Naniwa Aotoshi 2000


Intro

The Naniwa Aotoshi is a massive stone :8.25 x 2.75 x 2.25 inches. It is commonly referred to as "The green brick of joy" and usually gets very positive commentary :

Outstanding stone!! I've had some time to try a few different knives on it and I love it! Great, silky smooth feel with a very high polish for a stone rated 2k. I'd guess the finish is closer to 5k almost mirror finish with light pressure. Very fun to use but very easy to gouge if your angle isn't steady.

and :

The green brick is exactly what it says -- a joy to use. It is the best 2000 grit stone I've used in years. It produces good mud and does not dish easily. It will last me a lifetime, due to its size. You cannot go wrong with this stone. Absolutely great!

It is a very fine stone, especially if compared to traditional India and Crystolon or diamond stones where even the "fine" stones typically have grit sizes in the 20-30 micron range. In contrast the Aotoshi is sub-ten micron and is very muddy so it is very much a polishing stone vs a grinding stone. These terms are kind of vague but essentially it means that it is more suited to changing the surface finish vs changing the shape of a bevel.

As with most very fine stones, a high magnification shot (50X) shows a fairly solid surface as the porosity and grit size is too small to clearly resolve without much higher magnification. Note the extreme contrast compared to the Norton Economy Crystolon (silicon carbide) stone rated at 320 grit. The Norton grit is much more coarse and the porosity is very high and is easily visible.

Quality and Consistency

This is a high quality stone and has no issues which are common in inexpensive stones :

It can be used without any preparation and doesn't require soaking. However this is where one of the most common problems/complaints originates. It isn't uncommon to see some slight criticism about the stone loading. This is because the stone is simply massive and if it isn't given at least a light soak it will dry out in the grinding and then it starts to load.

Finish

The Naniwa produces a high polish, a hazy mirror and can easily do so on fairly hard and high carbide steels. It is a slightly muddy stone, however it is harder and not as muddy as the the Naniwa Superstone 400 or King 1000. The Naniwa Aotoshi will releases grit almost immediately even with very low pressure of 2-3 psi but it forms a very watery slurry.

The finish or level of polish is fairly high which would be expected from the high grit rating of the stone. Note the contrast between the finish from a Bester 700 and the Aotoshi 2000. The reason that the bevel finished by the Aotoshi is almost solid black is that it is almost mirror at that point and is over loading the sensors on the camera.

However while it does produce a high polish, it isn't actually a great stone for setting the final finish of the apex. The slurry it generates tends to round the apex as it ploughs through the watery grit. In addition it is possible to easily cut into the stone when grinding on the very apex. It can be used to set the final finish of the apex, but it is not an efficient choice for that compared to harder stones and it doesn't play to the strength of the stone.

However while the stone isn't that great for applying the final finish to the apex it work very well to prepare edges for the final micro-bevels. The edge from the Naniwa Aotoshi 2000 can easily have a micro-bevel applied with a very fine ceramic stone such as the Spyderco white (6-7 micron) and even the Spyderco Ultra-Fine (3-4 micron) .

In regards to using it as a prep stone, to minimize any burr formed from a harder stone, consider the edge on a very basic cheap kitchen knife (3Cr13) stainless steel. This knife was over ground to a visible burr with the Suehiro 'Chemical', a very coarse 320 grit wasterstone. It was just lightly ground with the Naniwa Aotoshi 2000 which not only left a much higher polish on the edge it also removed the burr. Due to the fact this stone is a little soft and generates a light watery slurry it does this fairly readily without having to use standard burr removal techniques :

Again, getting a final sharpness can be a bit tricky because of how easy an edge can dig into and then even cut a slice off the surface of this stone, unless a challenge is required, this is better used as a prep stone and then micro-bevel to finish with a harder stone. It is possible of course to use the Naniwa Aotoshi to set the finish of the final apex and even to apply a micro-bevel, it just requires some very specific technique.

In order to form a clean micro-bevel the simplest procedure is :

It might be thought to let the stone dry to prevent the slurry from impacting and rounding the apex. However if that is done with the Aotoshi then it just increases the chance that the knife can cut into the stone which is likely to damage the edge significantly. Edge trailing passes can be used however they can also produce a burr and leave the edge deformed and give it low edge retention.

In contrast, it is far simpler to apply that micro-bevel and set that final sharpness of the apex with harder stones.

Note even with care, the Naniwa Aotoshi can't match the initial sharpness of much harder stones such as :

But it is a matter of specific purpose. The Naniwa Aotoshi doesn't set a micro-bevel and the final finish well, but it grinds/polishes edges far better than those sames stones which are far slower.

Cutting Speed and Pressure

As a basic check on grinding steel, a basic kitchen knife (Everday Essentials Chef's knife) had the edge reset on a narrow bevel under low pressure :

Relative cutting speeds :

The much slower cutting speed of the Aotoshi is in well agreement with the abrasive size ratio which is approximately five to one. It obviously is too fine and would be too slow for any significant grinding, to remove any chips damage or adjust an edge angle.

Even on some very dull knives, a more coarse stone has to be used to be efficient. On an attempt to sharpen a few dull knives, the Naniwa Aotoshi could not generate a clean apex readily. A few minutes of grinding would leave the edges still reflecting light.

For comparison a few of the knives were sharpened on the much more coarse Bester 700 and required :

For dull kitchen knives then, the Naniwa Aotoshi could be expected to take 1500 pps and thus isn't a sensible choice for dull knives. But again, with edges applied by a more coarse stone it is an excellent choice to polish the edge and minimize the burr.

It is fast enough as a polishing stone to even allow refining even hard to grind steels efficiently. The images at the right show the edge at 50X magnification on a Maxamet 69/70 HRC blade, an extremely high carbide steel. It only took 250 pps to grind out all the coarse scratches from the Bester 700 and give the edge the finish of the Naniwa Aotoshi.

It is also fairly flexible in terms of the range of pressures which are functional. With very light pressure :

The stone generates a very light slurry and it slowly thickens. However even with very high pressure the stone is resistant to excessive fracture and so it maintains a high g-ratio (grinding efficiency).

The reason that the pressure has those wide ranges is that the contact length changes during the stroke.

There is an issue with high pressure grinding though as the stone has a weak enough bond and a very fine abrasive that with the load applied is half a pound or more then the apex can actual dig into the stone and cut off a slice. It only took a load of just 250 grams which is only half a pound to generate enough pressure so the edge would cut into the stone as soon as the angle drifted and the apex would make direct contact.

Now this can be avoided by using :

And in general it really isn't a problem for the kind of knives it was designed to work on. Japanese waterstones are in general designed for Japanese knives, no surprise there. Japanese knives generally have large flat bevels which produce very low pressure due to the large contact area and thus there are rarely problems of this nature. The cleaver for example is one such knife, very little belly and a very wide bevel and so the pressure on the stone is very low. It still works up a slurry, but there is no concern about digging/gouging into the face of the stone.

Steel suitability

One of the few complains about this stone is the occasional comment that it is more suitable, or in extreme cases, only suitable for "cheaper" steels, the low carbide stainless steels.

... it is picky about what steels it want to sharpen. Softer steels will get screaming sharp and have a nice semi gloss after sharpening , but your harder steels will still get kind of sharp but not really what I call sharp.

However it can readily grind and polish steels as different as :

In regards to final sharpness, it is hard to explain why such issues are often reported because there are often lack of details. However the main difference might be that the easy to grind steels simply grind much faster with less pressure as with high pressure the knife can cut into this stone.

In regards to polish and burr minimization it works equally very well on pretty much any steel. In regards to actually setting the apex it works equally poorly across the same compared to harder and less slurry based stones.

Summary

Naniwa Aotoshi :

Comments and references

Comments can be emailed to cliffstamp[REMOVE]@gmail.com or by posting to the following thread :

and/or the YouTube Playlist for Abrasives/Sharpening.

Most of the pictures in the above are in the PhotoBucket album.

1 : Conversion Chart Abrasives - Grit Sizes


Last updated :
Originally written: 09/09/2013
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