Blade Length and Forging Temperature: Craftsmen's Fire-Control Technique for Uniform Heat Treatment of Long Swords

Heat Treatment of Long Swords—Why Is It So Difficult?

In Japanese sword making, the recognition that "the longer the sword, the more difficult it is" is widely shared among bladesmiths. Heat treating blades uniformly when the blade length exceeds 70 cm in swords like the katana, or 90 cm or more in tachi or nagata naginata, presents entirely different technical challenges compared to short swords of around 30 cm blade length.

The essence of this difficulty lies in "temperature non-uniformity." When heating a long sword in a furnace, the temperature inside the furnace is not uniform, and temperature differences of 100-200°C or more can occur depending on location. These temperature differences directly result in non-uniform hamon (blade pattern) after hardening, warping defects, and in the worst cases, cracks or breaks.

Furnace Temperature Distribution—The Fundamental Challenge of Long Sword Heat Treatment

Temperature Distribution in Traditional Furnaces (Standing Furnace or Floor Furnace)

Traditional bladesmith furnaces (standing furnaces or floor furnaces) use charcoal as fuel and feature a bellows (fuigo) that supplies air. The furnace is typically smaller than the sword blade length, making it difficult to fit the entire long sword inside the furnace in some cases.

Temperature Distribution Problems:

• The center of the furnace (where the bellows air is directed) reaches the highest temperature
• The edges and entrance area of the furnace have lower temperatures
• The longer the blade, the greater the temperature difference between the base (ne) and the tip (kisaki)

Ideal Hardening Temperature

The temperature required for hardening a Japanese sword must exceed the 'AC3 point (approximately 800-900°C),' which transforms steel into austenite structure. If the temperature falls below this, proper martensitic transformation does not occur.

The problem is that it is difficult to maintain this temperature uniformly across the entire length of the blade within the furnace.

Bladesmiths' 'Fire-Control' Technique—Strategies for Uniform Hardening of Long Swords

Furnace Orientation and Blade Position

The first strategy experienced bladesmiths employ is 'how to position the sword in the furnace':

Angling the sword: By placing the sword at an angle to the furnace, the bladesmith utilizes different temperature zones within the furnace across the entire blade length.

Rotational heating: By periodically rotating the blade, the front, back, and sides are heated uniformly.

Multiple heating passes: Rather than heating the entire length at once, some bladesmiths divide the blade into two or three sections and heat them sequentially.

Methods of Observing Temperature—'Color of Flame' and 'Electric Furnace Instruments'

Temperature judgment by flame color (traditional method): When steel reaches high temperatures, it becomes "incandescent" and changes color.

Craftsmen observe the color of the blade in the furnace and instantaneously judge "where the current temperature is." This "skill of reading flame color" is acquired through experience and is tacit knowledge that is difficult to document or quantify.

Electric furnace thermometer (modern method): Most modern bladesmiths use electric furnaces and can measure furnace temperature with thermometers (thermocouples). However, even with electric furnaces, the temperature distribution inside the furnace is not uniform, and the judgment of experienced craftsmen remains necessary.

'Special Heating of the Tip Section (Kisaki Direction)'

In long swords, the tip (kisaki) direction tends to become cooler. Therefore, some bladesmiths:

Concentrated heating of the tip: Immediately before hardening, apply 'additional heating' to the tip direction to equalize the temperature.

Adjustment of clay application on the tip: By applying the hardening clay slightly thinner on the tip, the cooling rate during hardening is adjusted.

Warping Control Unique to Long Swords—The Relationship Between Hardening and Warping

Controlling 'warping (sori)' is also an important issue in hardening long swords.

How Warping Develops from Hardening

Before hardening, the sword is given a shape that anticipates "the change in warping when hardened" (called 'hardening form' or yakiiregata). During water quenching, due to differences in thermal expansion and contraction between the edge (high-carbon steel) and spine (low-carbon steel), the sword "curves with the edge on the outside (warping increases)." The hardening form is calculated in advance to "bend in the opposite direction" to compensate for this increase.

The Difficulty of Managing Warping in Long Swords

The longer the sword, the greater the change in warping during hardening, and it is more prone to becoming uneven. Particularly:

Differences in cooling rate between base and tip: When entering the water from the base versus from the tip, the cooling timing of each section differs, and this changes how the warping develops.

Prevention of bending deformation: There is a process called 'straightening (tame)' where, immediately after hardening (while the steel is still deformable at high temperature), the craftsman quickly inspects the sword and corrects any distortion using a wooden mallet or similar tool. This straightening process is particularly difficult with long swords.

Modern Bladesmiths' Long Sword Technique—Fusion of Tradition and Innovation

Modern bladesmiths designated as Living National Treasures or Important Intangible Cultural Property holders both inherit traditional techniques and utilize modern knowledge:

Data accumulation: An increasing number of modern bladesmiths record all data (furnace temperature, water temperature, changes in warping, final shape) from their completed swords and apply this 'data-driven' approach to improve future production.

Dialogue with modern materials science: Through information exchange with metallurgists, traditional knowledge such as "the method of reading flame color" is being scientifically verified to align with metal phase transformation temperatures, advancing the dialogue between tradition and science.

Conclusion—'Fire-Control' Is the Highest Art of the Bladesmith

The 'fire-control' required to uniformly heat-treat long swords with blade lengths of 70-90 cm is the highest art of the craftsman, one that cannot be captured in textbooks. Reading the color of the flame, predicting changes in blade warping, and fine-tuning the method of water quenching—the moment when all of these function as the culmination of the bladesmith's experience and intuition is the 'hardening' process.

This momentary judgment determines whether hundreds of hours of forging work culminate in a completed long sword or result in failure. The difficulty of long sword making is the fundamental element that makes the Japanese sword one of the most challenging bladed weapons in the world.