A significant aspect of a Japanese style blade is the hollowed back, the urasuki. The genius of the laminated blade, the uber-hard cutting edge married to a durable and resilient soft iron, is a great and wonderful thing. At least until you try to sharpen it. Even though the hard steel is an ideal minimum amount of material, it is still a very difficult piece of metal to abrade. That is why the urasuki is so important.
|Not perfect, but OK. This is a good start towards ito-ura.|
It's all about sharpening, isn't it? You can have the hardest metal, the greatest potential, but it means nothing if you can't get it (and maintain it!) sharp.
As you are sharpening a tool, the surface bevel is riding on a cushion of water/oil, metal waste, and shed abrasive grains. This is the slurry, and it acts as a very thin cushion between the blade and the sharpening stone. A little bit of slurry is a good thing, but it also acts as an interference layer, making large surfaces challenging to get flat. The blade "floats" on the liquid.
The larger the bevel surface, the less force is able to be applied, and contact between the tool and the stone is reduced. The blade and your arm are the force, and that is a fixed quantity. Distribute that limited amount of force over a large surface (a western style plane blade is a great example), and you have a recipe for some rough slogging.
There is no end to the number of articles extolling the virtues of the "flat back" and how to get it. Hand in hand are the discoveries of techniques that can be used to get around the onerous task. The reason that this simple task is so difficult is that you are applying a small amount of force over a huge area, the whole back surface of the blade. It takes time and patience, but at least you only have to do it once (hahahahahaha!!!! LIARS! Every time I see this comment I laugh. You work the back surface every single time that you sharpen!).
There is a common trick that works well to avoid the issue of flat back syndrome, the "ruler trick".
Brent Beach explains this best.
This technique uses a thin ruler as a means to slightly elevate the head of the blade, which concentrates all of the force on a small area, forming a slight secondary bevel. This works, but introduces a different set of concerns.
The importance of the flat back isn't usually well explained. A flat surface is a standard that you work towards. Repeatability is the key. Making a blade sharp enough to use is simplicity itself. A sloppy, half-assed, rounded front and back bevel will work fine, but for every time that you achieve the perfect edge, there are ten times that aren't.
Getting the back of a plane iron TRULY flat is nearly impossible, but you do your best. The ruler trick is a great means for a beginner to get a workable edge and is a great addition to any repertoire.
Japanese tools use a different work around, and that is the urasuki, the back hollow. Hollowing the back of a blade reduces the target surface area to a minimum, making the blade easier to sharpen. A nicely formed ura makes the job easier. The quality of the ura, the shape, says a lot about the values of a craftsman.
Want to learn about high quality standards? Look to So Yamashita and Japan-tool. Read everything.
A famous blade, perfect ito-ura, "Rangiku" made by Chiyozuru Sadahide.
Ito-ura is the flat at the bottom edge of the blade being the bare minimum. Ito-ura requires care to establish and an attention to detail to maintain. It results in the smallest amount of material that needs to be sharpened, and makes sharpening quick and easy. When you see ito-ura, you KNOW that the tool is sharp!
This is more common.
Notice that the surface area on either side of the hollow is large. That is the amount of area that is addressed at each sharpening. All things being equal, the second blade (Beta-ura) will require 10x longer to sharpen. At a smaller scale, the larger surface presents the same difficulties that a solid backed, western style blade presents, that of maintaining flatness and repeatable edge sharpness. In reality, the second blade will never be a sharp as the first.
Ito-ura is not about ridiculous, unpractical standards. Ito-ura is efficiency and speed. It is about care for your tools, and attention to detail, and doing a good job. It requires skill to achieve, and that is why it is important.
Des King studied under real craftsman, at a technical college in Japan, and has written **The Best**book about the building of Japanese Shoji screens.
Des has published an excellent tutorial on ura-dashi, and posted it on his blog. This will get you to closer to ito-ura.
Des King on ura-dashi (tapping out procedure)
Wow! I didn't intend to write all of this, it just sort of drained out. I was going to write about carving the urasuki using a sen (scraper), but.....