Summer and Winter Forging: How Seasons Shape the Swordsmith's Schedule and Steel Quality

"Winter Forging" Tradition — Why Winter Sword-Making Was Preferred

In the tradition of Japanese sword forging, winter (fuyu-kaji) has long been considered the ideal season for sword-making. The cool and dry working environment of winter has been regarded as advantageous to sword smiths in multiple respects.

The main reasons why winter sword-making was considered advantageous are the following two points.

• Stability of water temperature during quenching (yaki-ire): Quenching is the process of rapidly cooling a heated blade in water (or oil) to form the hamon and give the blade hardness. At this stage, the lower the water temperature, the greater the cooling speed becomes, and the hardening of the steel is promoted. If the ambient temperature in winter is low, the temperature of the water bucket is naturally maintained low as well, making the quenching effect more easily and stably achieved. Conversely, in summer, water temperature tends to rise, and the cooling effect may become uneven.
• Visibility of the forge's flame color (hi-iro): Before quenching, when heating the blade to the appropriate temperature (roughly 800-900°C), the sword smith judges the temperature by observing the color of the flames and the radiance of the blade with the naked eye. Compared to the bright conditions of summer, winter, with its early darkness, provides clearer visibility of the flame color, making it easier for the sword smith to detect subtle temperature changes.

Winter Forging in Practice — Advantages and Challenges of the Cold Environment

The working environment in winter has various effects on sword smiths. Since the heat of the forge warms the entire workspace, the drop in ambient temperature is partially offset in terms of perceived temperature. However, precision work while hands, feet, and face are cold presents challenges to maintaining concentration, and in delicate processes such as forge welding (tansetsu) and finishing forging, hand stiffness caused by cold can become a problem.

From the perspective of steel behavior, it is said that in winter, the shedding of oxidation scale during forging may be more uniform than in summer. This is because temperature and humidity affect the rate of surface oxidation of steel, though this mechanism remains an area of ongoing research from the perspective of modern materials science.

Moreover, winter coincides with the agricultural off-season, providing a practical reason why sword smiths could more easily secure time to concentrate in their forges. In traditional agrarian society, craftsmen often engaged in farming as a secondary occupation, and the practice of dedicating oneself to sword-making in winter, avoiding the busy farming season, naturally became established.

Summer Forging Challenges — Effects of High Temperature and Humidity on Steel and the Craftsman

Summer sword-making is sometimes called "summer forging" (natsu-kaji), but traditionally it tended to be somewhat avoided. The greatest challenge is water temperature management during quenching. When summer water temperatures exceed 20°C, the cooling speed differs compared to winter, and the effectiveness of quenching and the formation of the hamon tend to be affected. To compensate for this, sword smiths have employed the following measures:

• Add salt to water to adjust boiling point and cooling characteristics
• Place water buckets in cool locations

The hot and humid summer places significant physical strain on the craftsman. In the forge, the furnace temperature exceeds 1000°C, and work is performed amid intense radiant heat and flying scale. The addition of summer temperature and humidity increases the risk of heat stroke and causes severe exhaustion of physical strength and concentration.

However, among contemporary sword smiths, many craftsmen are not averse to summer forging, and improvements in furnace design, ventilation equipment, and water temperature management technology have made it possible to control seasonal effects more than in the past.

Season and the Sword Smith's Skill — Viewing Japanese Swords as a Dialogue with the Natural Environment

From the perspective of modern materials science, water temperature, ambient temperature, and humidity certainly have an effect on quenching results, but an experienced sword smith can make adjustments adapted to these changes. The superiority of winter forging is not absolute, and it is believed that much of the variation can be absorbed through the skill and experience of the sword smith.

The understanding of the Japanese sword as "a craft born from the fusion of the natural environment and the craftsman's skill" is well reflected in this relationship with the seasons. From the collection of sand iron to the entire process ranging from tatara iron-making, forging, quenching, and polishing, the changes of Japan's four seasons exert subtle influence. The sword smith continues to make swords while reading, compensating for, and sometimes utilizing these changes. When appreciating the Japanese sword with an awareness of the seasons, one's imagination regarding the environment in which that blade was created expands.

Sword-Making Calendar and Shinto Rituals — The Annual Cycle of the Sword Smith Marked by Seasons

Traditional sword smiths' work cycles often incorporate customs linked to Shinto rituals closely connected to swords and agricultural calendars. Rituals honoring Kanayago-kami (Kanayago-kami), the guardian deity of the forge, are conducted at forges throughout the country, and the timing of starting work and performing sword presentation carry ritual significance.

Throughout the year, the sword-making schedule traditionally follows this pattern:

Many contemporary sword smiths also observe this basic seasonal rhythm, and the tradition of winter forging is inherited not as mere superstition but as practical wisdom.

From the perspective of those who appreciate Japanese swords, being conscious of the "season in which the sword was made" is fascinating. When investigating the creation background of a celebrated sword, one discovers how closely the sword smith's working environment, technique, and belief are intertwined with the natural environment. When browsing swords in the TOUKENZA online catalog, imagining the seasonal activities of the craftsman behind the scenes is also one way to deepen one's appreciation of Japanese swords.