Skip to content
Material World
Element · Fe

Iron

The most-used metal on Earth by a wide margin, and the element that ended the Bronze Age.

Iron is the fourth most abundant element in the Earth's crust and the overwhelming majority of all metal produced. Its ores are common almost everywhere, which is precisely why it displaced bronze: bronze needs tin, tin is rare and comes from a few places, and a disruption to tin supply is a civilisational problem in a way that a disruption to iron supply is not.

Pure iron is soft and of limited use. What matters industrially is iron with a small and closely controlled amount of carbon in it — steel.

History

Ironworking requires higher temperatures than bronze casting and a different set of techniques, so its adoption was slower than the abundance of its ores would suggest. Once established, the abundance told: iron tools could be made everywhere, by anyone with ore and charcoal, without a trade route to a tin source.

Uses

Iron's use is steel, and steel is used for more than every other metal combined. Construction frames, reinforcing bar, vehicles, ships, pipelines, machinery, tools, appliances and packaging — the material is so pervasive that its consumption is used as an index of economic development.

Cast iron, higher in carbon and unforgeable, remains the material for engine blocks, machine tool beds and cookware, where its ability to damp vibration and hold heat matters more than its brittleness. Iron compounds are also pigments, water treatment coagulants and catalysts, most importantly in ammonia synthesis.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is found in

  • Chalcopyrite — mineral Wikidata
  • Hematite — mineral Wikidata
  • Magnetite — mineral Wikidata
  • Pyrite — mineral Wikidata
  • Fayalite — mineral Wikidata
  • Bauxite — rock · the iron oxides left behind when the aluminium hydroxides are dissolved out, and the reason bauxite is red
  • Tool steel — alloy · 80–97% · the balance
  • High-speed steel — alloy · 70–85% · the balance
  • Weathering steel — alloy · 96–99% · the balance
  • Invar — alloy · 63–65% · the balance
  • Wootz steel — alloy · the balance
  • Reinforced concrete — material · as the steel reinforcement, and its corrosion is how most reinforced concrete eventually fails
  • Mineral wool — material · from the rock, and the reason a slag wool exists at all
  • Aquamarine — mineral variety · ferrous iron for the blue, ferric for the yellow, and heating converts one to the other
  • Citrine — mineral variety · the colouring element, in a different oxidation state from the iron that makes amethyst
  • Tiger's eye — mineral variety · oxidised from the crocidolite it replaced, which is where the gold-brown comes from
  • Carnelian — mineral variety · as oxide dispersed through the chalcedony
  • Jasper — mineral variety · as oxide, and it is the impurities that make the material worth looking at
  • Peridot — mineral variety · also essential, and the source of the one colour peridot has
  • Lithium iron phosphate — compound · the redox metal, and the reason the material is cheap — iron is the commonest useful metal there is
  • Ferrite — material · as the oxide that is most of it, and the source of the magnetism
  • Arsenopyrite — mineral Wikidata
  • Chromite — mineral Wikidata
  • Columbite — mineral · substitutes within a solid solution — an individual specimen may hold little of it
  • Ilmenite — mineral Wikidata
  • Olivine — mineral · substitutes within a solid solution — an individual specimen may hold little of it
  • Pentlandite — mineral · substitutes within a solid solution — an individual specimen may hold little of it Wikidata

is a component of

  • Steel — alloy · 97–100%
  • Stainless steel — alloy · the balance
  • Cast iron — alloy · the balance
  • Neodymium magnet — alloy · 62–68% · the bulk of the alloy, and the reason it is cheap enough to put in a toy
  • Mischmetal — alloy · 0–30% · as ferrocerium, the lighter flint — the iron is what lets a steel wheel shave sparks off it
  • Electrical steel — alloy · 95–97% · the balance, and the magnetism — everything else in the alloy is there to manage iron's shortcomings as a core

is extracted from

  • Hematite — mineral · the most-used iron ore, though magnetite is richer Wikidata
  • Magnetite — mineral · higher iron content than hematite, and concentrated magnetically rather than by flotation Wikidata

is produced by

  • Smelting — process · as pig iron, high in carbon and brittle until refined

is associated with

  • Iron Age — event · the metal whose working defines the period
  • Industrial Revolution — event · smelted with coke from 1709, which uncoupled iron production from the growth rate of woodland

succeeded

  • Bronze — alloy · iron displaced bronze because its ores are common almost everywhere, where bronze depends on tin from a few places — an advantage of supply rather than of metal

is a source for

  • Vanadium — element · most vanadium is recovered from the slags of iron and steel making rather than from a vanadium mine

is used as

  • Structural engineering — application · as the basis of steel and cast iron, which is what nearly all iron becomes and the reason it is mined at all

is an alternative to

  • Aluminium — element · as the structural metal, which is the largest materials choice there is by tonnage. Iron is stiffer, cheaper and rusts; aluminium is a third the density, forms its own protective oxide, and costs several times more per tonne because winning it takes electricity rather than coke

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • PubChem
    US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors
  • CRC Handbook of Chemistry and Physics
    CRC Press / Taylor & Francis · Commercially published reference work, all rights reserved

Questions this page answers

Where it comes from, and what it becomes

Follow Iron back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Iron → is produced by (as pig iron, high in carbon and brittle until refined) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Iron → is extracted from (the most-used iron ore, though magnetite is richer) → Hematite
  • Iron → is extracted from (higher iron content than hematite, and concentrated magnetically rather than by flotation) → Magnetite
  • Iron → is produced by (as pig iron, high in carbon and brittle until refined) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite
  • Iron → is produced by (as pig iron, high in carbon and brittle until refined) → Smelting → takes as input (reduced with carbon to metallic tin) → Cassiterite
  • Iron → is produced by (as pig iron, high in carbon and brittle until refined) → Smelting → takes as input (reduced with coke in a blast furnace) → Hematite

These are the most distinct paths back. Iron can be traced through others besides.

Downstream — what it becomes

  • Iron → is a component of (the balance) → Cast iron → is an input to (as pig iron tapped from the blast furnace, carrying the carbon the process removes) → Basic oxygen steelmaking → produces (most of the world's primary steel) → Steel → is associated with (produced in small quantities long before it could be made reliably) → Iron Age complete chain
  • Iron → is a component of (the balance) → Stainless steel → is produced at (discovered here in 1913 by Harry Brearley, investigating gun barrel erosion, and found on a scrap heap because it had not rusted) → Sheffield → is associated with (Bessemer built his first steelworks here in 1858, at the point iron gave way to steel) → Industrial Revolution complete chain
  • Iron → is a component of → Steel → is associated with (produced in small quantities long before it could be made reliably) → Iron Age complete chain
  • Iron → is associated with (the metal whose working defines the period) → Iron Age complete chain
  • Iron → is associated with (smelted with coke from 1709, which uncoupled iron production from the growth rate of woodland) → Industrial Revolution complete chain
  • Iron → is a component of (the balance, and the magnetism — everything else in the alloy is there to manage iron's shortcomings as a core) → Electrical steel → is an input to (a box anneal of more than a day above 1,100 °C, during which a few correctly oriented grains grow enormously and consume the rest) → Heat treatment → produces (the same, and mostly without the autoclave, which is why it costs a fraction as much) → Glass fibre composite → is used in (the FR-4 laminate itself: woven glass cloth in flame-retardant epoxy, stiff, dimensionally stable when heated, and self-extinguishing) → Printed circuit board → is used in (and it is the change that made electronics manufacturable: assembly stopped being a wiring job and became a printing one) → Electronics manufacture

These are the most distinct paths onward. Iron ends up in others besides.