How Is the Shape of a Samurai Sword Forged?

Have you ever looked at a samurai sword and wondered—how does such a perfectly balanced, elegant curve come into existence? The truth is, shaping a katana isn’t just about hammering hot steel. It’s a meticulous process that unfolds in two major stages: Sunobe (素延) and Hizukuri (火造). Let’s dive into these traditional methods and see how Japanese swordsmiths bring steel to life.

Step One: Sunobe – Forging the Sword Blank

So, what exactly is Sunobe? Think of it as the stage where the swordsmith creates the foundation of the blade, often called the “sword billet.” At this point, the katana doesn’t yet look like the sleek weapon we imagine—it’s still raw, blocky steel. But inside, the most important relationship is already taking shape: the marriage between the softer core steel (shingane) and the harder outer jacket steel (kawagane).

To protect the carbon inside the steel, the billet is first coated in a clay and rice-straw ash mixture before it goes into the forge. Once it glows bright, the smith and his assistants begin hammering. Why assistants? Because forging alone can quickly become exhausting. An experienced swordsmith focuses on the precise striking points, using a small hammer to guide the assistant’s heavy hammer — a method that is both efficient and energy-saving. The smith uses a small hammer for precision, while helpers swing the massive sledgehammers to stretch the billet longer and thinner.

This heating and hammering cycle is repeated again and again. Slowly, the billet extends into a long, straight form—usually around 60 to 70 cm, just enough for a katana. The smith must constantly check thickness and width, because if the steel gets too thin in the wrong place, the blade will be ruined. Even at this stage, the tang (nakago) is hammered out, allowing the smith to clamp and control the billet as it’s worked further.

By the end of the Sunobe stage, the billet has become the sword’s skeleton. It already has a defined tang, tip, and overall proportions. The soft inner steel ensures resilience, while the hard jacket steel promises a deadly sharp edge. Though still rough, the surface is already surprisingly smooth, thanks to the rhythm of hammer on hot steel.

Step Two: Hizukuri – Shaping the Final Katana

If Sunobe is the skeleton, then Hizukuri is the flesh and muscle. Here, the swordsmith transforms the billet into the katana’s final body. The blade grows about 10% longer and up to 30% wider, and crucially, the cutting edge begins to take shape.

The smith reheats the billet to around 1100°C, striking while it glows yellow, and works until the steel cools to a dark cherry red at about 700°C. Strike at the wrong temperature, and the blade may crack or warp. Too hot, and the delicate balance between core and jacket steel could be destroyed. Precision is everything.

During Hizukuri, the smith defines all the iconic features: the kissaki (tip), the spine, the edge, even the ridge line known as the shinogi. The blade’s surface is thinned along the edge, gradually forming the sharp side that will later be polished to lethal perfection. Any unevenness—bumps, warps, or twists—must be corrected here, with hammer and fire, long before polishing ever begins.

Watching an experienced smith at this stage is mesmerizing. Each strike seems almost effortless, yet years of training go into controlling the steel. Apprentices may spend three to five years just learning how to hammer correctly before they’re trusted with shaping a blade. After all, steel doesn’t compress; when you hit one spot, the metal flows somewhere else. A wrong move can ruin the entire piece.

One more detail: the anvil and hammer are kept damp, so that when red-hot steel touches them, the steam blasts away impurities and scale. It’s a simple but ingenious way to keep the blade’s surface clean. As the smith moves gradually from tang to tip, and back again, the katana’s graceful curve is coaxed into being, section by section.

By the end of Hizukuri, the katana’s full shape is there to see: spine, edge, ridge, tang, and tip all perfectly balanced. The edge is still left slightly thick, about 2mm, to protect it for the hardening process later. But the blade is now unmistakably a katana—sleek, curved, and alive with potential.

Glossary of Key Terms

• Sunobe (素延): The billet stage, where the steel block is hammered into the sword’s rough foundation.
• Hizukuri (火造): The forging stage where the final katana shape is created.
• Kawagane (皮鉄): Hard outer jacket steel used for the blade’s sharp edge.
• Shingane (心鉄): Softer inner core steel that adds resilience.
• Nakago (茎): The tang of the sword, the part that fits into the handle.
• Kissaki (切先): The pointed tip of the katana.
• Shinogi (鎬): The ridge line running down the blade.