FAQ

This is because most blades of Japanese chisels and planes are made of two kinds of metals. The cutting edge and base is made of steel, which is forged on the soft iron called jigane. 

There are 2 reasons for this.The first reason is to protect the steel. Steel is very hard, but is also brittle at the same time. By laminating it with soft and tenacious iron, the brittle iron can be protected.

The other reason is that by replacing some of the hard steel with soft iron, you can sharpen the blade more easily and quickly.

The hollow of the back side of the blades helps you to save time and effort when you flatten the back side allowing you to flatten only the surrounding narrow area of it. 

Some say that multiple hollows are more beneficial because they increase the area in contact with the material. Others say that it just requires more time to flatten out the surface. My opinion? It’s just a matter of aesthetics...maybe (by Hisa)