Why steel and iron rust
Iron and carbon steel rust slowly in clean, dry air, faster in damp air, and fastest where a thin film of water sits on the surface. Conservators at the Canadian Conservation Institute (CCI) state that clean iron is stable at a relative humidity of around 50%, while above 65% rust damages it continuously. Corrosion is strongly accelerated by chloride and sulphate salts, which reach the metal from sweat and skin on the hands, from polluted air and from sea air. A fingerprint, conservators say, therefore leaves a mark that can practically no longer be removed without repolishing.
Carbon steel and stainless steel
Stainless steel contains at least 10.5% chromium, which forms a thin passive layer with oxygen that protects the metal from corrosion; the more chromium, the higher the resistance. This layer can, however, be damaged, so even a stainless blade should not be left wet or sweaty. Carbon steel has no such protective layer and rusts without oil and dryness.
After every use
Do not touch the blade with a bare hand; conservators recommend clean cotton or nitrile gloves. After training or viewing, wipe the whole blade with a soft cloth or paper and apply a thin, even layer of oil. Wipe from the base to the tip in one smooth stroke, never scrubbing back and forth. A Japanese sword is drawn edge up, the saya is held from below with the left hand, and the blade is drawn and sheathed slowly and without jerking.
Japanese sword step by step
Drive out the bamboo peg (mekugi) with a punch without using rough force, and carefully slide the hilt (tsuka) off the tang. Use the first sheet of soft paper (nuguigami) to wipe off the old oil and dust. Then apply uchiko, a finely ground whetstone powder wrapped in paper and cloth, using light, even tapping, never rubbing. Sources disagree here: some tap the packet directly over the blade, others advise sprinkling the powder onto paper or soft cloth first and never putting it directly onto the blade. They agree that overusing uchiko damages the polish and the hardening line (hamon). Wipe the powder off with a second, clean sheet of paper; mixing up the paper used for old oil and the paper used for the final wipe is a common mistake. Check whether rust has appeared, and lay the blade on a soft pad.
Apply chōji oil with a piece of paper folded to roughly 3 × 6 cm and soaked in fresh oil, using the same strokes as when cleaning. The layer should be thin, because any excess runs down into the saya. The tang only needs a light wipe with a finger. The Kyoto maker does not recommend mineral oils, gun oils or thin lubricant sprays for Japanese blades. Refit the hilt and press the peg in firmly, but without forcing it.
How often
One source states that after fresh polishing, oiling should happen roughly every 10 days for about half a year, and then at least once every six months. Another advises cleaning only after handling or when fingerprints and dust appear, because unnecessary cleaning wears the polish down.
Rust and how to remove it
Conservators distinguish stable rust — a red-brown, well-adhering layer that does not change in dry conditions — from active corrosion. You recognise the latter by bright orange crystals between the metal and the rust, by cracking and flaking of the surface, and by so-called weeping, where yellow-brown to orange droplets form on the surface above 55% humidity. Isolate such an object and keep it below 35% humidity.
For ordinary steel objects, the Canadian Conservation Institute (CCI) recommends brushing off dust and removing a thin, even layer of rust with very fine 000 or 0000 grade steel wool and a light machine oil, then wiping the surface with mineral spirits on a lint-free cloth and applying fresh oil. It does not recommend acidic liquid rust removers, wire brushes or abrasive methods, and warns against stripping the entire corrosion layer, because the metal will then rust again into unsightly orange spots. A magnet can help: an object that is only weakly attracted has already lost a lot of metal and is brittle.
For a polished Japanese sword, the approach is different. Sources on Japanese swords explicitly warn against removing rust yourself, for example by scraping it with a spatula or the edge of a coin, and advise handing the sword straight to a togishi (polisher).
Storage and humidity
The Canadian Conservation Institute (CCI) gives 30% relative humidity and below as ideal and 40 to 55% as an acceptable range; the Japanese sword maker recommends 40 to 50% and a room temperature of 20 to 22°C. Both stress a stable environment without fluctuations, which is why attics and cellars are unsuitable. In a closed cabinet, silica gel helps. Conservators wrap iron in acid-free, unbuffered paper and support it with polyethylene foam. Do not put naphthalene or camphor near the blade; sources on Japanese swords say they cause rust.
Sources disagree on storing blades inside their saya. Conservators advise storing iron objects next to their sheaths rather than inside them, and against using wooden cabinets, because wood releases organic acids. Japanese tradition stores the blade in a plain wooden shirasaya, ideally of paulownia wood, inside a cloth bag and lying horizontally. Even the maker warns against leaving the sword in its saya for long without airing it. Do not store the sword standing upright, as the oil would run down toward the tip. Protect the saya from direct sunlight.
Common mistakes
The most common causes of damage are bare fingers on the blade, storing a sweaty blade, thin sprays used instead of oil, acidic rust removers, wire brushes, a damp cellar, rubbing rather than tapping uchiko, and home scraping of rust off a polished sword.
Which weapons this applies to
This guide applies to the Japanese swords katana, wakizashi and tantō, to European swords, sabres and daggers with a steel blade, spearheads, axes, kukri, kris, and to steel or iron sai and jitte.