How a Katana Is Clay Tempered, Step by Step

clay tempering
Matsukage katana with a hand-forged, clay-tempered T10 carbon steel blade

Part 2 of our clay tempering series. New here? Read What Is Clay Tempering? first for the science, or start with our clay tempering overview.

Clay tempering takes only a few seconds at the moment that matters, and days of work to get there. Here is how a hand-forged katana goes from raw steel to a finished, clay-tempered blade, and how you can spot the real thing when you're shopping.

The process, step by step

1. Forge and shape the blade

The smith forges the steel to rough shape, drawing out the length, setting the bevels, and forming the point (kissaki) and the tang (nakago). The blade is left nearly straight. Most of its curve will come from the quench. It is then refined by filing and rough grinding so the geometry is even along its full length. Uneven thickness is a leading cause of warping and cracks later.

2. Normalize the steel

Forging leaves stress and an uneven grain in the steel. Before hardening, the smith heats and air-cools the blade several times to relax that stress and refine the grain. It's an unglamorous step, but it gives the quench a much better chance of success.

3. Mix the clay

The coating, called yakiba-tsuchi, is traditionally a blend of clay, powdered charcoal, and fine stone powder mixed with water into a thick paste. Every smith has a preferred recipe. The goal is a coating that sticks to hot steel, insulates well, and doesn't flake off when the blade hits water.

4. Apply the clay

This is where the hamon is designed. The smith spreads a thick layer over the spine and body, then brushes a very thin wash over the edge. The line where thick clay meets thin clay becomes the hamon. A straight line makes a straight hamon; a hand-shaped wave makes a wavy one. Thin strokes of clay drawn down toward the edge create the small streaks (ashi) that collectors look for. The clay is then left to dry completely.

5. Heat to hardening temperature

The blade goes into the fire and is heated evenly along its entire length. Traditional smiths often work in a darkened forge, or even at night, so they can judge the exact shade of red by eye. Too cool and the edge won't harden; too hot and the grain coarsens and the risk of cracking rises.

6. The quench

At the right moment, the smith plunges the blade edge-down into a trough of water (some smiths prefer warm water or oil, which cool more gently). The thinly coated edge hardens almost instantly; the clay-covered body cools a little slower and stays softer. In those seconds the blade often flexes one way and then settles into a deeper curve as the edge transforms. This is the moment the sword is either made or lost.

7. Temper

Fresh from the quench, the edge is hard but highly stressed. The smith reheats the blade gently, far below forging heat, to relieve that stress and trade a sliver of hardness for toughness. In Japanese this step is called yaki-modoshi.

8. Inspect and correct

The blade is checked for cracks, warps, and an even hardening line. Small bends can be corrected; a crack in the edge (hagire) usually means starting over. Even experienced smiths lose blades at this stage, which is one reason clay-tempered swords cost more than through-hardened ones.

9. Polish to reveal the hamon

Straight out of the quench, the hamon is barely visible. It comes out during polishing, worked through progressively finer stones until the hardened edge shows as a bright, frosted band against the darker body. A full traditional polish can take a skilled polisher many days. This is why every clay-tempered MoonSwords blade is hand-polished.

How to tell a real hamon from a fake

Because the hamon signals quality, some makers fake it with acid etching, masking, or wire-brushing on a blade that was never differentially hardened. Here's what to look for:

  • Tilt it in the light. A real hamon changes as you move the blade. A soft, misty transition zone appears and disappears with the angle. A fake one usually looks the same from every angle, like a printed line.
  • Look for activity. Real hamon often show tiny sparkling crystals (nie), a soft glow (nioi), or small streaks toward the edge (ashi). Fakes tend to be a flat, uniform frosted band.
  • Check where it starts. A real hamon begins near the notch where the edge meets the tang. One that runs onto the tang, or stops abruptly, is a warning sign.
  • Read the listing. A clay-tempered blade should say so clearly and name a suitable steel, such as T8, T10, 1095, or a carbon-steel Damascus. "Hamon pattern" alone, with no mention of clay tempering or differential hardening, deserves a question to the seller.

If you're unsure about a MoonSwords piece, just ask us. We'll tell you exactly how it was made.

Caring for a clay-tempered blade

The hardened edge and the polish that reveals the hamon both deserve gentle care:

  • Wipe the blade after handling and keep a light coat of oil on it. Our high-carbon steel care guide covers the routine.
  • Avoid harsh abrasives and metal polishes, which can dull the contrast of the hamon.
  • Deal with spots early and lightly; see Safe Sword Rust Removal.
  • Store horizontally in a dry place, out of the saya for long-term storage.

Find a clay-tempered blade

Clay-tempered, hand-polished blades run across our whole range:

Tags:  Clay tempering,  Hamon,  Katana,  Sword care