Iron

The metal of the core and the crust

Where iron comes from, and where it went

Iron is made in stars. Fusion releases energy only up to the nuclei of iron and nickel, which are among the most tightly bound of all; a massive star that has burnt its way down to an iron core has nothing left to burn, collapses, and scatters its iron in a supernova. That is why iron is the sixth most abundant element in the universe, and why so much of it went into the forming Earth.

While the young planet was still molten, the heavy iron sank. Most of it lies out of reach: the core is an iron–nickel alloy, and iron makes up about a third of the planet's mass. The slow churning of the liquid outer core is the dynamo that gives Earth its magnetic field — the same field an iron compass needle aligns with.

In the crust, iron is fourth by mass, after oxygen, silicon and aluminium. It is rarely found native; almost all of it is locked in oxides and silicates. The great ore deposits are the banded iron formations, laid down roughly 2.5 to 1.8 billion years ago, when oxygen released by the first photosynthesis rusted the iron dissolved in the oceans and dropped it to the sea floor in layers of hematite (Fe₂O₃) and magnetite (Fe₃O₄).

From meteorites to the Iron Age

The first iron people held fell from the sky. Beads from a grave at Gerzeh in Egypt, made about 3200 BC, were hammered from a meteorite; so was the dagger buried with Tutankhamun around 1323 BC. Meteoritic iron gives itself away by its nickel, which smelted ore does not carry, and the old languages knew where it came from: the Egyptians and the Sumerians both called it, in effect, metal from the sky.

Smelting iron is harder than smelting copper or tin. A charcoal furnace of the ancient world could not melt it; what it produced was a bloom, a spongy lump of iron and slag that had to be hammered hot to drive the slag out. Ironworking was known in Anatolia by the middle of the second millennium BC, but the metal stayed rare and costly until about 1200 BC, when it spread quickly across the eastern Mediterranean and the Near East. Why then is still argued over: the collapse of the Bronze Age trade routes that brought tin from afar is one explanation; better carburising of the bloom, which turned soft iron into hard steel, is another. China went its own way and, by the fifth century BC, was casting iron in moulds — a technique Europe did not master for another two thousand years.

Steel

Steel is iron with a little carbon, usually well under two per cent, and the difference is everything. Wrought iron, with almost none, is soft and tough; cast iron, with three or four per cent, is hard and brittle; steel, in between, can be hardened by quenching and toughened by tempering, and holds an edge. Smiths made it for millennia by heating iron in charcoal and hammering, and the crucible wootz of India, traded across the Islamic world, gave the watered blades of Damascus their fame.

Making steel cheaply is a story of the eighteenth and nineteenth centuries. Abraham Darby smelted iron with coke instead of charcoal at Coalbrookdale in 1709; Henry Cort's puddling furnace of 1784 turned brittle cast iron into wrought iron on an industrial scale. Then, in 1856, Henry Bessemer showed that a blast of air blown through molten pig iron burns out the carbon in minutes and heats the metal by itself. Steel rails, ships, bridges and skyscrapers followed. The Siemens–Martin open-hearth furnace of the 1860s and the basic oxygen process, first run at Linz in 1952, are its descendants. The world now makes about 1.9 billion tonnes of steel a year; by mass, iron is roughly nine-tenths of all the metal that is refined.

Iron in blood and rust

Iron gives up electrons easily and takes them back, which is why living things use it. Every molecule of haemoglobin carries four iron atoms, each cradled in a ring of haem, and each holds one molecule of oxygen on the way from lung to tissue; blood is red because of it. An adult body contains about four grams of iron, most of it in the blood, and recycles it carefully, since too little means anaemia — the most widespread nutritional deficiency in the world — and too much is a poison.

The same readiness to give up electrons is iron's weakness. In damp air it oxidises to rust, a hydrated iron(III) oxide that is flaky and porous, so that, unlike the tight oxide skin on aluminium, it never seals the metal beneath it; iron left outdoors simply keeps rusting until it is gone. The answers are coatings — paint, tin, zinc — or alloying: in 1913 Harry Brearley in Sheffield found that steel with a little more than a tenth of chromium keeps a bright surface, and stainless steel was born. Mars is red for the same reason an old nail is: its dust is rusted iron.