There are the good reasons for using a diamond stone as a nagura. This is not a novel idea – the intent here is to present a clear rationale for it. However, there’s also a significant practical problem involved.

All of this applies to synthetic finishing waterstones. I think most or all of this probably also applies to Japanese natural finishing stones but I defer to those with more knowledge about those.

The reasons for a diamond nagura:

1. It’s fast. The slurry is raised faster and the surface of the stone is refreshed faster than with any other type of nagura that I have tried. Whatever you perceive to be the benefits of these effects, as discussed in the previous post, they arrive faster with a diamond nagura.

2. The slurry consists of grit solely from the finishing stone; no new grit is added. This removes the uncertainty of introducing another grit, often unknown, from a stone nagura, along with the uncertainty of the amount of it that gets into the slurry based on the relative hardness of the bond in the main stone versus the nagura.

3. It is very capable of crushing the grit in the slurry. I first learned about this several years ago from So Yamoshito, a Japanese tool vendor in Australia and expert on Japanese natural stones. I wrote about it then. The rationale for specifying 1200 grit diamond is that it is fine enough to readily crush the loose fine grit in the slurry yet coarse enough to raise the slurry quickly. The latter effect is apparent.

I can’t directly prove the crushing theory. Furthermore, for it to be of value, the crushed particles would have to retain good cutting ability as finer particles. After working with this for years at the sharpening bench, it does seem borne out by the blade edges it produces.

By the way, what about just using the slurry created by flattening the finishing stone with a coarse diamond stone, say 220 grit? Yes, that’s pretty good but the crushing effect is better with the 1200 diamond. Also, a lot of water is used in flattening and the process tends to swipe the slurry off the finishing stone.

Now for the problem. When you rub a 1200 grit diamond stone, even an Atoma with its surface made of tiny dots of grit clusters, on the wet finishing stone, it sticks like crazy. This is very annoying and then it tends to carry away much of the slurry when you remove it.

I tried using smaller continuous surface diamond stones but they were no better. Then I tried a DMT 6″ x 2″ inch 1200 diamond stone with the “polka dot interrupted surface.” This reduced the sticking but still not well enough. It needed to be smaller.

The little DMT polka-dotted pocket stones were too thin to grip in my fingers. So I hacksawed a 2″ x 2″ section off the 6″ x 2″ stone, which you can see above. It works pretty well. The small size and the perforations eliminate most of the sticking.

Note that I do not consider flattening to be a function of the nagura. In fact, a reasonably evenly-distributed rub of the small diamond nagura should not significantly change the flatness that has already been established well by a coarse diamond flattening plate. I flatten stones at the end of a session when they are fully wet and so they are ready to go for the next use.

The best solution, I believe, would be a 1200 grit diamond nagura, about 2″ x 2″, with narrow channels extending to the edges that would reduce sticking and allow the slurry to flow away from the nagura and remain on the finishing stone. I am working on prototypes using 1″-thick ABS plastic for the base and various applied diamond surfaces. I’m hoping this results in a nagura that is the bee’s knees, but in any case I will report on this soon.

Several reasons are usually given for using a nagura on fine grit waterstones. These include: to raise a slurry, to remove unwanted deposits in natural Japanese stones, to refresh the cutting surface of the stone, and to flatten areas of the stone.

Let’s think about what’s going on when a nagura is used, recalling what we can directly sense at the sharpening bench.

The slurry

When the little nagura stone is rubbed on the finishing stone, a paste, or slurry, is generated. It is sometimes claimed that the slurry actually does the sharpening, but it seems questionable whether loose abrasive particles in the slurry are really cutting steel. There are microscopic photographs of blade edges and stone surfaces, but to my knowledge, no direct visual recording of the actual cutting action at a microscopic level. We can observe the effects but not the actions that produced them.

The thin edge of steel plows most of the slurry but perhaps some loose particles are held by the stone’s surface texture, enabling them to cut. Maybe it burnishes the steel. Maybe it creates a variable grit surface on a synthetic stone somewhat like in a natural stone.

In any case, we can sense that the slurry improves the feel and ride of the blade on the stone and reduces sticking, all helpful effects. So, whatever it is actually doing, the slurry at least feels good.

The next issues are what composes the slurry and what happens to it.

Are there particles of the finishing stone, the nagura stone, or both in there? Particles of the softer (more loosely bound) of the two stones will presumably predominate. This should be considered when the two differ in grit size. For example, a nagura that is softer and coarser than the synthetic waterstone with which it is paired will be probably be counterproductive.

With fine natural Japanese waterstones, nagura selection is an art unto itself. Consult a knowledgeable purveyor of these stones. The nagura also is used to remove defects in natural stones that can damage the blade edge. This function is, of course, not relevant for synthetic stones.

So, what happens to the particles in the slurry? Are they left intact or crushed to some degree? If the nagura could crush loose grit to a finer size, that would seem to be an advantage assuming these crushed particles retained their cutting ability.

The surface of the stone

We can see and feel that a nagura refreshes the surface of the stone by removing metal and glazing. Much like dressing a grinding wheel, cutting particles are better exposed at the surface, ready to cut steel.

As for flattening, there are better ways to do this accurately than with a nagura, though with natural stones a nagura might be helpful for some local flattening as it is used intermittently for its other benefits in the course of sharpening.

A solution

There may be more questions than answers here and you may be thinking that this is all a little bit interesting but enough already. I agree, I’d rather get back to woodworking. However, at least restricting the matter to synthetic waterstones, which most woodworkers use, there is a simple solution to all of this, to be discussed in the next post. The background discussion of this post will support why I think the solution makes so much sense.

I guess I can’t quite leave sharpening alone. About six months ago I switched to using a Naniwa Chosera 10,000 as my primary finishing stone.

I had been using a Shapton 16,000 glass stone for this purpose and could have left well enough alone because it is an excellent stone. It’s fast, the grit is very consistent, it’s convenient because it requires no soaking, and it gives a great edge. However, the drawback for me has always been the lack of excellent tactile feedback from the blade on the stone surface.

This is particularly important for freehand or semi-freehand finishing of the cambered edge of plane blades where the feel of the edge on the stone is critical. Even when using a honing guide for simple non-cambered edges, the delicate feedback near the end of honing is helpful and reassuring that the angle and edge are right.

I had read reviews claiming the feel of sharpening on the Chosera 10K stone was outstanding and that is just what I have found. That’s the big difference and it really matters. Moreover, it cuts just as fast as the Shapton, maybe faster, but is practically more efficient because I have fewer do-overs. There is also no tendency for the blade to skip as on the fine Shapton.

The Shapton is 16K and the Chosera 10K, so does this mean a step down in edge quality? Grit number is just one factor is producing edge quality. Others include the particle shape and how it fractures, the consistency of particle size, the density of the particles and how they present at the surface, and the properties of the binder. In practical shop use, the Chosera has been producing edges not one bit less sharp or otherwise of lesser quality than the Shapton. In fact, I think it’s better, if only because I can better feel those last whispery kisses of the sharp edge on the stone to get it just right.

The minor downside of the Chosera is that it needs pre-soaking. Various recommendations can be found for this including that soaking is optional. I’ve found it needs 15 minutes. Less soaking, 5 or 10 minutes, makes the stone too quickly drink up the water you splash on when starting to sharpen. When finished sharpening, I flatten it while it is still wet with the Shapton diamond lapping plate.

I still use a 1200 grit diamond stone as a nagura and remain convinced this enhances the action and feel of finishing stones including the Chosera 10K, as well as the quality of the finished edge. (More to come on this soon.) I did not find helpful the nagura that comes with the Chosera.

This is an expensive stone but at more than one inch thick, it will last a long time. The bottom line is that it has made my sharpening more assured because of the excellent feel.