Hi Martin,
As you might know, Shirasaya means a Saya made of unvarnished wood. Shirasaya is a Saya which is designed especially to keep a sword. Because it is used in unvarnished, the humidity in the Saya is naturally controlled and the blade does not rust easily. Even if the blade rusts, you can easily split the Shirasaya and clean the inside of it. For that purpose, a Shirasaya is made by pasting wood with weak glue. The history of Shirasaya is not thought to be so old. There is a theory that its origin was late Edo period. The upper samurai class had a lot of swords, and it can be thought that they began to use Shirasaya to keep important swords with care. It is also thought that Shirasaya have been widely used after 1876 when a law was issued not to wear a sword in public. After that most swords became things to be kept except for military use.
On the other hand a sword which was worn in daily life was always in Koshirae. And the blade was not oiled because it would deteriorate the Koshirae.
Ref; http://ja.wikipedia.org/wiki/%E9%9E%98