I have quite a lot of nagura and various slurry stones, it's the one area of jnats that I do know a bit about. Which is mostly because of razor honing, where you use very fine, slow and usually hard stones to finish on. The use of nagura slurry gradually diluted will speed the process up no end.
But having acquired various examples for that purpose has opened my eyes to their use for knives,
especially for polishing. So below are some thoughts concerning nagura as slurry stones, as opposed to flattening or surface dressing stones, for the latter I, like many people here, just use atomas.
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Here's a picture of some of my most used nagura which I'll talk a little bit about below.
Top: Chu, Gujo, Mikawa Asano Koma, Nakayama
Bottom: Tam O'Shanter, Blue Thuringian, Coticule Les Latneuses
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Japanese Nagura
Some things to note:
1.) There are certain types of stone that historically have been specifically quarried and used as nagura, but of course you can use any type of stone as a nagura, part of the fun is experimentation. Softer suita work well, I've broken stones up that had too much toxicity to make nagura, where you can work around or get rid of the toxic lines. The Nakayama is my pic above was sold as nagura simply because of the size, though it's also realtively soft and coarse for the mine...
2.) Ideally you want a nagura to be softer and slightly coarser than the base stone. If it's harder you'll just raise slurry off the base stone, if it's finer then the effect of the nagura slurry will get lost. But many nagura aren't actually that soft initially...
3.) This is important, and often not commented on or realised:
***Traditional Japanese nagura are soaking stones*** Tsushima, Chu, Gujo, Mikawa - they're all soaking stones, anything from 10 mins to about an hour will soften them up and make them better in use.
4.) These types of traditional nagura are katana polishing stones. It's why Mikawa benchstones are so frighteningly expensive, and why Asano have specific stamps for sword grade pieces. Small versions work very well for in-hand polishing on their own, as well as for slurry on other good polishing benchstones.
Tomo nagura - I don't often use tomo tbh. There's no real advantage to be gained over just using an atoma, except perhaps preserving the life of the base stone. If a stone comes with one, then by all means use it. But I'm not really in the game of cutting bits off the end of a stone in order to have a tomo (I have done, just rarely).
Chu nagura - Relatively light, porous, fast, and friable. The grit range overlaps with the coarser end of Mikawa. My stone above is quite a coarse and fast version. Would work very well as a knife edge sharpening stone too.
Gujo nagura - I wasn't aware of Gujo nagura until earlier this year noting a constant stream of high praise from
@gbupp, and it turned out I actually had one already. The stone in my picture above was my favourite nagura, but I had mistaken it for an exceptionally good piece of Mikawa, an easy thing to do as they can look and act very similarly. This is a flat out excellent polishing stone, and I would pay handsome money for a bench size version. As fine and fast as koma, but softer.
Mikawa nagura - The famous ones, and rightly so - Mikawa nagura are very good indeed. There are eight recognized types; Atsu, Ban, Yae Botan, Botan, Tenjyou 1, Tenjyou 2, Meijiro, Koma. I've nicked that list of Keith's website and tried to order it from coarsest to finest, though there is some overlap: Meijiro can be finer than Koma for instance. I also don't have all of those layers, only four I think. Mikawa can be exceptionally good for polishing though they can be quite hard, I tend to soak for at least half an hour before use. Perhaps someone like
@Steve56 might be able to offer some further thoughts here as he knows these stones very well, and not just in the context of razor use.
[NB. I don't have and have never used
Tsushima nagura, so I'm leaving out of my list above, others have talked about them anyway. I understand they are coarser than the above, and would be used as the first stone in a nagura polishing progression.]
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Non Japanese Nagura / Slurry Stones
Many types of traditional western whetstones are quite fine and relatively non-friable in comparison to a lot of jnats. There aren't many types that work particularly well as nagura, imo. Some that I do like:
Tam O'Shanter - Of all the other stones in all the world Tams are most like jnats to my mind. They're at the finer end of the natural stone spectrum, relatively slow, and for a lot of their history they were largely used as polishing stones. On their own they're rarely as good as a good Japanese polisher, but as nagura they work extremely well.
Thuringians - Thuris are extremely fine-grained stones, but softer than almost anything else at a comparable grit level. If you're looking for high level, mirror type polish then a Thuringian slurry stone on a hard and difficult-to-use awasedo is a very good option. The one in the picture is actually an Escher, but any Thuri will do.
Coticules and Belgian Blue - The hardness, speed, and grit level of yellow coticule varies as much as the whole world of jnats, so it's difficult to make generalizations here. But take my word for it - cotis and bbw can be quite exceptional nagura. The slurry stone in my picture above is from the Les Latneuses vein which to generalize in comparison to other cotis is; fairly soft, very very fast, and medium grit.
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Synthetic Nagura
Another thing I don't often do, but it can work. The slight issue with synthetic nagura is that much of the interest in using nagura comes from how the abrasives in the slurry round out (or 'break down' as people often call it). The Aluminium Oxide and Silicon Carbide used in synthetic stones is far harder than the Silicon Dioxide in natural stones, so it doesn't do the rounding out / breaking down in quite the same way.
Remember that you don't necessarily need to use a small stone in order to raise slurry, you can very well just rub one benchstone on another.
I wouldn't use synthetic slurry on a natural stone, or vice versa.
