Nihonto Storage and Preservation: Temperature, Humidity, Sword Boxes, and Long-Term Care Risks
* Image is for illustrative purposes only.Table of Contents
Main Causes of Japanese Sword Deterioration
The main causes of deterioration in Japanese swords are the following three.
- Oxidation (rust)
- Organic contamination (mold)
- Mechanical damage
These are primarily caused by failures in temperature and humidity control in the storage environment, as well as improper oil application and maintenance methods.
Proper Temperature and Humidity Management
The ideal storage environment should aim for temperatures of 15–25°C and relative humidity of 50–60%.
| Humidity Level | Effect |
|---|---|
| Above 70% | Rust progresses rapidly |
| 50% or below | Wood materials wrapping the blade (shirasaya and sword box) risk cracking due to drying and shrinkage |
Because Japan's summers are hot and humid, using both air conditioning and a dehumidifier is effective. In winter, during dry periods, humidification may also be necessary. In traditional homes with storehouses (kura), it has long been customary to conduct "mushi-boshi" (airing out) in spring and autumn to check the condition of the swords and disperse moisture. This practice remains an effective maintenance habit even today.
Choosing Sword Boxes and Sword Bags
The main tools and materials used for storing Japanese swords and their characteristics are as follows.
| Tool/Material | Characteristics and Precautions |
|---|---|
| Sword box (katabako, paulownia wood) | Insect-repellent properties, moisture absorption and release, and light weight make it ideal for preserving Japanese swords |
| Shirasaya (magnolia wood) | Light weight and moderate moisture absorption and release protect the blade |
| Synthetic storage cases | Poor ventilation and risk of moisture being trapped by sealing |
| Sword bag (katanabukuro, cotton or silk) | Unlike synthetic fibers, it has no adverse effects on the blade from static electricity or chemical components |
| Camphor and naphthalene | Components adhere to the blade and can cause discoloration, so avoid using them during storage |
Proper Use of uchiko and Clove Oil
Uchiko is a silk bag filled with fine powder of uchigumori whetstone, used to remove old oil. Uchigumori whetstone is a natural whetstone used in the final stage of Japanese sword polishing, and its fine powder works to absorb and remove oil from the blade surface. The usage method is as follows.
- Lightly tap the blade with the uchiko ball to apply the powder
- Wipe away with soft hoshogami paper
As a precaution, overuse is prohibited. Frequent use can cause fine scratches to accumulate on the blade surface, so for well-polished high-quality swords, use should be minimized, with a guideline of about once or twice per month. If you first wipe away the old oil with hoshogami paper to some extent before using uchiko, you can reduce the burden on the surface.
Clove oil (choji oil) is applied thinly to the blade as a rust preventative. The application procedure is as follows.
- Soak a small amount onto tissue or hoshogami paper
- Spread thinly and evenly over the blade
Thick application is counterproductive because dust adheres more easily. Because clove oil is a vegetable oil, it oxidizes easily over time, and if oxidized old oil remains on the blade, it becomes a cause of rust, creating a counterproductive effect. For this reason, periodic "oil replacement" (wiping away old oil and applying fresh oil) is necessary. Additionally, if clove oil seeps into the scabbard, it can cause "saya-zome" (scabbard staining), so be careful about the amount of oil and the timing of storing it in the scabbard.
Risks of Long-Term Storage and Inspection Frequency
The main cause of microscopic scratches (hike) that are invisible to the naked eye on the blade is friction that occurs when the blade moves inside the sword box during storage. This can be prevented by securing it inside the sword box or by storing it properly in a sword bag.
Neglecting maintenance for several years or more can lead to fatal rust. The difference in how to address rust depending on its severity is as follows.
| Degree of Rust | Treatment |
|---|---|
| Minor rust | Can be addressed with partial polishing by a togishi (sword polisher) |
| Deep rust | Full-scale polishing is necessary and costs become high |
In the case of prolonged absence (such as overseas assignment), consider entrusting storage to a trustworthy sword merchant or cultural property preservation specialist.
Entrusting to Specialists
When long-term storage and maintenance are difficult to manage on your own, entrusting it to a sword merchant or art preservation specialist becomes a practical option. For swords purchased from TOUKENZA, you can consult via the contact form listed in the online catalog.
Timing and Cost of Polishing
Swords that have not been maintained for a long time may require polishing (re-polishing). Polishing (re-sharpening) is handled by specialists called sword craftspeople or togishi (sword polishers), and the cost varies greatly depending on the condition, length, and depth of polishing of the blade. Cost estimates are as follows.
| Type of Polishing | Cost Estimate |
|---|---|
| Standard polishing | Approximately 50,000–150,000 yen per shaku of blade length |
| Full-scale polishing | Can be several hundred thousand yen |
After polishing, the blade regains its shine, but it is important to understand that polishing makes the blade thinner (a condition called "togi-gari", or polishing wear). Excessive polishing can change the blade's shape and diminish its artistic value, so for minor rust, it is important to monitor with clove oil and uchiko, seek expert opinion, and then decide on polishing.
Obtaining Maintenance Tools
The three-piece set of uchiko, hoshogami paper, and clove oil can be purchased from sword merchants or specialty sword tool shops. Since quality varies, it is preferable to choose products handled by trustworthy sword merchants. TOUKENZA also accepts inquiries about choosing maintenance tools via the contact form.
When using uchiko, clove oil, and hoshogami paper, it is ideal to learn by watching an expert's demonstration at first. To minimize the risk of damaging a precious blade through improper use, if you have any questions about maintenance, please submit them via TOUKENZA's contact form, and we can provide basic advice.
