- [Derek] 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!

- [Derek] 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.

(sword thwacks) (dramatic music)

(river noises)

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.

(drums and flute music)

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.

- [Derek] 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.

- [Derek] 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.

(gentle music)

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.

- [Derek] 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.

- [Derek] 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.

(serious music)

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.

(charcoal rattles)

(flame crackles)

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.

(charcoal rattles)

Oh. Oh boy.

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

(thunder rumbles)

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.

(serious music)

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.

- [Derek] 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.

(serious music)

- [Derek] 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.

(steelworkers clap)

- Oh, that's insane.

(slag sizzles)

(serious music)

(hammer pinging)

Whoa.

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.

(shovels scrape)

Just want you guys to know that it's 3:16 in the morning

and I'm still here and I'm really sleepy.

(subdued music)

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.

- [Derek] 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!

- [Derek] 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.

(crane whirs and clunks)

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

The result for all the hard work.

This is step one of making a Japanese sword.

(subdued music)

- [Derek] 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.

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.

(hammers clank) (Petr grunts)

- Woo. That was terrifying.

- [Derek] These days, electric hammers are used.

When the steel is flat enough,

it is then bent back on itself,

(fire crackles)

and it is then hammered again

to press the steel back together into a solid block.

(gentle music)

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.

- [Derek] 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)

- [Derek] About one third of all blades

shatter during the quenching process.

- You quench it once

and you thank the stars that you made it.

- [Derek] 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.

- [Derek] 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.

(hammer taps softly)

- [Derek] 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.

(serious music)

(subdued music)

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.

- [Petr] 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.

(katana thuds)

(playful music)

- [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.

(Petr exhales forcefully)

(katana thuds) - Oh.

Oh.

- Woo!

Like this is kind of the best day ever.

(subdued music)

- [Derek] 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.

(steelworkers speaking Japanese)

(steelworkers clap)

- [Derek] 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.

(both speaking Japanese)

- Do that enough times

and you might just make something beautiful.

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