used only for such things as tools and weapons.
The product made by the early workers in iron was wrought iron. Pure iron as such is only a laboratory curiosity and has no commercial use, but wrought iron is quite close to it. It has a fibrous structure: if a piece of wrought iron is nicked with a chisel on one side and then hammered back on itself it will tear and open out to show a structure that looks very much like that of a piece of wood. Wrought iron can be shaped by hammering it while it is hot (or in later years by passing it between rotating rolls) and if two pieces at the right temperature are hammered together they weld into one piece. It is possible to melt wrought iron but of no practical value, so it was never melted in practice: the iron was converted, or reduced, directly from the ore in what is therefore termed the direct reduction process.
The early equipment used to make wrought iron was as simple as the metal itself, consisting of a small furnace, heated by charcoal and called a bloomery, hand- or foot-operated bellows to blow the charcoal fire, and some tongs to hold the hot metal while it was forged into shape. Bloomeries varied in shape and size, though they all functioned in the same way. They were made of clay, which would resist the heat of the fire. Charcoal was lighted inside the bloomery and then, while a continuous blast of air was kept up by the hand or foot bellows (the operators taking turns), more charcoal and some iron ore were fed in by hand through a small aperture in the top. As the oxygen in the ore united with the carbon of the charcoal it became a gas, which burned off at the top of the bloomery as a light blue flame.
After a few hours all the oxygen had gone from the iron ore, and a small, spongy ball of iron, the bloom from which the bloomery took its name, remained. Then the front of the bloomery was broken open and the bloom was raked out and taken to an anvil for hammering to whatever shape was required. In common with workers in other trades, ironworkers relied on their practical skills, not on theoretical knowledge. Apprentices learned from their masters, or their own experience, how to judge when the bloom was ready inside the enclosed furnace, or how to choose the best ores from their appearance. Such craftsmanship was the basis of their operations until comparatively recent years, when scientific methods took over.
The bloomery could never have been operated on a large scale, even if mechanical power had been available. Some modifications were made to the process in some parts of the world and sometimes a waterwheel was used instead of manpower to work the bellows. Individual bloomery outputs grew a little, too, but no essential change in technology occurred in the three thousand years up to the fifteenth century AD.