Shigefusa 270mm kasumi yanagiba with custom, by me, desert ironwood handle

25 Nov 2010

Aizu vs Aoto - Medium finishing stones #1500-5000

In this post I will try to make a comparison between two of my main medium sharpening stones. I have chosen to do them together as they actually are more similar than different, even if this is not immediately apparent. One is the very well known Aoto, which I got from Tomohito Iida at Iida Tools, and the other is the less known Aizu, which was brought to my attention and delivered by Maxim at Japanesenaturalstones. Both are very substantial stones; the Aoto measuring 240x95x95mm and weighing in at almost 9 pounds and the Aizu measuring 210x80x67 weighing in at about 5,5 pounds.

Both stones are also quite rare, but for different reasons. The Aizu because the mining ceased in the 1950's and have not been mined since, the Aoto because of the massive size and the exeptional quality and grinding force which is very rare to find in an Aoto these days. Especially outside of Japan. To put it simple, this should be very close to "as good as it gets" regarding medium grit Japanese natural sharpening stones.

Looking at them side by side, they could not really be much more different from each other. The Aoto is a dark blueish to charcoal grey with what looks like olive green sesame seeds embedded in the grey. The Aizu is a solid chalky white with almost translucent olive green lotus or renge embedded in the white base. The Aoto has a beautiful skin on both sides while the Aizu is sawn with a large, coarse circular saw leaving a beautiful almost sculptural surface on five sides. I think they are both really visually stunning in their own way. 

In the left picture below you can see the white board with translucent olive green renge spots of the Aizu stone and on the right, the dark grey board also with olive green spots on the Aoto. Actually I was a little bit surprised to find the spots with almost exactly the same color in these two stones. I am not sure that the olive spots are the same material in both stones, though... 

Except for the obvious differences in appearance between these two stones, the biggest difference is noted immediately when starting to sharpen on them. Initially, starting out with a clean wet surface, the two stones feel very similar in grit size. A bit like a #1200-1500 man-made stone. However, I could almost immediately feel that the Aizu was a harder stone than the Aoto, requiring a lot more work to build a slurry. Both of the below pictures show the slurry produced after about 50 passes on each stone. The Aizu to the left, still with the surface of the stone as the main grinding factor, and the Aoto to the right, already with a dense slurry smoothing the feel of the stone.

This difference makes the Aizu a more aggressive stone by working the stone surface for a longer period of time before the slurry develops to a buffer or lubricant for the sharpening process. A very good thing about the Aizu being significantly harder is that I feel a lot more confident when sharpening chisels or kanna blades with very narrow blade roads, which can easily dig in to the softer surface of the Aoto. The Aizu is a lot more resistant to this "digging" thus a lot more forgiving to my lack of experience in sharpening these kinds of blade. Aizu is therefore my preferred medium sharpening stone for blades with narrow and flat bladeroads like kanna blades and chisels. For knives, the Aoto gives me the better feel and a more velvety response during sharpening.

So, lets take a look at the final results. After about 100 passes on each stone, both stones had built a slurry dense enough to effectively buffer the stone surface from the steel. I did about 50 more passes with full slurry before comparing the results. Samples below are 20mm across. You can see the scratch pattern of the Aizu to the left is a little bit more pronounced showing a few more clearly visible scratches than the sample from the Aoto to the right. The explanation for this might be that even after the slurry was fully developed the Aizu felt a little more "grindy" compared to the very creamy Aoto. Probably because the Aoto particles break down even more than the Aizu particles. The scratch pattern from both stones was just about equally easily erased by the Oohira Asagi reviewed in one of my previous posts. These stones can actually be used as finishers for most stainless and daily use knives. An edge off either of these stones will easily beat the main stream factory edge and making the edge a lot more wear resistant.

It is very hard, if not impossible, to decide a winner between these two magnificent stones. Both stones have unique qualities making them highly useful and desirable. The hardness of the Aizu is great with kanna blades and chisels and the softness of the Aoto is great with most knives. The difference in grit size and sharpening result is marginal between the two, the main difference being the difference in hardness of the surface.

If you only sharpen kasumi or stainless knives, I would probably go for the Aoto. If you sharpen both knives and chisels or kanna equally often, you will need both. If I have to choose, I would probably go for the harder Aizu, due to my lacking sharpening skills on kanna blades and chisels. If you only sharpen kanna or chisels I would be very happy to live with just the Aizu. Fortunately I am the very happy owner of both.

Maxim at Japanesenaturalstones tells me that the Aizu also is a lot more effective than the Aoto on Honyaki blades. Truth said, I am actually quite happy with my Aoto's effect on my Honyaki knives, and so far I have not had the time to test the Aizu with my Honyaki. However, I have no reason at all to question Maxim's experience on this. He is a trained professional with significant skills and knowledge on japanese natural stones and the application of them on different kinds of steel. If you are interested in further reading on the Aizu, Maxim also has a very good description on his web page (se link on the left menu).

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