I thought I might add a few comments and some theoretical considerations. So I think it is useful to consider particle size of the sprays as a very important criteria to compare apples to apples and oranges to oranges. Secondly I'd like to broadly classify what sorts of stones do what.
So let's take the case of an Arkansas stone, presumably either a surgical black or better yet a translucent. I consider these to be in the range of 3k or so grit - 5k might be stretching it, roughly a 4 micron particle ignoring particle geometry for the moment. What I've found useful for this is a 4 micron particle - CBN or diamond actually does enhance the edge produced on a number of steels, particularly abrasion resistant steels, partially because it increases the abrasion rate by increasing slurry density by abrading the stone and secondarily by increasing the ability to abrade more abrasion resistant steels. But this produces greater sharpness and an edge suitable for knives, but not razors. I wouldn't consider an edge off of an Arkansas stone for shaving either with or without compounds on it. I also consider the type of slurry generated by this stone to not be one which breaks down rapidly or at best minimally. Using a very fine particle like .25 microns on a 4k stone is a 16x difference and too extreme of a difference.
Now if we consider stones like coticules, Thuringians / Eschers, and probably other hones (DTs, etc) that seem to work best using sequential dilutions where loose particles give a coarser edge than embedded ones and the particles themselves (garnets, etc) aren't particularly susceptible to breakdown, I suspect that break down won't be enhanced significantly by continuing to use the slurry rather than diluting it. I will test this as it just might break down these particles given the hardness difference between garnet and CBN or diamond. No verdict yet.
Now with Japanese synthetic waterstones, I don't see particle size reductions as significant, but I do see increased slurry density as the stone abrades more quickly. I also see a positive difference in edge performance. Regarding particle compound size I tend to match CBN or diamond particles with the stone grit - a 1 micron particle on a 16k stone, half micron on a 30k stone, 15 microns on a 1k stone, etc. Going significantly finer is of minimal benefit.
Now on to Japanese naturals. For coarser stones - arata like Ohmura and middle grits (nakado) like binsui and Igarashi, I tend to go equal to slightly finer grits. This works quite nicely with aoto stones as well. For polishers or awasedo, I like to divide into two groups - those with pores or su, typically suita layer stones and those with more homogeneous non-layered structures like various tomae layer stones.
In this video Matt used a Nakayama Asagi. Stones like this or Ozuku Asagi or even the softer Yaginoshima Asagi I consider ideal candidates for this technique.
In the best of cases, I would probably assign a grit range to these stones under 64k or a half micron particle, again ignoring the particle geometries of these Japanese naturals vs synthetics. Most will fall in the 8k - 30k range. So we can choose to match grits and get a result where the two particle groups are in a competitive relationship and some of the advantageous properties of the natural stone finish are defeated and you get the shiny bevels like you might see from an aluminum oxide finish or a pure diamond or CBN finish as seen in one of Matt's micrographs presented early on in his presentation.
But if you go with a significantly finer particle - tenth micron and below, now we have a different phenomena. These harder particles do several things:
1. break down the natural stone slurry at a more rapid rate into a finer slurry. This yields a finer NATURAL stone finish as seen in his later micrographs in the video. This is a key feature both for producing a finer sharper edge, but secondarily - primarily for straight razor honers - maintaining the comfort levels associated with natural stone edges. By using significantly finer particles, you aren't replacing the natural stone finish but rather enhancing it. And as Matt shows, the edge characteristics do change to a more precisely formulated edge.
2. Using tomonagura or matching slurry stones isn't contraindicated with using compounds - in fact just the opposite. I personally prefer a tomonagura based slurry over a slurry generated with a diamond plate as high a 1200 grit and certainly below that. Higher grit lapping plates and films is yet another story, but I still maintain that a tomonagura slurry is best and certainly prefer it as final finishing over nagura (botan koma etc) mixed slurries. A tomonagura generated slurry is certainly a candidate for CBN or diamond enhanced or doped slurries.
3. Regarding grit contamination using diamond / CBN slurries, here are my thoughts. If one uses a similar particle size, yes the likelihood of getting these particles stuck in the su is high and I wouldn't recommend it. Similarly I wouldn't recommend naguras - Koma Botan Chu, Tsushima, etc - on suita type stones as they are coarser particle sizes too. But for less porous stones, it seems to not be a problem, particularly the more muddy ones where the sprays are in the slurry moreso than left embedded in the stone's surface. On the less porous stones where naguras are appropriate I feel quite comfortable with the use of sprays too - Nakayamas, Ozukus etc.
4. If one uses particle sizes significantly smaller than the stone's particle size range, grit contamination is less of an issue for several reasons. One, it flies below the radar and doesn't interfere with the stones' characteristics. If there is a remaining fraction left, it is miniscule. Remember we are only talking about using a single pump or even less of spray to begin with - not matching volumes of stone slurry with similar amounts of compound. As an aside, you must use water soluble compound formulations on Japanese waterstones. There are exceptions but let's skip this for now. On other stones that can be used with oil, you might get away with oil type suspensions, but I haven't tried this yet. Two, if the particles get into the su they seem to be easily removed. This is tru because if they get into the su they su have a blend of the stone's own particles and the sprays as only a small fraction of the mix and they are so much smaller that they don't fill up a su with a single large particle or clump of them.
I hope this contributes to the discussion and as always I'm interested to see how these concepts play out in actual use on straight razors. I've recently gotten reports of successful use on an Okudo stone (suita or tomae?) and would appreciate knowing what sorts of results you get. This particular razor was a dollar razor, so how this works out on various steels, particularly the more abrasion resistant ones is something of particular interest to me.
Just as an update, here's the feedback Matt got from the person's razor honed in the video:
"Just finished a shave. Very nice.
I had a 2 days worth of growth, slight tugging with WTG pass but not bad. Very smooth for the rest of passes.
ATG felt good. Min to no burn with AS.
He said there was a slight pull on his first pass. My opinion was that my edge compared to his was the CBN edge was a more keen shave with all the comfort, much more than a synthetic edge. I tried an edge with .5 cBN in the slurry and it seemed it may have been even better. Too soon to tell as it was a different razor. "
So it may turn out that going to a slightly coarser cbn / diamond spray for a first pass like 0.5 microns and following this with a finer spray in a fresh slurry may produce an even more interesting edge.
Looking forward to seeing what you guys come up with. If I can be of help, just ask.
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