Controlling Blade Curve in Heat Treatment: How Climate, Season, and Humidity Shape the Sori and Require Correction
* Image is for illustrative purposes only.Table of Contents
One of the essences of the beauty of Japanese swords is "sori" (curvature). The unique curved shape through which the blade flows is not merely an aesthetic value but a functional curve that realizes the dispersal of force during cutting and the stability of the blade edge. However, creating and controlling this "sori" accurately is one of the most difficult fields within sword forging techniques. The impact that environmental conditions during quenching—temperature, humidity, season—have on blade shape is greater than one might imagine, and sword smiths have dealt with this challenge over hundreds of years.
Why is sori born?
The sori of Japanese swords is automatically generated during the cooling process of quenching. Because the cooling speed differs between the surface of the blade (edge side) and the back side (spine side), internal stresses of different strengths are generated. The edge side undergoes more rapid cooling, martensite transformation progresses completely, and large volume expansion occurs. On the other hand, the spine side cools somewhat more gradually, and ferrite components tend to remain. Because the expansion and elongation of the edge side becomes larger than the spine side (ridge side), the blade curves toward the spine side.
From the ancient sword period through the Edo period, sword smiths intuitively understood this mechanism and adjusted the "sori" through the selection of cooling liquid and adjustment of quenching speed. However, environmental changes due to weather and season were always a challenge that disturbed this delicate balance.
Impact of season and outside temperature on sori
Quenching is the momentary work of submerging a heated red blade into cooling liquid. At that time, outside temperature affects the temperature of the cooling liquid itself.
| Season | Cooling Liquid Temperature | Impact on Sori |
|---|---|---|
| Winter | Around 5°C if well water, around 15°C even if tap water | Temperature difference between edge side and back side becomes more pronounced, and strong sori is produced |
| Summer | Even well water can exceed 20°C | Cooling speed drops, temperature difference becomes relatively small, and only weak sori is produced |
Among old swords, many have what is called "winter-forged" strong sori, which reflects the characteristics of winter quenching. During the Edo period, sword smiths employed various ingenious methods to produce strong sori during summer, such as pre-cooling the cooling liquid (embedding ice, quenching during nighttime).
Humidity and environmental impact at the tempering stage
Further, as a subtle factor, there is humidity. Immediately after quenching, the blade has an extremely hard martensite microstructure, but at the same time is in a state where internal stress is at maximum. During the subsequent "tempering" (heating at 200°C to 400°C) to relieve stress, it is known that ambient humidity affects the tempering process.
In high humidity environments, the surface of the blade tends to be covered with fine condensation, and heat conduction changes slightly. This is a change so small that it appears negligible to the eye, but it can result in a 0.5mm to 1mm difference in the final sori shape after tempering. Particularly in short swords like kosigatana, this 1mm difference significantly affects appearance, so sword smiths considered humidity as well.
Regional climate differences and sword smith innovation
Climate differences across the Japanese archipelago also affected sword smith technique.
| Region | Climate Conditions | Sword Smith Response/Characteristics |
|---|---|---|
| Cold regions (Aizu, Etchu, etc.) | Can take advantage of low-temperature environment | Little effort required for cooling liquid temperature management; ideal cooling speed can be obtained from natural conditions alone, allowing the targeting of strong and accurate sori. Swords from the ancient period in Aizu and Etchu are known for their strong and beautiful sori |
| Warm regions (Kyoto, Osaka, etc.) | Environmentally disadvantageous | Deep wells were dug to keep water temperature constant, and there was a practice of concentrating quenching during the coldest time of day (early morning) |
Modern sword smiths' environmental control and correction techniques
Today's professional sword smiths use thermometers, hygrometers, and even infrared cameras to precisely control the quenching environment.
Workshop facilities that maintain constant room temperature, cooling devices that control the temperature of cooling liquid, temperature curve records in electric furnaces for tempering—everything is visualized. Even under such conditions, contemporary sword smiths know that a 1°C temperature difference due to seasonal variation affects the final sori, and their correction techniques are a scientific backing of the craftsman's years of experience.
Wisdom of traditional correction techniques
Edo period sword smiths made fine adjustments to the blade before quenching to correct "sori deviation" caused by environment.
- Winter: Knowing that strong sori tends to emerge, they would pre-adjust the blade to have slightly weaker sori at the forging stage
- Summer: Anticipating the reduction in sori during quenching, they would pre-apply stronger sori
Additionally, many sword smiths performed "sori adjustment" by gently bending the blade in a specific direction during the period after quenching until it cooled completely, a technique based on experience and extremely subtle. Even today, master professional sword smiths retain the same technique, which is valued as an area that cannot be replaced by machines.
Conclusion
Sori is one of the elements that best expresses the individuality of a sword. Its formation is not merely a physical result of quenching but the very process of the sword smith in dialogue with the natural environment. Innovations according to season, adaptation to regional climate, fine correction techniques—all of these have been refined through centuries of trial and error by sword smiths. Even today, the finest swords are born only at the fusion of such traditional knowledge and modern science.
