Thread by @jasoncrawford

Another “boring” topic I'm fascinated by: steel.

Iron & steel were once shrouded in myth. Now they are everywhere around us.

If someone from ancient times were brought to the present, our abundance of high-quality metal would make them think we were gods.

How did this happen?

Before the Iron Age, the metal was known only in the form of meteorites fallen to Earth. It was considered a gift from the gods; ancient words for steel translate roughly as “metal from heaven”. King Tut was buried with a meteoric iron dagger.

Iron is abundant in the Earth's crust, but not in pure form. Instead, iron is found in the form of *ore*. A common form of ore is a rust-colored rock—not a coincidence, because both ore and rust are iron oxide.

To get any metal from its ore, you have to heat it at thousands of degrees to separate the elemental metal from the oxygen, a process called *smelting*.

The oldest way of smelting iron is in a furnace like this one. Dump in ore, charcoal and lime, and let it burn for hours.

If all goes well, a spongy mass of iron will collect at the bottom, called a “bloom”. Remove it with tongs and pound it with a hammer. This smooths it out and gets out impurities (such as sulfur and phosphorus, which combine with the lime to make a waste product called “slag”).

The iron can then be worked with tools, which is why this form of the metal is called “wrought iron”. This is what a blacksmith does, using hammer and anvil. The metal is softer when hot, so you return it to the forge constantly—literally striking while the iron is hot.

Wrought iron, though, is typically softer than bronze. For a tool or weapon, say a sword, you want something harder.

Ancient blacksmiths found that heating the iron again in charcoal will harden the outside, good for a cutting edge. Steel is basically this hardened form of iron.

If you want to make the metal hard all the way through, you can harden the outside of many thin strips, then pound them together. This also creates aesthetic patterns, and the technique is called pattern-welding. (Japanese swordmaking used a related technique.)

There were also ways to make a strong steel that is homogenous instead of layered, but they were difficult, time-consuming and expensive. One way was to heat the iron in charcoal for days or weeks, called “cementation”. This is a cementation furnace in Sheffield.

Another process, from ancient India, was to melt the iron in crucibles together with other materials, producing “wootz steel”. This was shipped to Damascus, where it was made into legendary swords.

In the Middle Ages, furnaces were built larger to smelt bigger heats of metal, for efficiency. A larger furnace needs more air, leading to more and larger bellows; ultimately the bellows had to be water-powered. This is called a blast furnace, for the blast of air it needs.

When a furnace gets big enough, and burns long and hot enough, the iron reaches the melting point and runs liquid out of the furnace.

But this form of iron cannot be worked, like wrought iron. It is brittle, and breaks when hammered. A nuisance, thrown back to be remelted.

Except there is one thing you can do with liquid iron: you can cast it in molds. So this form of iron is called “cast iron”. Early cast iron products included church bells and cannons (popular during the religious wars of the 15th & 16th centuries).

And just like there were ways to harden wrought iron, we found ways to refine cast iron and make it less brittle. Heating in a “finery forge”, exposed to the air, would turn cast iron into wrought iron or steel.

In 1784, Henry Cort created an improved refining process called “puddling”. This used something called a “reverberatory furnace”, and involved turning and stirring the iron with a long pole.

But refining cast iron was still slow & expensive. We needed better methods.

What was going on inside the iron? Why was wrought iron soft and malleable, while cast iron was hard and brittle? Why did steel have the best of both worlds?

Was something getting into the metal to harden it, or being removed? And if there was a foreign substance, what was it?

Philosophers and scientists had been wondering about this since the dawn of the Iron Age. Aristotle thought that iron was hardened through purification, and that the forging process removed impurities. Not a bad hypothesis, but like many pre-scientific theories, it was wrong.

Réaumur experimented by melting iron in crucibles with various materials, and found in 1722 that steel is not a pure form of iron, but a “dirty” one. Bergman isolated the substance in 1781 but identified it as “phlogiston”, which was theorized to be in all combustible materials.

One tiny flaw in Bergman's theory: the phlogiston model of combustion is wrong, and phlogiston does not exist. (Whoops!) In 1786, Monge, Vandermonde and Berthollet correctly identified the mystery substance as *carbon*.

The fuel itself was getting into iron and hardening it!

We now know that wrought iron is < ~0.1% carbon, cast iron is > ~2.1%, and the sweet spot in the middle is steel.

So refining cast iron is a process of removing carbon. How to do that more efficiently?

Enter Henry Bessemer.

Bessemer knew that oxygen unites readily with carbon. And existing refining methods exposed cast iron to the air.

So: why not just use a *lot more air*?

That's exactly what Bessemer did. He built a converter to take molten iron and blow air up through nozzles in the bottom.

When he first did this, the converter erupted like a volcano, shooting fire out the top. It was rapidly oxidizing the carbon.

It worked so well that the process which had formerly taken days or weeks was reduced to about twenty *minutes*.

Bessemer lowered the price of steel by over 80%, from £40 to £6–7 per long ton, which revolutionized manufacturing. Andrew Carnegie brought the process to America in the 1870s.

Cheap steel arrived in the age of railroads, and steel rails were its main product at first. Cast iron couldn't be used for rails—they were brittle and broke. Wrought iron rails needed to be replaced frequently, in some places every few *months*. Steel rails lasted *years*.

It was also used in construction: a steel skeleton could take the weight of a building off the masonry, allowing for more floors and more windows, which led to the age of skyscrapers.

On the farm, John Deere's steel plow helped break up the tough prairie soils of the Midwest.

There have been many developments since, such as electric furnaces (good for melting scrap), rolling machines, and “continuous casting”. The blacksmith at his anvil is now part of a romanticized past, seen only in Renaissance faires, Colonial Williamsburg, or Game of Thrones.

Without steel, the modern world would be unrecognizable. Imagine NYC without its skyline, cars with wooden paneling, all our food in heavy, brittle glass jars. There would be no powered flight and certainly no space program.

The metal may no longer be mythical, but it is still marvelous.

More detail and image credits here: rootsofprogress.org/iron-from-myth…

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