Edited 2025-09-02 10:56:02 to fix unprintable characters
Monday answer
Answer: Rust and skin acid are the main culprits. A koshirae is a dress scabbard and because it is for show or presentation it is lacquered and does not breathe. Continued storage in the display scabbard will result in rust--especially in any humid environment. To combat this, a resting scabbard, also called a sword's pajamas, the shirasaya is made. It is of wood from the Ho tree, honoki, Magnolia obovata. It is a wood soft so as to minimize marking, regular grain so it can be worked, when seasoned (typically ten years) it has virtually no sap, and it is a natural rust inhibitor (remember the blades are carbon steel). Note: You might find it interesting that never does sandpaper, ever touch a scabbard. Only a horsetail reed is used on the exterior so as not to leave sandpaper grit. The rest is chisel, saw, and plane. Powder applied to the blade is ushiko powder, identical to the powder used in the absolute final polishing. Also the glue to hold the two only slightly unequal hollows of the scabbard is made from rice paste and applied only to the outside edge. Rice glue will not draw moisture and can be sprung open to clean out the opening should it be needed. Each wood scabbard has an internal pocket at the tip of the sword blade where oil can accumulate and not remain on the blade. Note: a good sword hangs inside its scabbard (wedged in by a finely fitted habaki a metal band collar that fits around the base of the blade and mates against two notches that are the beginning of the tang.) Because the scabbard and the habaki fit so precisely, the blade of a fine sword doesn't normally rest on the sides or edges of the inside of the scabbard. It "floats" wedged in by the fit of the habaki. In addition, when the blade is resting in the shirasaya a separate, wooden, identically shaped blade is placed in the dress scabbard, koshirae, to help keep it shaped properly.
If ever given the opportunity to open a sword wear cotton gloves and do it slowly by pushing opposite thumbs on the scabbard and grip then withdraw it resting only the back of the sword on the scabbard, never the side or edge, as the swords are so incredibly sharp and polished it could cut the insides of the scabbard, form a splinter and cause a scrape/scratch when reinserted. Lastly, a good sword is never owned. The possessor is only the latest steward of the blade, which is, I am told, the same burden as a high maintenance woman. Be certain you know what you're getting yourself into.
Link: [url]http://www.hi-net.zaq.ne.jp/osaru/e_kosirae.htm[/url] about koshirae and shirasaya