Swords and Seasons — Care Wisdom from Japan's Four Seasons

Japanese Seasons and Japanese Sword Care

Japan's four seasons have an extremely significant impact on the storage of Japanese swords. Located in a temperate monsoon climate, Japan experiences major fluctuations in weather conditions throughout the year—from the high humidity of the rainy season to winter dryness. This intense climate variation is a key factor that makes storing Japanese swords one of the most challenging art preservation tasks in the world.

A sword blade is a subtle alloy of iron and carbon, and is extremely sensitive to changes in humidity and temperature. When humidity rises, rust forms; sudden temperature changes cause condensation. Understanding the precautions needed for each season and implementing practical measures are essential knowledge for passing precious swords down to the next generation.

Spring (March–May): The Season of Pollen, Yellow Dust, and Temperature Fluctuations

Spring is marked by the scattering of pollen and yellow dust, as well as large daily temperature swings between morning/evening and daytime—a "variable period." When taking out a sword, be sure to ventilate the room thoroughly and take care that fine particles do not adhere to the blade. During this season when windows are frequently opened and closed, storage in a sword-specific case is recommended.

Early spring is also a time when the sword oil applied the previous winter becomes prone to oxidation. Make it a seasonal routine to carefully wipe away old oil and reapply fresh sword oil. If you need to transport the sword, keep it in its case during transit and save appreciation for after you have reached your destination and the environment has settled.

Rainy Season through Summer (June–August): The Season Requiring the Most Careful Attention

From the rainy season through summer is the period requiring the greatest care in sword management. When humidity exceeds 80%, red rust rapidly forms on the blade. Even in modern homes, moisture tends to accumulate in closed storage spaces more than in outdoor air, so caution is essential.

Dehumidifier use is strongly recommended, and ideally one should run continuously 24 hours. Moisture can easily accumulate inside the shirasaya, so the sword should be removed and aired at least twice monthly. Store the koshirae separately from the blade, and regular ventilation is important. Air conditioning "dehumidify mode" alone often has insufficient capacity, so the introduction of a dedicated dehumidifier is recommended. During the rainy season, make it a habit to check the humidity meter in your storage location every weekend.

Autumn (September–November): The Optimal Period for Maintenance and Comprehensive Inspection

Autumn brings relatively stable weather and is the ideal season for sword maintenance and inspection of the overall storage environment. Refresh the sword oil and check the shirasaya for cracks, warping, and splits. It is also an excellent opportunity to remove moisture accumulated during summer from the entire blade.

Choose a clear day with good air circulation, remove the sword from its shirasaya for several hours, and allow it to air dry naturally indoors to confirm and maintain the blade's condition. Direct sunlight is absolutely forbidden. Under soft indirect light, check for early signs of rust and changes to the ji-tetsu (ground steel). By identifying problems during the autumn inspection, you can make complete preparations heading into winter.

Winter (December–February): Addressing Dryness and Temperature Changes

Special attention must be paid to the dryness caused by heating during winter. When air conditioning or kerosene heaters are in use, indoor humidity can drop to the low 20s percentage-wise, and extreme dryness can cause shirasaya to crack or warp. Using a humidifier to maintain storage location humidity in the 40–60% range is important.

Sudden temperature changes cause condensation. When moving a sword from a warm room to a cold entryway, first place it in a location where outdoor and indoor temperatures are similar before relocating it. When heating is turned off at night, temperature drops by morning easily cause condensation, so checking the storage location's condition before bed is an effective habit.

Basic Maintenance Cycle: Four Times Annually as a Guide

By performing seasonal maintenance, a sword can be kept in optimal condition. Plan maintenance four times yearly, with the turning of each season as a guide.

1. Use uchiko powder to wipe away old sword oil
2. Observe the blade's condition and check for rust, blade chips, and damage
3. Apply fresh sword oil thinly and evenly (such as clove oil)

If the oil is too little, rust prevention is inadequate; if too much, it causes condensation inside the shirasaya.

After handling the sword for appreciation or photography, always wipe away skin oils from your hands before applying oil and storing the sword in its shirasaya. Avoid touching the blade with bare hands when possible; use clean cloth or gloves.

Annual Record Management and Long-Term Preservation

Installation of a digital thermohygrometer at the sword storage location is strongly recommended. Choose a model with recording capability and check the data weekly. Keeping a simple log can be beneficial for quickly identifying the cause when abnormal reading fluctuations occur.

Selection of the storage location is also important. Avoid the following places and utilize storage stands or cases with adequate ventilation:

• Locations exposed to direct sunlight
• Near air conditioning or heating vents
• Deep inside closets where moisture easily accumulates

When storing multiple swords, ensure sufficient space so the blades do not contact each other.

Japanese sword storage is not merely "storage" but "environmental maintenance." By actively responding to seasonal changes, you can deliver the sword's true beauty to those viewing it a hundred years in the future. That is both the true responsibility and the joy of owning a Japanese sword.