A shirasaya is not a katana, but rather a type of traditional Japanese sword mount or scabbard. It is a wooden scabbard without any ornate fittings or decorations, typically made from plain wood or sometimes lacquered.

The shirasaya is primarily used for storage and protection of the sword blade when it is not in use. It consists of two pieces: the saya (scabbard) and the tsuka (hilt or handle). The blade is fitted into the saya with no tsuba (guard) or other embellishments, resulting in a simple and minimalist aesthetic.

While the shirasaya is not exclusive to katanas, it is commonly associated with them. Katanas are often placed in a shirasaya for long-term storage or to showcase the beauty of the sword's blade itself without distractions from decorative fittings.

It's important to note that when a katana is mounted in a shirasaya, it is not ready for immediate use in combat. The shirasaya is primarily a display or storage mount, and the sword would need to be remounted in a proper katana koshirae (mountings) with the necessary fittings and handle wrap (tsuka-ito) to be functional for wielding.

So, a shirasaya is a type of scabbard associated with Japanese swords like the katana, but it is not a katana itself.