Hello and welcome to my blog! In this article I will discuss my use of Japanese natural sharpening stones, known as tennen toishi, and how they fit into my workshop.
The traditional use of Japanese natural sharpening stones begins with sword polishing, dating back as far or possibly further than the Kamakura period. Japanese swords were polished both for functional reasons and aesthetic reasons. Polishing the blade reveals the structure of the steel which allows the blade smith to improve their efforts and further refine their blades. Luckily, Japanese tool users are the beneficiaries of centuries of these efforts both in regard to blade smithing and sharpening.
Japanese tools have a lamination point between the hard steel and soft backing. When the bevel is polished the unique qualities of those materials are revealed. In this article I will refer to type of finish on the steel know as a kasumi finish. A kasumi finish refers to the misty look is created in the jigane (soft backing) when compared to the polished hagane (hard steel) when worked with certain finish stones.
As a tool user what I want in a sharpening stone is slightly different from that of a knife sharpener or sword polisher. Generally speaking, and with some important exceptions, as a tool user I’m maintaining a flat bevel on Japanese and western tools. A knife user, or sword polisher, would be interested in maintaining a ‘hamaguri’ or clam shell shaped bevel. I have rounded or hamaguri bevel tools, gouges and some plane blades, that I also maintain and so I have stones to accommodate them as well, but I will focus on flat bevel tools in this article.
I have a few goals in mind, ideals, that I want in the stones that I own and use. I want a stone that works fast for it’s respective grit range, maintains its flatness and builds a light slurry with use or a slightly heavier slurry when worked with a diamond plate.
The first stone in the lineup is a stone I use primarily for curved tools such as the jack plane, the Ikarashi stone. This stone creates a thick slurry very quickly with use and dishes with relative ease making it ideal for curved work. I can take a chip out of a curved edge in a hurry with this stone. The grit range is approximately 1,000-2,000 grit.
This stone is one that I use to work both curved and flat bevels depending on how I use the stone. If I slurry the stone with a diamond plate, I can use the stone very effectively for curved bevels. However, if I let the stone build a light slurry in use I can work flat bevels. This is a medium stone, so it’s about 3,000-5,000 grit. Grit ranges are approximate for tennen toishi because the grit breaks down with use and becomes finer. If I slurry the stone it will act slightly coarser, but will be more forgiving.
This stone is a ‘medium-finisher’, it builds a much lighter slurry than the Tsushima nagura and remains flatter in use. I estimate this to be in about a 5,000-7,000 grit range.
The Shinden suita is a true finishing stone. This stone is used to create a fine edge, good for most finish planing, paring, and detail work. Most often when I’m using this stone I first build a light slurry so that it can cut the hard edge of a Japanese chisel or plane iron. This stone acts very similarly to about an 8,000-10,000 grit range.
The final stone in my range for flat bevel tools is a Nakayama asagi, this stone creates an incredibly fine slurry when worked with a 1200 Atoma plate. When I want to put a bright polish on wood, I use this stone to work the edge. It is capable of extremely fine razor edges, most edges off of this stone and light plain stropping can pass the ‘hanging hair’ test. It’s considered to be comparable to 30,000 grit, but the actual particles are no finer than 8,000 grit.
Hakka Kiita Koppa
The last of my stones is this small Hakka Kiita koppa. Koppa is a reference to the size of the stone, being a very small stone. This stone is a finish stone for curved edges, not something I need to use on a jack plane, but for a fine gouge I would use this stone finish the bevel.
Next I have very small sized Uchigumori stones, these stones are a medium-fine range used to create a kasumi finish (contract between the jigane and hagane) on Japanese round bevel tools and knives. This is only needed on curved bevel tools and knives as the kasumi finish is created in normal sharpening on flat bevel tools. These stones can be broken up into ‘finger stones’ but I prefer to leave them as is, create a thick slurry and work the bevel that way.
To provide context I’ve worked the same chisel with synthetic stones, 1k, 3k, 8k and 13k and Tsushima, Yaginoshima, Shinden and Nakayama. The chisel used on this test is an old stock (1980’s) Kikuhiromaru in White steel 1 that is very hard at the edge. To keep the edge shape perfectly consistent from stone to stone I’m using a guide, in normal sharpening this is not necessary, however I want to show the finishes in the most easily repeatable process that I can provide.
All of the stones are flattened to .001″ before sharpening and used with their respective best practices. I won’t provide the maker of the synthetic stones since they’re simply used a basis for comparison, I’m making no attempt to provide that they’re not producing a good edge, as they do provide a good edge and they cut steel quickly.
1000 Grit Synthetic
This finish is a nice, fairly even scratch pattern and shows a distinction between the jigane (soft steel) and hagane (hard steel).
3000 Grit Synthetic
It actually produces a very distinct difference between ji and ha, and the obvious streaks on the hagane from a very consistent abrasion.
The stone which I typically begin with natural stones is the Tsushima, I will normally jump to the Tsushima from a 1000 synthetic. From the Tsushima you see just barely a difference between the hagane and the jigane with consistent scratch pattern in both.
8000 Grit Synthetic
This is a beginning to show a mirror polish, it looks like a streaky mirror, very little distinction between ji and ha.
It shows an obvious difference between the jigane and the hagane with a very consistent scratch pattern and what almost looks like bead blasting on the hagane.
This shows like a mirror from a decent distance and while the jigane and hagane are barely discernible, I would not consider this a kasumi finish because of how brightly the jigane is polished.
This starts to show the pattern that exists in the jigane. Some light scratching still shows but it is mostly a very even finish. The hagane has a mercury like appearance. This is where the difference between the two types of abrasive become very apparent.
Finally the Nakayama asagi, this is the highest level of finish that I can produce on flat bevel tools. It creates a nice even finish, sparing a few very light streaks which can only be seen up extremely close. It produces a mercury finish on the steel. It is barely discernible from the finish produced by the Shinden suita but proves slightly finer.
And the view from a normal distance to see the finish produced.
I want to give credit and say ‘Thank you’ to David Weaver, Chris Hall and So Yamashita for being absolutely incredible sources of information in addition to being good friends and for encouraging me to take this road.
Thank you for visiting!