From: veritasium
Japanese swords are renowned for their strength and sharpness, capable of slicing a bullet in half [00:00:04]. The traditional method of making these swords has remained virtually unchanged for hundreds of years, with everything done by hand [00:00:30]. Despite their age, they are still considered among the best in the world [00:00:38]. One 16th-century sword was appraised at $105 million, making it the most expensive sword ever built [00:00:52].
Raw Materials: Iron Sand
Steel, the primary material for these swords, is an alloy of iron, which is the fourth-most-common element in Earth’s crust [00:03:01]. While most of the world’s iron comes from banded iron formations in sedimentary rock [00:04:01], Japan’s volcanic geology means it has barely any of these sedimentary iron oxides [00:04:16]. This is likely why Japan was late to steel production [00:04:22].
Instead, Japanese sword makers source iron oxides from igneous rocks like granite and diorite [00:04:49]. As mountains weather, these iron oxides break apart and wash downstream, becoming part of the sand [00:04:57]. Due to their density, iron oxides accumulate where rivers change direction or speed, with heavier iron sinking to the bottom [00:05:07]. To amplify this effect, diversions were deliberately created in rivers to increase iron concentration [00:05:25]. This method can yield iron sands with up to 80% iron oxides by weight, which is more concentrated than high-quality iron ore and has fewer impurities [00:05:44].
Smelting: The Tatara Method
In the Shimane province of Japan, a smelter is lit for only one night each year to produce steel using the Tatara method, a process that dates back 1300 years [00:01:09]. Only steel made this way is used in the best Japanese swords [00:01:25].
The process begins with ceremonial prayers and the lighting of the fire by a Shinto priest [00:01:38]. Workers remain at the smelter for at least 24 hours [00:01:46].
Historical Context of Steel Production
While sword making in Japan dates back about 3000 years with bronze swords [00:01:59], Japanese sword makers shifted to steel 1200 years ago during the Heian period [00:02:45]. Bronze, an alloy of copper and often tin, has a low enough melting point to be smelted in regular pottery kilns but is too soft to hold a sharp edge [00:02:27]. Steel artifacts in Anatolia (modern-day Turkey) are nearly 4,000 years old [00:04:26], but in Japan, metals were imported until the 8th century [00:04:34].
Chemical Process
Heating iron oxides to over 1,250 degrees Celsius breaks the bonds with oxygen, yielding pure iron [00:06:00]. However, pure iron is softer than bronze [00:06:08]. The key is adding a small amount of carbon to iron, which creates the incredibly strong alloy known as steel [00:06:28]. This is why charcoal, which is basically pure carbon, is used for heating [00:06:21]. Alloys are generally stronger than pure metals because different sized atoms reduce the ability of atoms to slide past each other [00:06:42].
Iron sand is mixed with water before being added to the flame to prevent it from flying straight up, but too much water could cause the kiln to explode [00:08:13]. The proper consistency is determined by feel [00:08:31]. Over several hours, hundreds of kilograms of charcoal and iron sand are added [00:09:00].
To achieve the necessary high temperatures (up to 1500 degrees Celsius, just below iron’s melting point of 1538 Celsius), a strong, steady supply of oxygen is required [00:09:22]. Historically, this was provided by huge foot-operated bellows, requiring continuous effort from many men [00:09:29]. Today, electric bellows are used [00:09:42].
Impurity Removal (Slag)
During smelting, impurities like sulfur, phosphorus, and silicon oxides combine with carbon from the charcoal [00:10:17]. These melt at a lower temperature than iron, becoming liquid and flowing to the bottom as slag [00:10:26]. Slag is periodically removed from the smelter [00:10:35].
Resulting Steel Block
After approximately 21 hours of smelting, involving 614 kilograms of iron sand and 670 kilograms of charcoal, the process is complete [00:12:09]. Traditionally, the steel block would be broken out, but now a crane is used to dismantle the smelter [00:12:33]. The result is a 100-kilogram block of steel, iron, and slag [00:12:51]. Only about a third of this block is of high enough quality for sword making [00:12:58]. The steel is then sorted by quality and carbon content, often by eye, which is part of the certification exam for swordsmiths [00:13:21].
Forging Process
The steel is sent to one of about 300 swordsmiths in Japan [00:13:31]. The forging process begins by heating the steel in a coal oven with hand-pumped bellows until it is soft and malleable [00:13:48].
Flattening and Folding
The master swordsmith flattens the steel using hammers [00:13:54]. In the past, this involved the swordsmith and three apprentices, with the swordsmith setting the rhythm and apprentices using large mallets [00:13:59]. Today, electric hammers are often used [00:14:23].
Once flattened, the steel is bent back on itself and hammered again to press it into a solid block [00:14:33]. This folding process serves two critical purposes [00:15:06]:
- Spreads Impurities: It distributes impurities like silicon, sulfur, and phosphorus uniformly throughout the steel, ensuring consistent strength without weak points [00:15:09].
- Creates Grain: It gives the steel a grain, reinforcing the sword in the direction it will be hit in combat [00:15:21].
As a bonus, exposing the steel to air during folding causes a small amount of oxidation, creating darker colored steel that forms beautiful patterns when folded [00:15:30]. While some swords have over a billion layers (achieved with about 30 folds), most swords are folded 10 to 13 times, resulting in a few thousand layers of steel [00:15:42].
Different Carbon Content
A blade is not made from a single block of steel. The carbon content dictates how hard the steel is [00:16:09]. Higher carbon percentages result in harder, more rigid, but also more brittle steel prone to chipping [00:16:30]. Swordsmiths use steel with different carbon contents for various parts of the blade [00:16:44]:
- Edge: High carbon steel for hardness and rigidity to maintain a sharp edge [00:16:51].
- Spine: Lower-carbon steel for flexibility to prevent breaking [00:16:58]. This is achieved by welding together pieces of steel with different carbon contents [00:17:04].
Quenching Process
After the sword is hammered into its straight blade shape [00:17:28], it is covered in a layer of clay: thick for the spine and thin for the blade edge [00:17:31]. It is then heated in a furnace and rapidly cooled in water, a process known as quenching [00:17:40].
Formation of Different Steel Types
The varying thicknesses of the clay layers cause different cooling rates [00:17:47]:
- Spine: The thick clay causes slow cooling, allowing carbon atoms to leave the iron matrix. This forms ferrite (low-carbon steel) and cementite, combining to create perlite, a mostly soft and ductile form of steel that constitutes the sword’s spine [00:17:59].
- Edge: The thin clay layer leads to rapid cooling, trapping more carbon in the lattice. This forces the lattice structure to change from cubic to tetragonal, forming martensite. Martensite is incredibly hard due to the stress from trapped carbon, making it ideal for the sword’s edge [00:18:32].
Iconic Curve and Hamon
The tetragonal lattice structure of martensite also takes up more space, causing the edge of the blade to expand relative to the spine. This expansion creates the iconic curve of a samurai sword [00:19:00].
The boundary between these different types of steel is visible in a finished sword as a difference in color, known as hamon (literally “edge pattern”) [00:19:15]. Swordsmiths can create intricate patterns like dragons in the hamon [00:19:31].
It is a challenging process, as about one-third of all blades shatter during quenching [00:19:40]. After quenching, the sword is placed back in the forge to evaporate remaining water and loosen some crystal structures, making the sword less brittle [00:19:51].
Polishing and Sharpening
Once forged, the sword is sent to a polisher. The polishing and sharpening are done by hand using whetstones of different coarsenesses [00:20:17]. This meticulous process can take up to a month for a single sword [00:20:24]. Occasionally, swords are also engraved with beautiful patterns, though this is rare [00:20:47].
Cultural Significance and Artistry of Japanese Swords
The creation of a Japanese sword involves immense care, attention, and expertise at every step, from gathering iron sand to smelting, forging, and sharpening [00:22:53]. The artifacts produced are of such high quality that they are still prized centuries later [00:23:13]. The process highlights how swords can be considered works of art, embodying deep care, attention to detail, and love for the craft [00:23:27].
“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.” [00:23:31]
This article was brought to you by Henson Shaving, who apply aerospace precision manufacturing to create high-quality razors. [00:23:50]