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Thinning Nakiri

Discussion in 'Sharpening forum' started by BKLNPaul, Jan 23, 2020.

  1. I’m new around here. And relatively new to the world of J knives, in general. I’ll write a fuller introduction soon, but first a question:

    I have a nakiri in a kurouchi finish. (An aside: There’s the kurouchi part, then a grey part that drips down and ultimately exposes the blade steel. What is the grey part called?) My knife came with a relatively skimpy amount of blade steel exposed. (White #2, if that matters.) Then the knife sacrificed a lot of steel to the cause of my early sharpening education. The result is that I have very little blade steel left exposed.

    It won’t be the next sharpening session, maybe not the one after that, but the day will soon come when I will have to thin the knife. But the knife is so thin already! It’s can’t be much more than a millimeter at the spine. I’ve watched all the thinning videos I can find, but I haven’t seen anything that will solve my problem. If I hold the knife at four degrees (hard enough, given that my stroke is not without some wobble), I don’t think that’ll be low enough to hit the grey part. Any lower and I might as well hold the knife flat against the stone, and I don’t think that’ll be good for anyone.

    Do I just sharpen the blade steel and the grey stuff together, into one bevel? Won’t that defeat the purpose of buying the nice blade steel in the first place?

    I’m not even sure that I actually need to thin the knife as much as just get rid of 1/4 inch of grey stuff. Can that be done?

    I’m tempted to give up and take it to a pro for sharpening. But I want to learn and, eventually, I’ll have to know how to do this. All comments/criticism/advice welcome and appreciated.

    Flummoxed In Fort Greene.
  2. Toothpick

    Toothpick #2 since day #1 Founding Member

  3. Taylor

    Taylor Professional Craftsman Founding Member

    Hard to tell exactly what you're talking about without pictures, so I'll share one of mine that's already on my computer, though it's not the greatest for showing it either. Shown is the kuroichi cladding. If you look closely, there are two different grays extending to the edge of the knife after the kuroichi finish. The first duller gray after the kuroichi is the softer steel cladding the harder steel core. This softer layer is called Jigane. The harder layer of steel is called Hagane. These two terms are Japanese, in the states, I often hear them referenced as cladding and core steels.


    Below is a picture of another kuroichi cladd knife taken from the perspective of the choil (the curved section extending from the handle to the heel, and referenced as such because it does not have a bolster). Thinning this knife would be to flatten the exposed section near the edge. That section should be flat on both sides of the knife, though if you only sharpened at that angle, the edge may be too thin to be useful. Hence, most sharpening is at a slightly more obtuse edge that can handle more obtuse. Neither of these knives will have to be thinned beyond that, as they are slightly concave after that bevel. The concavity leads to the max thickness of the bevel getting thinner and thinner as progressive thinning happens. Thinning convex or flat ground knives means starting from the spine, and removing material all the way to the edge, but most kuroichi knives I've seen are slightly concave. If you want to look at the grind, hold a flat edge (a business card works well here) against the side of the knife, and see where it makes contact and doesn't by looking for light coming through the gaps. Hope that answers at least some of your questions.

  4. Taylor, thank you for your thorough and engaging response. You not only answered my question but you gave me new terms to throw around. Jigane. Higane. If only I knew someone who understood what I was talking about.

    Confession: This knife was $65 from CKTG. It barely buys me admission to this forum. So when I took your grind test, I didn’t expect much, thinking a) How complex can this grind be? For $65, I don’t think that anyone with -san after their name was laboring over this knife. And b) This knife is too thin to play many tricks with the grind. Try to make it concave and you’d quickly grind through to the other side.

    But when I held the knife against a ruler, I was surprised to see light shining through. Starting about halfway down the height of the knife, it begins to curl up. By the time it reaches the edge, there is a small but perhaps significant gap between the blade and the ruler. Hold it on the other side, the edge touches the ruler and the steel bows up behind it.

    Could this be a poor man’s concavity? Or did someone step on it at the warehouse?

    Besides this bend, the steel seems to be a uniform, i. e., it’s a flat grind. Following Taylor’s Third Law (there’s gotta be two already), that means starting from the spine and working down.

    I was inching towards a similar conclusion, but for less elegant reasons. I couldn’t think of a way to isolate the jigane (or higane) and remove metal from them without involving the whole knife.

    I think that we’re both saying that thinning this knife spells doom for the nice kurouchi finish. That’s a high price to pay. I know that it’ll eventually wear off, but I want to enjoy this knife in it’s prime, while it still has its casual sprezzatura.

    In my original post, I mentioned that while the problem was looming, things are okay for the moment. So I will officially bury my head in the sand. When the day comes that it can be ignored no longer, I’ll do . . . something. Maybe take some sandpaper to the jigane and Try to buy myself a quarter inch of time. Taylor, I promise to confer with you before taking that step.
  5. I skimmed through this thread pretty fast so maybe I missed something. Did OP say if it is single bevel or double bevel nikiri? I thought that was the difference between a usuba and a real nikiri. Maybe someone smarter than me can comment on that. I know that I would think about this problem differently depending on the bevel. There are also concave backs on single bevel Japanese knives that are easy to screw up if you don't lay it flat on the stone.
  6. It's a double bevel knife. Like you, I always thought that usuba was single bevel, but I can't claim to be smarter than anyone on that subject.

    I still think that the whole knife is too thin (and too cheap) for anyone to be messing around with fancy concave grinds. And I don't see one when I use Taylor's test.

    Although I'm open to suggestions, I think the only answer only answer is to take some sandpaper to the jigane. Or higane. If I could push it back a 1/4 inch, I'd be okay for years. By then, I'll be swimming in denka no hoto.
  7. Taylor

    Taylor Professional Craftsman Founding Member

    Nakiri's are double bevel and Usuba's are single bevel. Occasionally you will find Nakiri's in the states labeled as Usuba, though I think this is due to inaccurate translations. Off topic, but there's a great extra scene in Monty Python's The Holy Grail, where they take a scene, translate it to Japanese, and then Subtitle it back into English. Amazing how badly things can change.

    Back on topic, there can still be concavity in double bevels, and it's not necessarily ground into the knife (hammers can achieve this as well. Still doesn't take away from knowing the geometry of the blade, and just seeing what's going on.

    You could thin with sandpaper, but most people will just use the stones. May seem confusing that you could thin with stones, but being precise with where you are applying pressure, while not using too much can give quite a bit of freedom in where you are removing metal. I'll try to do a diagram in the next few weeks to model this, but I'll start a new thread on it.
  8. “Being precise with where you are applying pressure, while not using too much . . . “

    Sounds like more delicacy than you’ll find in this corner of Brooklyn.
  9. Rick

    Rick aka Pensacola Tiger Founding Member Gold Contributor

    Here's a trick that the razor sharpening people have been using for years - wrap the spine with several layers of electrical tape so that when you lay the side of the knife on the stone the layers of tape raise the blade to the thinning angle you want. You're making a quick and dirty guide for the thinning angle. Don't use any pressure on the spine so you don't abrade the electrical tape too fast. The tape should raise the side of the blade enough that you don't scratch the kurouchi finish. If you start wearing through the tape, replace it before the side of the blade touches the stone.

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