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Iron is the sixth most abundant element in the universe. It’s so abundant because it’s forged by stars as part of their natural lifespan. When stars die they usually erupt in a supernova explosion which expels it into space. where it finds its way into asteroids and planets. It often sinks into their core, with some deposits left over in their crust.

On Earth, it’s been relatively simple to extract from the ground and melt into new shapes, which has been pretty useful for tribes of early humans.

The ‘Iron Age’ defines the era of the ancient city states of Athens, Sparta, Troy, Carthage, and the Persian Empire. In mainland Europe, Celts built hill forts to defend their small agricultral communities. In India, the Vedas were being written. In every one of these cities you could hear the clank of the blacksmith’s anvil.

Iron’s disadvantage is that it is melts at a higher temperature compared to copper and bronze, so most early civilisations used them first before they developed the technology to use iron.

The earliest known iron goods were iron beads found in Egypt, made from metorite iron. The first iron was extracted from ore in Mespotamia and Turkey from ‘laterite’, a rusty-red rock that contains iron. By modifying their furnaces to burn hotter and by adding charcoal, they sparked a chemical reaction which gave them lumps of iron which they beat into the shape of tools and weapons. Trade and cultural adoption began the era known as the ‘Iron Age’.

Although iron is harder than metals like copper or bronze, it’s main disadvantage is that it is more brittle. Over time our use of iron has become advanced to solve this problem. We’ve purified it and created mixtures with other substances that are stronger than their constituent parts. Substituting charcoal with coke produces steel, one of the hardest substances yet developed.

But it’s main advantage is that unlike the tin required to make bronze, iron is abundant.

Iron age weapons

This abundance comes from a unique property of iron, and it is the reason why it’s made by stars. It has the highest β€˜binding energy’ of all the elements.

It is like iron’s nucleus lies at the bottom of a valley. Moving up the slope on the left are elements lighter than iron like hydrogen, carbon, and oxygen. On the right are heavier elements, like uranium. Moving towards iron unlocks energy, like rolling a ball down the slope.

If you’re on the left, you have to increase the size of the atom. On the right, you have to decrease it.

Stars have so much internal pressure that their hydrogen atoms get fused together, which increases their size, creating new elements and releasing energy.

An atomic bomb bombards uranium atoms until they split, decreasing their size and releasing energy.

But when you reach iron at the bottom of the valley, you need to spend energy to make any change, like pushing the ball back up the slope.

The ‘iron valley’

Being able to generate energy by fusing atoms together like stars has been a holy grail of engineering since the 1940’s, as it may be a practically unlimited source of clean energy.

A star’s heat and light energy comes from pushing light elements like hydrogen and helium towards iron

Stars cycle through their material, first fusing the lighter elements like hydrogen and helium, then heavier elements, until they reach iron.

As stars can only fuse atoms, not break them apart, when a star starts producing lots of iron it’s a sign that it is near the end of its life. Depending on its mass, it may erupt as a supernova or a planetary nebulae, in which the chaos of the explosion will fuse a wild assortment of other elements together, creating the heavy elements like lead and uranium.

The supernova explosion, in effect, pushes the ball up to the other size of the valley, imbuing it with lots of energy that we later use when we split atoms like uranium. But splitting atoms has the side effect of producing unstable atoms that release harmful radiation as they degrade, called nuclear waste. As this waste may outlast civilisation, it’s not the ideal way of making energy.

Even though iron signals the end of a star’s life it’s at the center of our civilisation, as we use it for everything from the steel supports of skyscrapers to cultery.

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Ben McCarthy

Ben McCarthy

Ben is the Founder of Discover Earth and the author of the Big Ideas Network.