This is a video about how Japanese swords are made, swords that are strong enough and sharp enough to slice a bullet in half.

The access we got for this video is incredible. We were able to film everything from gathering the iron sand to smelting the iron, forging the sword, to sharpening and polishing it.

They even let us use it.

- [Petr] That is so cool!

The method of making these swords has remained virtually unchanged for hundreds of years, with everything done by hand.

They are still considered to be among the best in the world.

- The Japanese made a weapon that was the absolute pinnacle for their style of warfare and the materials they had at hand.

These swords are held in such high regard that one from the 16th century has been appraised at $105 million, making it the most expensive sword ever built.

In the Shimane province of Japan there is a smelter that is lit for only one night each year where steel is made in the same way it was 1300 years ago.

It's known as the Tatara method, and only steel made in this way ends up in the very best Japanese swords.

And we were invited to come film it.

Just after 9:00 AM, the ceremonial prayers are said and the fire is lit by a Shinto priest.

Everyone that will be working this smelter will be here for at least the next 24 hours. That includes Veritasium producer Petr.

- I'm committed. We're gonna do this. It's gonna be fun.

Sword making in Japan goes back about 3000 years, but in those days, swords were made out of bronze.

We're not sure how people first learned to smelt metal, but it was likely related to pottery.

- In that you were using these rocky oars to make glazes and such for pottery under very controlled atmospheres.

And then find maybe the potters found metallic beads in the bottom of the furnaces that they were firing it. This possibly gave them the idea.

Bronze was discovered before steel because it's an alloy of copper, and usually tin, both metals with low enough melting points that they can be smelted in regular pottery kilns.

The problem with bronze is that although it can be sharpened, it's too soft to hold an edge for long.

So Japanese sword makers shifted to steel 1200 years ago in the Heian period.

This is what most people would recognize as a Japanese sword. It's made of steel with a curved blade.

Steel is an alloy of iron, the fourth-most-common element in Earth's crust.

The oceans of the world used to be rich with dissolved iron.

But two and a half billion years ago, cyanobacteria started photosynthesizing and creating oxygen.

The iron reacted with that oxygen precipitating out of solution to be deposited at the bottom of the ocean.

Incidentally, the cyanobacteria were poisoned by the oxygen that they themselves produced so it's thought that when levels got high enough, they died off, and as a result, oxygen levels dropped and iron no longer precipitated out of solution.

Then the cyanobacteria could multiply again and the cycle repeated.

That's why most of the world's iron is found in layers of sedimentary rock called banded iron formations.

Each layer of iron was formed during a global flourishing of cyanobacteria that infused the ocean with oxygen.

The majority of the global iron supply comes from these banded iron formations because of their high concentration of iron, up to around 60% iron oxide by weight.

But Japan, with its mostly volcanic geology, has barely any of these sedimentary iron oxides.

And this is likely why the country was late to the steel production game.

Archeologists have found steel artifacts in Anatolia, which is modern day Turkey, that are nearly 4,000 years old.

But in Japan, metals including steel, were imported from China and Korea up until the eighth century when Japan started making its own steel.

So where did they get the raw ingredients?

Well, igneous rocks like granite and diorite still contain iron oxides, just in much lower concentrations.

But as the mountains are weathered, these iron oxides are broken apart and washed downstream. Eventually they become part of the sand.

The Japanese noticed that because iron oxides are denser than other minerals in the sand, they accumulate in places where the river changes direction or speed.

The heavier iron sinks to the bottom and the lighter material is washed away.

To amplify this effect, they deliberately created diversions in the river to increase the concentration of iron.

- What do you do is you dam off a section of river and then you drag sand into it.

Because iron is heavier than the other parts of the sand, it is the thing that gets left behind and everything else gets washed downstream.

With this method, you can get iron sands with 80% iron oxides by weight. That's more concentrated than high-quality iron ore.

And since it has fewer impurities, it's an excellent source for high-quality steel.

If you heat up those iron oxides to over 1,250 degrees Celsius, you can break the bonds with oxygen and get pure iron.

But pure iron is actually softer than bronze. So in its elemental state, iron provides no advantage.

But nature gave humans a lucky break.

One of the few ways you can heat something up to 1,250 degrees is with charcoal, and charcoal is basically pure carbon,

and if you add just a little bit of carbon to iron, it creates an incredibly strong alloy: steel.

- Yeah, a lot of people see it as a heat process. I see it as a chemical process.

Alloys are usually stronger than pure metals because they contain different sized atoms, and this reduces the ability of atoms to slide past each other when an external force is applied.

- So I've just been given gloves, other gloves, and a towel.

So things are very much getting real. I'm genuinely quite worried.

Here is the room with all of the charcoal that we're going to be using overnight. There's just bags and bags of this stuff.

There's a Buddhist saying: "Before enlightenment, chop wood, carry water. After enlightenment, chop wood, carry water."

So we're lining up on the four corners, I guess.

Oh. Oh boy.

Didn't do a great job of that. (chuckles)

So the rain is coming, so we're quickly getting all of the charcoal out and then measuring it.

So each bag of these is 10 kilos. Okay.

So with the iron sand, it is mixed together with water because if you don't mix it with water and you put it on the flame, it just flies straight up.

But if you mix it with too much water, then there is water that's gonna heat up. It's gonna become water vapor, and the whole kiln could explode.

Terrifyingly enough, they do this by feel.

They mix in enough water until the iron sand is clumpy.

But again, if it's too much, the whole thing could explode.

Okay, put some iron in.

It is just past four in the afternoon, and over the last couple of hours we have added 250 kilograms of charcoal and nearly 60 kilograms of iron sand.

So yeah, it's a slow process, but I think we're starting to get somewhere.

I have no idea because obviously the thing is hidden, but it should be growing.

To achieve the high temperatures required to make steel, you need a strong, steady supply of oxygen.

For hundreds of years, this was provided by huge foot-operated bellows.

It would've taken an around-the-clock, full-body effort by many men to maintain the furnace's temperature.

- When I came here, I was a little bit sad that the bellows were electric.

I really wanted to, you know, have this proper experience, have this proper workout of stepping on these bellows for 24 hours.

The temperature inside the smelter gets up to 1500 degrees Celsius, just below the melting point of iron, which is 1538 Celsius.

So the iron being smelted isn't liquid, but it's soft and malleable enough to clump together into a big block of iron.

No matter how high quality the iron sand is, there will always be some impurities, like sulfur, phosphorus, and silicon oxides.

They combine with carbon from the charcoal and melt at a lower temperature than iron, so they become liquid and flow to the bottom.

This is known as slag.

After many more hours of adding charcoal and iron sand, it is time for the first removal of the slag.

Before the first removal of slag, another prayer is said.

- Oh, that's insane.

So for the last three hours, there's been three processes that we've been doing.

One is adding the charcoal, two is adding the iron sand, and three is opening up the smelter from the bottom to break apart the impurities so they can flow out.

Just want you guys to know that it's 3:16 in the morning and I'm still here and I'm really sleepy.

So it's currently six o'clock in the morning the next day.

We've been smelting for 21 hours.

I'm exhausted, but the sun is about to come out and it's been pretty amazing, I gotta be honest.

We gotta close these doors really quick before they get mad at me.

At 9:00 AM the next morning, the smelting is complete.

A total of 614 kilograms of iron sand and 670 kilograms of charcoal were added to the smelter.

At this point, in a traditional smelter, the only way to get the steel out would be to break it apart.

These days, a crane is used to take the smelter apart.

- [Petr] Oh wow, okay. Oh!

And what is left to show for all that hard work is a 100 kilogram block of steel, iron, and slag.

Only around a third of this block is high enough quality to be used in sword making.

- Oh, that's insane. That's so cool.

The result for all the hard work.

This is step one of making a Japanese sword.

The steel is sorted by quality and carbon content, which is also done by eye, in fact, this is one of the exams you need to pass to be certified as a swordsmith.

in fact, this is one of the exams you need to pass to be certified as a swordsmith.

Then, the different grades of steel are sent out to one of 300 swordsmiths around the country.

Only 30 do it as their full-time job, and one of them is Akihara Kokaji, who we went to visit next.

This is when the forging of the sword begins.

In a coal oven with hand-pumped bellows, the steel is heated until it is soft and malleable.

Then using hammers, the master swordsmith flattens out the steel.

In the old days, this would've been done by the swordsmith and three apprentices.

The swordsmith using a smaller hammer, would set the rhythm and the apprentices would use big mallets to flatten the steel.

- Woo. That was terrifying.

These days, electric hammers are used.

When the steel is flat enough, it is then bent back on itself, and it is then hammered again to press the steel back together into a solid block.

So why go to all this effort flattening the steel, only to fold it back on itself and end up with a chunk of steel the same size as before?

Well, because folding does two very important things.

First, it spreads out the impurities like silicon, sulfur, and phosphorus. It spreads them out throughout the steel.

This ensures a uniform consistency without any weak points.

Second, it gives the steel a grain.

After folding the sword, it is now reinforced in the direction that it will be hit in combat, and as a bonus, the steel is exposed to the air. So there is a small amount of oxidation creating a darker colored steel, which when folded makes beautiful patterns.

There are some swords which have more than a billion layers.

Now this doesn't mean the sword has been folded a billion times since every fold doubles the number of layers, so you only need about 30 folds to get a billion layers.

But usually a sword is folded 10 to 13 times, resulting in a few thousand layers of steel.

Now a blade isn't made from a single block of steel.

The carbon content affects how hard the steel is. So different carbon percentages are used in different parts of the blade.

Because carbon atoms are much smaller than iron atoms, they can fit inside the crystal lattice of iron.

These trapped carbon atoms then apply an outward force to the lattice putting the steel under stress.

The higher the carbon percentage, the harder and more rigid the steel.

But this hardness comes at a cost. The steel becomes brittle, making it more likely to chip and shatter rather than bend.

So what swordsmiths do is they use steel with different carbon contents for different parts of the blade.

The edge is always high carbon steel to make it hard and rigid so it can maintain a sharp edge for a long time.

But the spine is usually made of lower-carbon steel, which allows the sword to flex without breaking.

This is done by welding together pieces of steel with different carbon contents.

- So we have about a 15-minute break because you know it takes a while for the iron to heat up and then meld together, and then we're back in there.

It's very hot. It's very, very hot in there.

It's kind of unbelievable that he can do this for four hours at a time.

After the sword is hammered into shape, which is a straight blade, it is covered in a layer of clay, a thick layer for the spine, and a thin layer for the blade itself.

It's then heated in the furnace and then rapidly cooled in water, a process known as quenching.

Now, because the layers of clay have different thicknesses, the rate of cooling is faster for the edge than the spine.

When the steel is heated, carbon enters the iron lattice,

and since the spine of the sword is covered in thick clay, it will cool slowly, giving time for the carbon atoms to leave the iron matrix.

This will lead to a very low-carbon steel called ferrite,

but the carbon atoms which have left the matrix will be caught by other iron atoms and created a type of steel known as cementite.

The combination of ferrite and cementite is known as perlite, and it's a mostly soft and ductile form of steel, though parts of it are hard due to the cementite.

So per light forms the spine of the sword.

In contrast, the very thin layer of clay on the blade means that it cools very rapidly, so more of the carbon is trapped in the lattice.

This forces the lattice structure to change from cubic to tetragonal making a form of steel known as martensite.

Since the trapped carbon puts stress on the lattice, martensite is incredibly hard, exactly what you'd want for the edge of a sword.

The tetragonal lattice structure of martensite also takes up more space so the edge of the blade expands relative to the spine, curving the sword backwards.

The iconic curve of a samurai sword comes from the formation of martensite.

You can actually see the boundary between different types of steel in a finished sword by the difference in color.

This is known as hamon, which literally means edge pattern.

- At the Victoria Albert Museum in London, there is a Japanese sword that has a very detailed little dragon in the hamon,

and I've looked at it many times. I don't, okay, I don't know how he did that. (laughs)

About one third of all blades shatter during the quenching process.

- You quench it once and you thank the stars that you made it.

The sword is then placed back in the forage to evaporate any remaining water.

This also provides a little bit of energy to loosen some of the crystal structures making the sword less brittle.

- And that's about the extent of the tempering process on a Japanese sword, which that that might be enough to relax things a bit, but they kept the edge much harder than you would've in the West.

After the sword is forged, it is sent to a polisher.

The polishing and sharpening of a sword is also done by hand with whetstones of different coarsenesses.

It can take a month to sharpen and polish a single sword.

- One of the things that I love is that like, this table is sloping down and the entire floor over there is sloping down.

So when you like add the water, all of the residue and all the water, you know, flows downhill so it's not perfectly flat.

Sometimes the swords are also engraved with beautiful patterns, though this is quite rare.

And after all that, the sword is done.

To learn how to use a Japanese sword, Petr got a lesson from a master, Takara Takanashi.

He is the 10th-generation student of Miyamoto Musashi, a legendary samurai.

Musashi killed his first opponent in single combat at the age of 13.

He spent the rest of his life perfecting his sword-fighting, inventing a new technique with two swords.

Musashi fought in more than 60 duels to the death, and he won every last one of them.

There is a story about a duel that took place during a snowstorm.

As he faced his opponent, katana outstretched, Musashi was so calm and kept his sword so still that snowflakes began to accumulate on the thin edge of the blade.

- So during the lesson, I thought I would get to use a katana, but instead we spent the entire time learning how to take the blade out of its sheath and then put it back in.

So when I actually got the chance to use a katana to slice through some things, I was deeply unprepared.

- [Producer] Well, it looks like it's your turn.

I'm so scared.

Okay, so this has been an amazing day.

We've looked at some beautiful katanas, and now these wonderful people are letting me use one of their just unbelievably beautiful pieces of art to chop some things.

- Oh. Oh.

- Woo!

Like this is kind of the best day ever.

There really is something remarkable about Japanese swords.

The amount of care, attention, and expertise that each step requires, from the gathering and refining of the iron sand to the smelting, to the forging and sharpening a sword, each step takes so much time and skill.

It's incredible that all these things were discovered by trial and error to produce artifacts of such high quality that they are still prized centuries later.

Before I made this video, I didn't really appreciate that swords can be art.

To me, it's a good reminder that whatever you do, you should do it with deep care, attention to detail, and love for the craft.

Do that enough times and you might just make something beautiful.

Hey, this part of the video was brought to you by Henson Shaving.

You know, making high quality things is hard, and as far as I'm concerned, Henson make the highest-quality razors in the world, the katanas of the shaving world, if you will.

I've been using their AL-13 razor for the last year, and it's the best razor I've used by a long margin.

One of the reasons for that is Henson really understands the physics of shaving, because as aerospace machinists for 20 years, they understand how to cut things effectively.

They get precision.

Human hair is much harder than the skin it's growing out of, and as a result, typical razors drag and pull on the hair while cutting.

What a multi-blade razor does is it assumes that the first cut will be ineffective, and so they build in extra blades to cut the hair before it can snap back into position.

And that does result in a good cut, but it also increases friction leading to razor burn and skin irritation.

Henson carefully studied that problem and realized that you didn't need to add extra blades, you just needed to properly support a single blade.

Virtually every razor on allows for too much play in the blade, but Henson doesn't.

The result is an effective cut without the additional friction on your skin.

Seriously, a Henson is truly a great razor.

It'll last you a lifetime.

You will get a better shave with less irritation, and you'll also save money in the long run.

So head over to hensonshaving.com/veritasium or scan the QR code here and enter code "veritasium" for a hundred free blades with a purchase of any razor.

I wanna thank Henson Shaving for supporting Veritasium, and I wanna thank you for watching.