Process of Finishing Shirasaya
A shirasaya is a simple storage scabbard carved from a single piece of wood. Most often is it made from Honoki (Japanese Bigleaf Magnolia). The first stages are similar for any type of saya (scabbard), but the shirasaya involves slightly thicker wood and different finishing processes as the natural wood will be the final surface.
The goal of shirasaya is to provide a stable blade storage that is effectively airtight. All of the joints and meeting of koiguchi and tsuka should be as clean and tight as possible for this reason. As a general rule the tsuka should slide on until about 1mm gap remains below the habaki and then a single strike on the palm should take it the rest of the way home. When replacing the saya it should also slide on until about 1mm remains between the tsuka and saya and then moderate pressure should complete the closure. Some shirasaya do not even have mekugi and are carved to proper tolerances for a friction fit only.
Historically a blend of pure choji or clove and camellia oil would be used on the blade. These are oxidizing oils and would slightly lower the amount of oxygen inside a very small space as they react over time. However the reaction causes the oils to thicken requiring frequent removal and replacement. Most modern sword oil is primarily mineral oil based with just a hint of choji oil for the correct scent and therefore does not oxidize but will not harden over time either. This means uchiko is no longer necessary to abrade off old oil that may have partially hardened.
Carving the Inside
Traditionally Hounoki (Japanese Bigleaf Magnolia) is used to make all types of sword handles and scabbards for several very good reasons. It is not overly hard or brittle which allows it to be hand carved and provides some slight compression, it is non-acidic, non-porous, low-moisture, low-pitch/resin, non-oily and can be glued well, stable once seasoned, relatively tight/straight grained, non-abrasive, resists splitting, and is locally grown/available in most of Japan.
The wood is split into halves and divided between handle and scabbard, the inside is carefully carved out to make room for the tang and the blade. When the fit is right, the halves are glued back together again using sokui (rice paste glue), and wrapped tightly until dry. For more details on these earlier steps, see: Aikuchi Tanto Koshirae | Making a Futokorogatana.
Shaping the Outside
Once the glue has dried and the halves are made into one again, the outline of the shirasaya is sketched and the rough dimensional shaping begins with a kanna. The shape of the fuchi, kashira, koiguchi and kojiri are then carved with a kiridashi and the block slowly reduced with a kanna to connect the profiles. Even in the case of a rounded profile a faceted shape is formed first to ensure even surfaces. Final shaping is done with a small kanna and then the surface smoothed and adjusted with a fine flat rasp if necessary.
The mekugi is carefully drilled with a kiri and filed to final dimensions and the mekugi is carved from bamboo or buffalo horn. Tokusa (horsetail grass) attached to a wood block is used to smooth and polish the surface of the shirasaya as well as soften the lines slightly. Finally some shavings and a cotton cloth are used to buff a thin layer of natural ibota wax onto the surface to add protection and shine.
Depending on the wood, a smooth antler tip may occasionally be used as a burnisher to press strongly over the entire surface, a small area at a time.