What does the bolster of wa handle for?
Japanese kitchen knives generally have a bolster(a part of wa handle) made of buffalo-horn, plastic or plywood for bolster which material is harder than its wooden handle.
These days, there are Japanese kitchen knives without bolster. So what does the bolster for?
To prevent from cracking
When you attach the knife blank to wa handle, some might have experienced, the handle may crack or break. Even a skilled craftman carefully attach it, especially for deba knife (pointed carver) which tang is thick. Also, it might crack or break by pressing the interface, the top of the handle, when using it a long-term.
The bolster would help to prevent from cracking. To implant the materials, such as brass, on the top of handle is effective the prevention of cracking as well.
To take a knife blank out wa handle
When you take knife blank out wa handle to sharpen the knife, you need to hammer the top of the handle. It is reinforcement for handle not to break. Nowadays, it is mostly fixed to wa handle with resin and bond. It is rare to take a knife blank out from handle for sharpening.
Decoration for a Wa handle
Japanese kitchen knives is derived from Japanese sword. There is a part as an interface between handle and the blade of a sword such as Tsuba and Habaki which features highly practical and excellent decorativeness. It is replaced with bolster for kitchen knives and knives.