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Go to Brilliant.org/SciShow to learn how you can take your STEM skills to the next level! [INTRO♩]. Today, Damascus blades are famous for their intricate, swirling patterns.

But in ancient times, these Middle Eastern swords were mainly known for their sharpness. It’s said that they were so sharp, they could cut silk in mid-air something European swords definitely couldn’t do. So, you would imagine that these blades were popular… but blacksmiths made the last batch of true Damascus swords back in the 1800s.

And after that, despite the efforts, nobody was able to forge these blades. What happened? Well, to answer that, we need to take a closer look at how they were made.

The two biggest things that set Damascus swords apart were how European and Asian blacksmiths melted their iron, and also where they got their iron in the first place. The iron ores that we pull from the ground are mostly a mixture of iron, along with impurities like oxygen, sulfur, and silicon. So, if you want a blade made with iron, you need to remove some of those impurities.

One way to do that is to heat the ore beyond its melting point. But for iron, that’s way easier said than done. Metals like copper or bronze melt at relatively low temperatures; you can even melt them in ceramic bowls!

But iron takes a bit more work. To melt it, you need to crank up the heat to 1540 degrees Celsius and for much of history, we just didn't have the technology to do that. Instead, a special kind of smelter, known as a bloomery, was used.

This is a type of clay furnace that can get pretty hot over 1400 degrees Celsius, in some cases. While this doesn't create liquid iron, it does create a kind of spongy iron mass known as a bloom – and what blacksmiths did with that bloom, once they had it, made all the difference. When European blacksmiths worked with the bloom, they often ran into the problem of carburization.

That’s where unwanted carbon from their charcoal fire got diffused into the ingot the piece of relatively pure metal. Now, you do need some carbon to make steel. But if you overdo it, the metal becomes brittle.

So It was necessary to control the amount of carbon in the steel and on a separate note, you also had to know what temperature to forge the blade at. European blacksmiths didn't understand either, and as a result, their blades were often brittle. But for Damascus blades, Asian blacksmiths had a different strategy: crucible steel.

They closed the bloom in a little clay vessel called a crucible, along with carbon-containing materials like wood or leaves to give them only the carbon they wanted. Enclosing the bloom also meant that they could actually melt it because as the carbon slowly diffuses into the bloom, it lowers its melting point, and the iron starts to liquify. But it takes a while, and some blacksmiths even heated the crucible for days.

After melting the iron, it was then cooled very slowly to form iron carbide crystals, a mixture of iron and carbon, which created the structure necessary for skilled smiths to create the distinct swirling pattern of Damascus blades. It wasn’t just the crucible that made Damascus steel so strong, though. It also had to do with where the metal came from.

Although they’re called Damascus blades, the raw material didn’t come from the city of Damascus, now in Syria. Instead, the ingots were made in India and Central Asia then transported to Damascus and forged into things like blades. And that made all the difference.

See, these ingots had special impurities like vanadium, a rare element in the Earth’s crust. Just because of how elements in the crust happened to be distributed, vanadium deposits are found around places like China and Russia, which is close to where blacksmiths made crucible steel at the time. And vanadium served as a place where the iron and carbon could form crystals rather quickly, a process known as nucleation.

Scientists first took interest in this property in the 1960s, but at that time, samples of Damascus steel were hard to come by, so they couldn’t study it in-depth. In 2009, though, German researchers finally got samples of Damascus sabres. And when they took a closer look at these blades, they realized that trace elements like vanadium were more than just a nucleation site.

In their study, the scientists took their sample and dunked it in acid to see what they might find inside. And after a week, what was left behind were carbon nanotubes! Carbon nanotubes are what they sound like: tiny, rolled-up sheets of carbon molecules.

And because the carbons are held so tightly together, they don’t dissolve in the acid. The authors suspect that the vanadium and other impurities in the iron served as a catalyst, speeding up the formation of these nanotubes inside the blade. This means that blacksmiths were also nanotechnologists before it was cool, using carbon nanotubes to strengthen the blades!

But it might also have been part of the reason for the swords’ demise, since blacksmiths back then didn’t know about the vanadium and other small impurities present in the ores making the swords harder to replicate as time went by. That also means that most of the “Damascus blades” you find online aren’t real Damascus steel. The beautiful swirling pattern isn’t from impurities like vanadium, but mostly from welding different steels together.

Which isn’t nearly as cool! But you know what is a current technological marvel? Brilliant!

If you’ve enjoyed all this cool engineering and chemistry, you might like to keep learning with a course from Brilliant. There is even a course on predicting chemical reactions, which I bet blacksmiths back then would have found helpful! But Brilliant doesn't only have courses in engineering and chemistry they also have ones on math, computer science and engineering so they have something for everybody.

If you want to check them out, click the link in the description down below or visit Brilliant.org/SciShow. [OUTRO♩].