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The 'metal of heaven'
It's not known when human beings first discovered how to smelt iron ore to produce useable iron. The oldest-known iron objects, found at two sites on the banks of the Nile, are beads made of iron derived from meteorites, which date from around 6,000 years ago.
Meteoric iron was widely known throughout the ancient world, where it was cold-worked (if it didn't break in the process) to produce spear tips, cylinder seals and ornaments. The Egyptians even called iron the 'metal of heaven' because of its association with meteorites.
More costly than gold
The first known smelted iron objects date from around 3000 BC and appeared in increasing numbers throughout the Near and Middle East during the next millennium. Iron remained extremely expensive, though – more costly than gold – and its use was largely restricted to prestigious ceremonial or ritual objects until the latter half of the second millennium BC. Even then, it did not replace bronze (an alloy of copper and tin) as the metal of choice for weapons and tools.
It was not until around 1200-1000 BC that iron working became widespread with the onset of what we now call the Iron Age in the Near and Middle East. The switch from bronze to iron was probably triggered by disruptions to the supply of tin.
Once established, the use of iron was to revolutionise the ancient world, not least in terms of weaponry. By the end of the second millennium BC, for example, the Greeks had developed the technique of 'carburisation' – adding carbon to iron – to harden iron weapons by heating them over charcoal. The armies that made best use of the new technology – in protective armour as much as in swords and other weapons – went on to dominate their enemies.
Iron Ages
The 'Iron Age' did not begin everywhere at the same time. Those who understood how to carry out the apparently magical transformation of stone into metal tried to keep their secrets for as long as they could. Iron working didn't spread beyond the Middle East and Mediterranean until much later than it first appeared there.
The British Iron Age, for example, didn't begin until about 700 BC, and widespread iron working and use didn't occur in Britain for several centuries after that. In China, the widespread use of iron emerged in the 6th century BC. In the Americas, iron working was unknown before the arrival of Columbus (although meteoric iron objects have been found there). This was one of the main reasons why the European invaders were able to conquer the native population with such ease.
Wrought iron
With the exception of China, virtually all of the iron produced before the 14th century was what we know as wrought iron. This is produced by heating iron ore in a furnace, so that the ore is 'reduced' (the oxygen forced out of the raw iron oxide).
Impurities in the ore must also be removed by 'slagging', whereby a high enough temperature has to be reached in the furnace to liquify and remove as much of the slag as possible. The resulting 'iron sponge' or 'bloom' is then worked by heating in a forge and repeatedly hammering and folding the heated product to force out the remaining slag and consolidate the pure iron.
Steel
The wrought iron produced in this way still contains a proportion of slag and other impurities. But the process leaves a very low carbon content, which means that the iron cannot be hardened to the same extent as steel (which is iron containing about 0.3-1.2% carbon).
Early ironworkers, however, discovered that they could get a much tougher product by heating wrought iron on a bed of charcoal and then quenching it in water or oil. This resulted in a steel coating that made the end product both harder and less brittle.
Cast iron
Cast iron (also known as pig iron, after the ingots, or 'pigs', in which it was cast) is an alloy of about 96.5% iron and 3.5% carbon. It requires more sophisticated and hotter furnaces to produce than wrought iron, and although the Chinese developed such furnaces from the 6th century BC onwards, the technology did not reach Europe until very much later.
Some of the earliest cast iron produced in Europe came from furnaces in Sweden in the 12th-14th centuries AD. Increased demand for cast objects such as cannon balls from the 14th century onwards helped to spread the technology throughout the continent. In time, with improved knowledge and better furnaces, ironworkers learnt how to refine the iron further to produce steel.
Stumbling into the future
The early ironworkers understood none of the physics and chemistry involved in producing workable iron and steel. They learnt by trial and error, were fiercely protective of what they knew and often mixed together practical techniques with ritualistic fantasy. They stumbled across the production of steel haphazardly and inconsistently. Knowing nothing about the role of carbon in producing hardened steel, they nonetheless managed it simply because they used (carbon-rich) charcoal to heat their ore.
The techniques that they discovered laid the foundations for the bigger and better blast furnaces of the industrial revolution. Ultimately, the 'metal of heaven' was to transform life on earth.