1This is a video about how Japanese swords are made,
2swords that are strong enough and sharp enough to slice a bullet in half.
3The access we got for this video is incredible.
4We were able to film everything from gathering the iron sand to smelting the iron,
5forging the sword, to sharpening and polishing it.
6They even let us use it.
7The method of making these swords has remained virtually unchanged for hundreds of years,
8with everything done by hand.
9They are still considered to be among the best in the world.
10The Japanese made a weapon that was the absolute pinnacle for their style of warfare and the materials they had at hand.
11These swords are held in such high regard that one from the 16th century has been appraised at 105 million dollars,
12making it the most expensive sword ever built.
13In the Shimane province of Japan, there is a smelter that is lit for only one night each year
14where steel is made in the same way it was 1300 years ago.
15It's known as the Tatara method,
16and only steel made in this way ends up in the very best Japanese swords.
17And we were invited to come film it.
18Just after 9 AM, the ceremonial prayers are said and the fire is lit by a Shinto priest.
19Everyone that will be working this smelter will be here for at least the next 24 hours.
20That includes Veritasium producer Petr.
21- I'm committed. We're gonna do this. It's gonna be fun.
22Sword making in Japan goes back about 3000 years,
23but in those days, swords were made out of bronze.
24We're not sure how people first learned to smelt metal,
25but it was likely related to pottery.
26- In that you were using these rocky ores to make glazes and such for pottery under very controlled atmospheres.
27And then find that maybe the potters found metallic beads in the bottom of the furnaces that they were firing it.
28This possibly gave them the idea.
29Bronze was discovered before steel because it's an alloy of copper, and usually tin,
30both metals with low enough melting points that they can be smelted in regular pottery kilns.
31The problem with bronze is that although it can be sharpened, it's too soft to hold an edge for long.
32So Japanese sword makers shifted to steel 1200 years ago in the Heian period.
33This is what most people would recognize as a Japanese sword.
34It's made of steel with a curved blade.
35Steel is an alloy of iron, the fourth-most-common element in Earth's crust.
36The oceans of the world used to be rich with dissolved iron.
37But 2.5 billion years ago, cyanobacteria started photosynthesizing and creating oxygen.
38The iron reacted with that oxygen precipitating out of solution to be deposited at the bottom of the ocean.
39Incidentally, the cyanobacteria were poisoned by the oxygen that they themselves produced
40so it's thought that when levels got high enough, they died off,
41and as a result, oxygen levels dropped and iron no longer precipitated out of solution.
42Then the cyanobacteria could multiply again and the cycle repeated.
43That's why most of the world's iron is found in layers of sedimentary rock called banded iron formations.
44Each layer of iron was formed during a global flourishing of cyanobacteria that infused the ocean with oxygen.
45The majority of the global iron supply comes from these banded iron formations
46because of their high concentration of iron, up to around 60% iron oxide by weight.
47But Japan, with its mostly volcanic geology, has barely any of these sedimentary iron oxides.
48And this is likely why the country was late to the steel production game.
49Archeologists have found steel artifacts in Anatolia, which is modern day Turkey,
50that are nearly 4,000 years old.
51But in Japan, metals including steel, were imported from China and Korea up until the 8th century
52when Japan started making its own steel.
53So where did they get the raw ingredients?
54Well, igneous rocks like granite and diorite still contain iron oxides, just in much lower concentrations.
55But as the mountains are weathered, these iron oxides are broken apart and washed downstream.
56Eventually they become part of the sand.
57The Japanese noticed that because iron oxides are denser than other minerals in the sand,
58they accumulate in places where the river changes direction or speed.
59The heavier iron sinks to the bottom and the lighter material is washed away.
60To amplify this effect, they deliberately created diversions in the river to increase the concentration of iron.
61- What do you do is you dam off a section of river and then you drag sand into it.
62Because iron is heavier than the other parts of the sand,
63it is the thing that gets left behind and everything else gets washed downstream.
64With this method, you can get iron sands with 80% iron oxides by weight.
65That's more concentrated than high-quality iron ore.
66And since it has fewer impurities, it's an excellent source material for high-quality steel.
67If you heat up those iron oxides to over 1,250 degrees Celsius,
68you can break the bonds with oxygen and get pure iron.
69But pure iron is actually softer than bronze.
70So in its elemental state, iron provides no advantage.
71But nature gave humans a lucky break.
72One of the few ways you can heat something up to 1,250 degrees is with charcoal,
73and charcoal is basically pure carbon,
74and if you add just a little bit of carbon to iron,
75it creates an incredibly strong alloy: steel.
76- Yeah, a lot of people see it as a heat process.
77I see it as a chemical process.
78Alloys are usually stronger than pure metals because they contain different sized atoms,
79and this reduces the ability of atoms to slide past each other when an external force is applied.
80- So I've just been given gloves, other gloves, and a towel.
81So things are very much getting real.
82I'm genuinely quite worried.
83Here is the room with all of the charcoal that we're going to be using overnight.
84There's just bags and bags of this stuff.
85There's a Buddhist saying: "Before enlightenment, chop wood, carry water."
86"After enlightenment, chop wood, carry water."
87So we're lining up on the four corners, I guess.
88Oh. Oh boy. Didn't do a great job of that.
89So the rain is coming, so we're quickly getting all of the charcoal out and then measuring it.
90So each bag of these is 10 kilos.
91So with the iron sand, it is mixed together with water
92because if you don't mix it with water and you put it on the flame, it just flies straight up.
93But if you mix it with too much water,
94then there is water that's gonna heat up. It's gonna become water vapor, and the whole kiln could explode.
95Terrifyingly enough, they do this by feel.
96They mix in enough water until the iron sand is clumpy.
97But again, if it's too much, the whole thing could explode.
98Okay, put some iron in.
99It is just past four in the afternoon, and over the last couple of hours we have added 250 kilograms of charcoal
100and nearly 60 kilograms of iron sand.
101So yeah, it's a slow process, but I think we're starting to get somewhere.
102I have no idea because obviously the thing is hidden,
103but it should be growing.
104To achieve the high temperatures required to make steel,
105you need a strong, steady supply of oxygen.
106For hundreds of years, this was provided by huge foot-operated bellows.
107It would've taken an around-the-clock, full-body effort by many men to maintain the furnace's temperature.
108- When I came here, I was a little bit sad that the bellows were electric.
109I really wanted to, you know, have this proper experience, have this proper workout of stepping on these bellows for 24 hours.
110The temperature inside the smelter gets up to 1500 degrees Celsius,
111just below the melting point of iron, which is 1538 Celsius.
112So the iron being smelted isn't liquid,
113but it's soft and malleable enough to clump together into a big block of iron.
114No matter how high quality the iron sand is,
115there will always be some impurities, like sulfur, phosphorus, and silicon oxides.
116They combine with carbon from the charcoal and melt at a lower temperature than iron,
117so they become liquid and flow to the bottom.
118This is known as slag.
119After many more hours of adding charcoal and iron sand,
120it is time for the first removal of the slag.
121Before the first removal of slag, another prayer is said.
122- Oh, that's insane.
123Whoa.
124So for the last three hours, there's been three processes that we've been doing.
125One is adding the charcoal,
126two is adding the iron sand, and three is...
127opening up the smelter from the bottom to break apart the impurities so they can flow out.
128Just want you guys to know that it's 3:16 in the morning
129and I'm still here and I'm really sleepy.
130So it's currently six o'clock in the morning the next day. We've been smelting for 21 hours.
131I'm exhausted, but the sun is about to come out
132and it's been pretty amazing, I gotta be honest.
133We gotta close these doors really quick before they get mad at me.
134At 9 AM the next morning, the smelting is complete.
135A total of 614 kilograms of iron sand
136and 670 kilograms of charcoal were added to the smelter.
137At this point, in a traditional smelter, the only way to get the steel out would be to break it apart.
138These days, a crane is used to take the smelter apart.
139And what is left to show for all that hard work is a 100 kilogram block of steel, iron, and slag.
140Only around a third of this block is high enough quality to be used in sword making.
141- Oh, that's insane.
142That's so cool.
143The result for all the hard work.
144This is step one of making a Japanese sword.
145The steel is sorted by quality and carbon content, which is also done by eye,
146in fact, this is one of the exams you need to pass to be certified as a swordsmith.
147Then, the different grades of steel are sent out to one of 300 swordsmiths around the country.
148Only 30 do it as their full-time job,
149and one of them is Akihara Kokaji, who we went to visit next.
150This is when the forging of the sword begins.
151In a coal oven with hand-pumped bellows, the steel is heated until it is soft and malleable.
152Then using hammers, the master swordsmith flattens out the steel.
153In the old days, this would've been done by the swordsmith and three apprentices.
154The swordsmith using a smaller hammer, would set the rhythm
155and the apprentices would use big mallets to flatten the steel.
156- Woo. That was terrifying.
157These days, electric hammers are used.
158When the steel is flat enough,
159it is then bent back on itself,
160and it is then hammered again to press the steel back together into a solid block.
161So why go to all this effort flattening the steel, only to fold it back on itself
162and end up with a chunk of steel the same size as before?
163Well, because folding does two very important things.
164First, it spreads out the impurities like silicon, sulfur, and phosphorus.
165It spreads them out throughout the steel.
166This ensures a uniform consistency without any weak points.
167Second, it gives the steel a grain.
168After folding the sword, it is now reinforced in the direction that it will be hit in combat,
169and as a bonus, the steel is exposed to the air.
170So there is a small amount of oxidation creating a darker colored steel,
171which when folded makes beautiful patterns.
172There are some swords which have more than a billion layers.
173Now this doesn't mean the sword has been folded a billion times
174since every fold doubles the number of layers,
175so you only need about 30 folds to get a billion layers.
176But usually a sword is folded 10 to 13 times, resulting in a few thousand layers of steel.
177Now a blade isn't made from a single block of steel.
178The carbon content affects how hard the steel is.
179So different carbon percentages are used in different parts of the blade.
180Because carbon atoms are much smaller than iron atoms,
181they can fit inside the crystal lattice of iron.
182These trapped carbon atoms then apply an outward force to the lattice putting the steel under stress.
183The higher the carbon percentage, the harder and more rigid the steel.
184But this hardness comes at a cost.
185The steel becomes brittle, making it more likely to chip and shatter rather than bend.
186So what swordsmiths do is they use steel with different carbon contents for different parts of the blade.
187The edge is always high carbon steel to make it hard and rigid so it can maintain a sharp edge for a long time.
188But the spine is usually made of lower-carbon steel, which allows the sword to flex without breaking.
189This is done by welding together pieces of steel with different carbon contents.
190- So we have about a 15-minute break
191because, you know, it takes a while for the iron to heat up and then meld together, and then we're back in there.
192It's very hot. It's very, very hot in there.
193It's kind of unbelievable that he can do this for four hours at a time.
194After the sword is hammered into shape, which is a straight blade,
195it is covered in a layer of clay, a thick layer for the spine, and a thin layer for the blade itself.
196It's then heated in the furnace and then rapidly cooled in water,
197a process known as quenching.
198Now, because the layers of clay have different thicknesses, the rate of cooling is faster for the edge than the spine.
199When the steel is heated, carbon enters the iron lattice,
200and since the spine of the sword is covered in thick clay, it will cool slowly,
201giving time for the carbon atoms to leave the iron matrix.
202This will lead to a very low-carbon steel called ferrite,
203but the carbon atoms which have left the matrix will be caught by other iron atoms and created a type of steel known as cementite.
204The combination of ferrite and cementite is known as perlite,
205and it's a mostly soft and ductile form of steel,
206though parts of it are hard due to the cementite.
207So perlite forms the spine of the sword.
208In contrast, the very thin layer of clay on the blade means that it cools very rapidly,
209so more of the carbon is trapped in the lattice.
210This forces the lattice structure to change from cubic to tetragonal
211making a form of steel known as martensite.
212Since the trapped carbon puts stress on the lattice, martensite is incredibly hard,
213exactly what you'd want for the edge of a sword.
214The tetragonal lattice structure of martensite also takes up more space
215so the edge of the blade expands relative to the spine, curving the sword backwards.
216The iconic curve of a samurai sword comes from the formation of martensite.
217You can actually see the boundary between different types of steel in a finished sword by the difference in color.
218This is known as hamon, which literally means edge pattern.
219- At the Victoria Albert Museum in London,
220there is a Japanese sword that has a very detailed little dragon in the hamon,
221and I've looked at it many times. I don't, okay, I don't know how he did that.
222About one third of all blades shatter during the quenching process.
223- You quench it once and you thank the stars that you made it.
224The sword is then placed back in the forage to evaporate any remaining water.
225This also provides a little bit of energy to loosen some of the crystal structures making the sword less brittle.
226- And that's about the extent of the tempering process on a Japanese sword,
227which that that might be enough to relax things a bit,
228but they kept the edge much harder than you would've in the West.
229After the sword is forged, it is sent to a polisher.
230The polishing and sharpening of a sword is also done by hand with whetstones of different coarsenesses.
231It can take a month to sharpen and polish a single sword.
232- One of the things that I love is that like, this table is sloping down and the entire floor over there is sloping down.
233So when you like add the water, all of the residue and all the water, you know, flows downhill so it's not perfectly flat.
234Sometimes the swords are also engraved with beautiful patterns, though this is quite rare.
235And after all that, the sword is done.
236To learn how to use a Japanese sword, Petr got a lesson from a master, Takara Takanashi.
237He is the 10th-generation student of Miyamoto Musashi, a legendary samurai.
238Musashi killed his first opponent in single combat at the age of 13.
239He spent the rest of his life perfecting his sword-fighting, inventing a new technique with two swords.
240Musashi fought in more than 60 duels to the death, and he won every last one of them.
241There is a story about a duel that took place during a snowstorm.
242As he faced his opponent, katana outstretched,
243Musashi was so calm and kept his sword so still that snowflakes began to accumulate on the thin edge of the blade.
244So during the lesson, I thought I would get to use a katana,
245but instead we spent the entire time learning how to take the blade out of its sheath and then put it back in.
246So when I actually got the chance to use a katana to slice through some things, I was deeply unprepared.
247Okay, so this has been an amazing day.
248We've looked at some beautiful katanas, and now these wonderful people are letting me use one of their...
249just unbelievably beautiful pieces of art to chop some things.
250Like this is kind of the best day ever.
251There really is something remarkable about Japanese swords.
252The amount of care, attention, and expertise that each step requires,
253from the gathering and refining of the iron sand to the smelting, to the forging and sharpening a sword,
254each step takes so much time and skill.
255It's incredible that all these things were discovered by trial and error to produce artifacts of such high quality
256that they are still prized centuries later.
257Before I made this video, I didn't really appreciate that swords can be art.
258To 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.
259Do that enough times and you might just make something beautiful.