Hand-tool woodworking in every tradition requires maintaining sharp edges. While I had no problem using sharpening jigs, I wanted to teach myself traditional freehand sharpening to understand its benefits. I quickly learned that I had underestimated the skills needed to sharpen a 70mm blade.
In retrospect, sharpening a smaller plane would have made things easier for both me and my sharpening stones. In addition, Japanese plane blades are constructed differently than modern Western irons. They have high-carbon steel as the cutting edge, laminated with soft iron that forms the bulk of the body (such as in early Western planes). The steel is heat-treated to be quite hard for good edge retention, but that hardness can make sharpening (especially lapping the back) difficult. To solve this problem, the Japanese blacksmith hollows out the back of the blade using a scraper just before heat-treating; that leaves far less surface area to sharpen on a stone. I had no problems with this until, after repeated sharpenings, I eventually ran out of flat land on the back.
I learned that the hollow on the back of the blade, known as urasuki, has to be maintained with a flat perimeter around the edge. This maintenance involves two processes: uradashi (tapping the steel out with a hammer) and uraoshi (flattening). The first time a person does this, the fear can be paralyzing because poor technique can easily crack the blade due to the brittleness of the hardened steel. But if I was going to use these tools to their potential, along with learning how to sharpen my blade, I had to muster my courage to learn to do this.
–Kim Choy, excerpt from “10,000 Hours: A Journey into Japanese Woodworking,” in Issue Five