How the Structure of a Katana Is Forged?

A katana isn’t just a sword made from one type of metal. It’s actually a carefully crafted structure made from layered and combined steel types. Inside, it has a softer iron core called shingane, which adds flexibility and impact resistance. Outside, it has a harder steel jacket called kawagane, which gives the blade its sharpness and edge retention. This structure is created using a forging method called Tsukurikomi (造込), which literally means "construction" or "built-up forging".

What Kinds of Steel Are Used in a Katana?

When forging a Japanese sword, two kinds of steel are used: a high-carbon outer steel (kawagane) and a low-carbon inner steel (shingane). Both are created through the same process of heating, folding, and hammering, but the original steel they’re made from is different. That’s because they each serve a different purpose in the final blade.

Shingane is softer and more ductile. It helps the sword absorb shock and prevents it from snapping during powerful blows. On the other hand, kawagane is harder and less malleable. It forms the sword’s outer layer and creates the beautiful grain pattern on the surface known as jigane. Its high carbon content is also essential for producing the hamon— the wavy temper line that appears after quenching.

How Is the Katana’s Structure Forged?

The smith combines the soft core and hard jacket to form a single piece of composite steel. One of the simplest and most traditional ways of doing this is a method called Kobuse (甲伏) forging. But there are also more complex methods used by different schools of swordsmithing. For example, the Honsanmai (本三枚) technique involves joining four pieces of steel: a hard edge steel (for the blade edge), a soft inner core, and two hard outer plates on the sides.

During the late Edo period (1790–1876), smiths experimented with all sorts of combinations, hoping to recreate the excellence of older swords.

In Kobuse forging, the process begins by shaping the hard kawagane into a wide rectangular plate. After removing any temporary tang that was welded on for easier handling, the smith hammers the kawagane along its centerline to form a U-shaped channel. This is done using special rounded hammers and tongs to hold the steel in place.

Once the U-shape is ready, one end of it is hammered closed. This end will eventually become the sword tip.

Next comes the softer shingane. This is forged into a long, slender bar with a pointed tip. It's inserted into the U-shaped kawagane shell. One important note— the pointed end of the shingane doesn’t reach all the way into the closed end of the kawagane. That’s because the tip of the katana needs to be especially hard, and that part must be pure kawagane.

After forming the shingane, the smith welds on the tang (the part of the blade that goes into the handle).

Then comes a crucial step—welding the two steels together. The surfaces of both shingane and kawagane must be in full contact with no gaps. To achieve this, the smith heats them and sprinkles borax on the surfaces. Borax acts as a flux: it prevents oxidation by shielding the hot metal from oxygen and helps the two steels fuse tightly.

Working quickly and precisely, the smith inserts the shingane into the U-shaped kawagane and hammers them together into one composite billet. This becomes the blade body. The kawagane wraps around the shingane, forming the outer surface and cutting edge. The U-shaped bottom becomes the future edge of the sword.

The result is a strong and flexible sword: the soft shingane gives it resilience, while the hard kawagane forms a sharp edge and beautiful hamon on the surface.

Common Katana Forging Techniques

The Honsanmai method uses three types of steel (in four pieces) to make a composite blade: a soft inner core (shingane), two hard side plates (kawagane), and a specially selected high-carbon edge steel called hagane. The hagane is placed along the edge side to form an exceptionally hardened cutting surface.

The joints between the hagane and kawagane can sometimes produce beautiful patterns like kinsuji (golden lines) or sunagashi (sand drifts) on the surface of the blade. These patterns are often visible in a polished sword and admired as signs of quality and craftsmanship.

In contrast, the Kobuse method is simpler. It wraps a single soft core inside a U-shaped hard outer shell. As the forging continues, the bottom edge hardens and reveals a distinct hamon after quenching. This makes the katana both a functional weapon and a work of art.