Rust Prevention and Corrosion Control in Japanese Swords: Coating Science and Long-Term Storage Mechanisms

Long-term preservation of Japanese swords requires rust prevention techniques. This article provides a detailed explanation of the chemical principles of rust prevention, the functionality of uchiko (polishing powder), and optimal storage environment settings.

Uchiko (Polishing Powder): Materials Science

Uchiko is a powdered maintenance material that has been lightly struck onto the blade surface since ancient times. Its main components are as follows.

Components of Traditional Uchiko

The main component of white uchiko is fine powder from uchigumori whetstone sealed in silk cloth (deer leather). By lightly striking it onto the blade surface, the whetstone powder functions as a grinding material that adsorbs and removes old oil remaining on the blade surface. It contains no oil, and the correct procedure is to apply oil coating with camellia oil and similar oils after uchiko maintenance.

When using uchiko, it is important to strike the blade uniformly and lightly. Applying too much force may cause the powder to dig into the blade, potentially causing damage, so it should only be struck lightly.

After striking, carefully wipe away excess powder and old oil with soft wiping paper (nuguigami). If this wiping step is skipped, whetstone powder may remain on the blade surface and become a factor attracting moisture, so uchiko application and wiping with nuguigami must always be performed as a paired operation.

Modern Rust-Prevention Materials

Synthetic uchiko developed in recent years sometimes uses polyethylene glycol (PEG) or silicone-based rust-prevention oils. While these have the following advantages, they are sometimes avoided by collectors who prioritize tradition.

• Low volatility and resistance to oil oxidation
• Low tackiness and does not remain on the blade
• Rust-prevention effects persist over extended periods

Considerations in Choosing Uchiko

Both traditional uchiko and modern synthetic uchiko share the common role of preparing to remove old oil from the blade and replace it with fresh oil. The choice between them is often determined by whether one prioritizes appreciation protocol or ease of handling, and no clear superiority can be assigned. In either case, one must understand that rust prevention is not to be expected from the uchiko itself, but rather from the subsequent oil coating and management of the storage environment, which are the true foundations of rust prevention.

Storage Environment and Relative Humidity

Rust formation on the blade progresses rapidly at relative humidity (RH) of 60% or higher. The key to rust prevention is "maintaining a low-humidity environment."

Ideal Humidity Management

It is recommended to place desiccant materials (such as silica gel) inside the sword box and replace them monthly. Special attention is needed during the rainy season (May–July) and winter (December–February, when condensation risk is high).

Choosing a Storage Location

When choosing where to place the sword box, attention must be paid not only to humidity meter readings but also to the surrounding environment. Rooms facing external walls or rooms near water sources such as kitchens and bathrooms tend to have locally higher humidity even indoors and should be avoided. Also, rather than placing the box directly on the floor, setting up shelves in positions that ensure air circulation reduces the impact of moisture from the floor. Regarding the material of the sword box, the traditional use of ho (magnolia) wood is said to be because it has low resin content, does not easily damage the blade, and possesses humidity-regulating properties that absorb and release moisture.

Oil Film Formation Mechanism

The oil film applied to the blade surface (especially camellia oil) physically blocks contact with oxygen. This film provides chemical protection as fatty acids in the oil form weak chemical bonds with the iron surface, physical protection as the oil film prevents the ingress of moisture and oxygen from the air, and changes over time, gradually oxidizing and darkening (forming black rust) within six months to a year.

Black rust (ferroferric oxide) is more stable than red rust (ferric oxide) and also works to prevent further corrosion. However, regular oil coating (1–2 times per year) is performed for aesthetic reasons.

When applying oil, the basic procedure is to apply a small amount of oil to soft cloth and spread it across the entire blade surface to form a thin, uniform film. Applying excessive oil may cause oil to pool at contact points with the habaki (blade ferrule) and tsuba (hand guard), potentially attracting dust and dirt, so it is better to use a small amount and lightly wipe away excess oil with wiping paper.

Seasonal Rust-Prevention Measures

Rust Prevention for Metal Fittings (Metalwork)

Rust prevention for metal fittings such as habaki (blade ferrule) and mekugi (retention pin), not just the blade, is also important. While these have small surface areas, moisture easily accumulates in gaps, making pinhole rust likely to develop.

Measures include applying uchiko to the metal fittings before storage, applying oil coating every six months, and applying fresh oil after completely removing degraded uchiko. Since metal fittings may differ from the blade in material and surface treatment, it is preferable to check individual condition and perform maintenance accordingly, rather than simply applying blade maintenance procedures.

Precautions During Handling

Often overlooked in discussions of rust prevention is the handling during appreciation itself. Touching the blade with bare hands allows salt and moisture from skin perspiration and oils to adhere, potentially causing rust to develop at that point. When holding the blade, the basic protocol is to hold the tsuka (handle) and avoid directly touching the blade itself, which is also rational from a rust-prevention perspective. When it is unavoidable to touch the blade, it should be done through clean cloth or gloves, and the touched areas should be quickly wiped with wiping paper to remove oils and moisture.

What to Do When Rust Develops

Despite diligent daily maintenance, rust can develop due to sudden changes in storage conditions or gaps in maintenance. When rust is discovered, one should avoid attempting to remove it using grinding compounds or sandpaper by one's own judgment. The blade surface bears important information for appreciation and authentication, such as ji (surface pattern) and hamon (temper pattern), and improper grinding may damage these and affect the value assessment in authentication documents. When rust is discovered, one should first stop its progression by adjusting the moisture and oil condition of the surface with a dry cloth, and then consult with a specialist such as a togishi (blade polisher) regarding condition assessment and appropriate treatment.

Summary

Long-term preservation of Japanese swords is achieved through the combination of removing and adsorbing old oil with uchiko, subsequent oil coating with camellia oil and similar materials, and maintenance of a low-humidity environment. From a materials science perspective, the grinding and adsorption action of uchigumori whetstone powder and oxidation prevention by oils form the foundation of the rust-prevention mechanism, supported by humidity management of the storage environment. Through seasonal measures and regular maintenance, durability over centuries is ensured. Careful attention during daily handling and consulting with specialists rather than attempting self-treatment when anomalies are noticed are also important elements supporting long-term preservation.