Effective "grit size" of natural stones

[Mr.Wizard]

I included estimated equivalent-grit-performance values for what I perceive as some of the more common natural whetstone types on my chart. I know that these estimates will always be rough, especially for hard stones where the received finish is important, and for types that act as different grits when handled differently. Nevertheless I think there is at least some value in charting these as long as the uncertainty is made apparent. Perhaps the next revision of the chart should have a natural stone column with range bars or similar notation to better illustrate this variability.

I have not used any natural stones myself and rely on sources like this Info 20M article. I would appreciate feedback on the estimates from the referenced article, as well as any suggestions for addition, removal, or improvement. If anyone cares to attempt providing equivalent grit ranges (assumed JIS unless stated otherwise) for particular natural stones I would appreciate that too.

If certain natural types are particularly consistent in performance I may wish to include them, even if they are not particularly common.

Conversely if it is deemed that presently included types are simply too variable or dependent on surface finish to have any meaningful grade assigned, even in the form of a range, I should remove them.

 

[1315]

I think this will be a great reference. Id likely try more naturals, but I'm not fluent in the jnat lingo, so i often have no idea what the stones application is from 1 to 4 Japanese words strung together.

 

[cotedupy]

I included estimated equivalent-grit-performance values for what I perceive as some of the more common natural whetstone types on my chart. I know that these estimates will always be rough, especially for hard stones where the received finish is important, and for types that act as different grits when handled differently. Nevertheless I think there is at least some value in charting these as long as the uncertainty is made apparent. Perhaps in the next revision of the chart should have a natural stone column with range bars or similar notation to better illustrate this variability.

I have not used any natural stones myself and rely on sources like this Info 20M article. I would appreciate feedback on the estimates from the referenced article, as well as any suggestions for addition, removal, or improvement. If anyone cares to attempt providing equivalent grit ranges (assumed JIS unless stated otherwise) for particular natural stones I would appreciate that too.

If certain natural types are particularly consistent in performance I may wish to include them, even if they are not particularly common.

Conversely if it is deemed that presently included types are simply too variable or dependent on surface finish to have any meaningful grade assigned, even in the form of a range, I should remove them.

Ha, that was you made that chart? I hadn’t read the small print / put 2 and 2 together, but I use and consult it often. Thank you for your work, it’s a fascinating and invaluable resource!

I wouldn’t be the guy for jnat ratings, but I do know quite a lot about European and American natural stones. I’ll ping ya a message in the next few days…

 

[cotedupy]

Though obviously the very big difficulty when attempting something like this is that in practice a silica based stone with micron size ‘X’ doesn’t equate to the same finish as an AlOx or SiC stone of micron size ‘X’. On hard steel; SiO2 abrasives will have a higher JIS equivalent, when compared Al2O3 or SiC abrasives.

You’d have to leave out the physical particle size I imagine…(?)

 

[Mr.Wizard]

@cotedupy I am glad that you find my chart helpful and I welcome your help to improve it.

The majority of the chart is expressly about physical particle size rather than abrasive performance as explained in the README, wherein I note e.g. "Soft but sharp abrasive may work very fast on soft metal yet be ineffective on high hardness steel." In the case of natural stones however I don't think actual abrasive particle size is particularly useful, due to differences of hardness, shape, and structure. To that end I have accepted that natural stones will have to be charted based on some fairly arbitrary functional equivalences, dressed and used on steel as considered appropriate by an experienced user.

To pick one that I know you have experience with, in part 4 of the INFO 20M article the Coticule is described with "Grit between 800 and 16,000 depending on the slurry thickness." Do you generally agree with that? It is useful and informative to present it with such a range on the chart? Can it actually work practically throughout that range when applied to an appropriate steel?

 

[stringer]

@cotedupy I am glad that you find my chart helpful and I welcome your help to improve it.

The majority of the chart is expressly about physical particle size rather than abrasive performance as explained in the README, wherein I note e.g. "Soft but sharp abrasive may work very fast on soft metal yet be ineffective on high hardness steel." In the case of natural stones however I don't think actual abrasive particle size is particularly useful, due to differences of hardness, shape, and structure. To that end I have accepted that natural stones will have to be charted based on some fairly arbitrary functional equivalences, dressed and used on steel as considered appropriate by an experienced user.

To pick one that I know you have experience with, in part 4 of the INFO 20M article the Coticule is describe with "Grit between 800 and 16,000 depending on the slurry thickness." Do you generally agree with that? It is useful and informative to present it with such a range on the chart? Can it actually work practically throughout that range when applied to an appropriate steel?

I was just writing about this. Here's what I have started.

The first problem is particle size doesn't mean much with natural stones. They aren't sieved to a particular grit and different abrasive types will behave different irrespective of size.

The second problem is the natural variability. Take washitas for instance. I have specimens that behave anywhere from similar to a Shapton glass 500 to a Shapton glass 8k. I have two aizu. One is about 800 the other 2k. I have coticules that range from 1k to 4k.

All that being said, I do think there's something valuable as a general ballpark/range to help people who are getting started or to compare different classes and families of stones.

 

[Mr.Wizard]

The second problem is the natural variability. Take washitas for instance. I have specimens that behave anywhere from similar to a Shapton glass 500 to a Shapton glass 8k. I have two aizu. One is about 800 the other 2k. I have coticules that range from 1k to 4k.

Let us examine the washita situation.

Are these classified in any useful way like "hard" and "soft" that can narrow the range? Is e.g. the Lily White Washita a consistent product or can it be all over the map?

Right now I have "Washita" at the coarse end of that range, I think originally based on a chart from Norton. Are these more common with the finer types being rarer? Were old stones finer than new ones, or vice versa?

If nothing can be done to narrow that range or break it into more specific types would you still want to see it included in the chart?

 

[captaincaed]

I use that chart and love it as well! Nothing can ever be fully complete but it’s a really good place to get started and refer others.

This is a pretty good primer on common natural stones used for razors, and their abrasive geometry. It talks a lot about why it’s complex.

I can’t speak to the “truth” of any of this, but I think it’s a good way to start thinking about it. Most of this guys videos are long form and sort of stream of consciousness, but this one in particular is fairly well structured and content-dense. Worth a watch.



For my money coticules probably compare most directly to synthetic stones, with a very hard abrasive suspended in a softer binder. But the range is pretty broad. It’s almost like having a pile of unlabeled synthetics that are all nearly the same color, all in the 1-10k range, with 4-8k being the peak of the distribution.

But yeah it’s a mess.

 

[captaincaed]

Right now I have "Washita" at the coarse end of that range, I think originally based on a chart from Norton. Are these more common with the finer types being rarer? Were old stones finer than new ones, or vice versa?

Vintage Pike Lilly Whites (top end of the quality range along with the vanishingly rare Rosy Red grade) were divided into soft/fast and hard/fine. There were other “lower” Quality grades that were a total mixed bag. Generally harder from my limited sample, but less consistent mineral composition. Generally woodworkers liked the soft and coarse stones since they did the heavy lifting before good synthetic stones were widely available. The hard/fine stones could refine the edge, but so could cheaper slates. The value was in the speed.

Later Nortons had Lily White but I don’t think they distinguished between hard and soft.

So “not really”. It’s a mess.

But I do think it’s worth the effort to try. The other Arks generally perform in narrower bands than washitas, and Jnats have a long tradition of grading and classification to at least give some sense of range. But I’m the last person that should give an opinion. Maybe @Badgertooth @Steampunk or @maxim will come out of the woodwork.

 

[ethompson]

I do really, really dislike putting grit numbers on jnats because the surface finish and edge behavior of synths and jnats is so different. But it’s the single thing I get asked about the most so I may as well give this a go.

 

[Mr.Wizard]

This is a pretty good primer on common natural stones used for razors, and their abrasive geometry. It talks a lot about why it’s complex.
I can’t speak to the “truth” of any of this, but I think it’s a good way to start thinking about it. Most of this guys videos are long form and sort of stream of consciousness, but this one in particular is fairly well structured and content-dense. Worth a watch.

I watched the whole presentation. It did not introduce me to any new concepts but I think it's a pretty good overview. He covers a number of the concepts that I try to explain myself when people want to compare abrasive performance based only on particle size, like sphericity, angularity, and cutting point density, though I think friability didn't get the time it deserves. For what it's worth his characterization of particle size analysis rather misses the mark, as the kind of measurement he implies is taking place simply is not, with the arguable exception of Dynamic Image Analysis.

Though he disparages giving grit numbers for natural stones nearly all his time is spent talking about physical particle size and why it isn't a good way to compare natural stones, and I don't disagree with that at all, or I would be seeking microscopic analysis rather than soliciting practical knowledge from user experience. Since he is comfortable saying "this stone" is finer than "that stone" he obviously is not against all forms of ranking, and ranking really is what I hope to achieve here, where and to the degree that is possible. Any mapping from ranking to grit numbers is of course imprecise and assumption-dependent, but the aim of my chart is to provide a reasonable summary and a starting point, rather than attempting the impossible of one-to-one performance translations with far too many unknown variables.

 

[stringer]

This is grossly oversimplified.
But here is the Arkansas family, according to me.

Washita 500-8000
Soft Arkansas 600-1200
Medium Arkansas 1200-2000
Hard Arkansas 2000-4000
Translucent Arkansas 4000-6000
Surgical Black Arkansas 6000-8000

 

[captaincaed]

I watched the whole presentation. It did not introduce me to any new concepts but I think it's a pretty good overview.

Knowing that light only comes from heat, I’ll try to be a little controversial for the sake of exploring your ideas.

In my mind, both the “low/high end” and “most likely” grits make up the whole picture. Plus, what experience is the average user likely to get, without too much nuance or too many stone-specific tricks.

Coticules - 1-10k range, peaking in the 6k “island of disappointment” that is neither really fine enough for shaving nor course enough for tough produce skins. Does great in paper towel though. But very fast, an easy stone to love and chase.

Belgian blue 3-8k range, peaking around 3-5k. Often more useful edge for the kitchen, but slower. Rarely can be razor finishers, but there are often better.

Thuringian German slates - 10-15k, peaking around 12k. Pure, slow finishers but reliably produce a great shaving edge.

Fine European slates e.g. Water of Ayr, La Lune, Yellow Lake, Black Shadow 10-12k, peaking around 10k.

Medium slate e.g. Tam o Shanter, Vermont Green, Green Shadow no experience but I hear lower around 6-8k.

Washita 400 - 4k, peaking around 3k
Soft Ark 800 - 1k
Hard Ark 800-2k but slower
Translucent Ark & Black Ark. Nearly the same around 10-15k, peaking around 12k. Average user will get around 8k results. Because they’re so hard, less than perfect angle control often ends up dulling a knife edge, but it can really improve a razor. Pure, slow finisher.
…
Unless you slurry one, then it cuts incredibly fast since it’s basically pure silica. Finishes around 2-3k, toothy.

Cretan ~ 2k light grey, ~5k dark grey. Limited experience.

Charnley Forrest - Supposed to be novaculite, similar to hard Arks, maybe not quite as fine, but I don’t have experience.

——

Bless you for sticking with the vid. I need to skim or read between the lines to get the best value out of those. Since you mentioned not having used many naturals I want sure how far down the rabbit hole you were.

While it’s frustrating I think there’s a reason why many natural stone users shy away from generalizations, since the minute we do, someone shows us an exception.

 

[ethompson]

As mentioned previously - I don't like using grit ratings for JNats. The people I respect most in this space don't do so either. Particle size is a small, largely irrelevant part of the picture compared to binder makeup, abrasive composition / concentration, surface behavior / prep, etc when it comes to these stones. Jnats are best compares to other JNats, not so so much synthetic stones. However, without that broad experience to draw on, it's hard to contextualize so I dos see why assigning a grit rating is so desirable. The below is based on my experience, which while less than some, is broader than most at this point. There are always exceptions to every generalization with Jnats, so these notes should be taken as very rough guidelines, not gospel.

Grit is based on edge performance on a surface that is neutral - neither overly burnished nor intentionally roughed up. How stones polish can very dramatically from edge behavior based on the surface, so I included subjective notes on that rather than numeric assignments. I'll stick to slightly wider ranges to account for differences in stone type.

Coarse / Mid-grit stones

Stone​	Grit	Polish
Aizu / Ikarashi / Kasabori	2-3k	slightly hazy near kasumi to burnished and detailed - visible scratch pattern
Mikawa Nagura	2-4k	slightly hazy to brightest burnished mirror - visible scratch pattern in hard light
Tajima	3k	bright near kasumi to burnished mirror - visible scratch pattern
Natsuya, new	1-2k	coarse / hazy to more burnished or closer to kasumi
Nastuya, sandy	500-1k	coarse, hazy
Natsuya, Sun Tiger	2-3k	slightly hazy near kasumi to burnished and detailed - visible scratch pattern
Ueno / Numata	1.5-3k	slightly hazy near kasumi to burnished and detailed - visible scratch pattern
Aoto, Tanba	1-2k	coarse, hazy
Aoto, Kouzaki	2-4k	slightly hazy near kasumi to burnished near mirror - visible scratch pattern
Kurama	2-3.5k	slightly hazy near kasumi to burnished near mirror
Amakusa	400-1k	coarse / hazy to coarse / burnished
Binsui	600-2k	coarse / hazy to slightly hazy to near kasumi - very variable
Omura	200-600	coarse / hazy

Finishing Stones (Kyoto Area)

Stone	Grit	Polish
Akapin	2-4k	very hazy, strong Kasumi on hard steel / visible scratch pattern on soft cladding
Western mine soft suita	4-7k	hazy, Kasumi affect - usually stronger contrast, core becomes more white to grey, not mirror | scratch free possible
Western mine hard suita	6-8k	brighter Kasumi, but still hazy | scratch free sometimes possible
Eastern mine friable suita	6-9k	bright kasumi, not as white core steel as soft western mine suit might be, but not mirror | scratch free possible
Eastern mine hard suita	8-11k	more towards mirror, very glossy with lower contrast, still kasumi though | scratch free sometimes possible
Western mine soft homogenous layer (tomae, aisa, etc)	3.5-7k	hugely variable, but usually a plain kasumi with white core steel and dark clad with few details, sometimes a visible scratch pattern on cladding
Eastern mine soft homogenous layer (tomae, aisa, etc)	5-8k	usually a plain kasumi with white core steel and dark clad with few details, sometimes a visible scratch pattern on cladding | scratch free possible
Western mine hard homogenous layer (tomae, aisa, etc)	7-10k	very bright detailed polishes, tends towards Kasumi rather than mirror
Eastern mine hard homogenous layer (tomae, aisa, etc)	7k-12k	very bright detailed polishes with lots of range - some will maintain kasumi with highest detail and others will be as close to mirror as stones can get also with highest detail
Atagoyama	5-8k	consistent but typically bland Kasumi with okay color and not much detail

Some stereotypes:

• Eastern mine (Okudo, Nakayama, etc) stones tend to be harder and finer than Western mines (Maruo, Ohira, etc)
• Suita tend to be faster than homogenous layer stones of the same refinement range
• Homogenous layer stones tend to be more consistent and predictable than suita of the same refinement range
• harder stones tend to behave finer than softer stones

 

[Desert Rat]

Seems to be a lot of over lap and redundancy with Washita's, Turkish, Hindostan's and in the upper limit also with the natural razor hones.
I have plenty of room here should y'all decide to stream line a bit...

 

[Censere]

This pertains to JNATS -
What Edward provide is very valuable information that one only gets after going through many many stones.
This is gross oversimplification on top of what Edward described in more detail above - one rule of thumb for me

Two general directions of finish below. As a whole, because Eastern stones tend to be harder and finer, you are more likely to achieve (2) with them
1. kasumi diffuse light towards different directions, correlates with coarser (and usually softer) stones
2. mirror is more uniform reflection of light, correlates with finer (and usually harder) stones

One major thing that has been intriguing me is the contrast. I've seen a darker clad usually correlating with coarser stones. However, there is a style of reflective and high contrast "eastern" finish that I love, with reflective dark tint on the cladding and a shiny core. I've mostly seen it done with eastern stones. This is likely achievable with alterations to technique like water mgmt, slurry, etc., but I've seen Okudo Suita, Okudo Tomae, and some Kiitas do this readily.

Curious if this is due to higher concentration of certain elements or particular shapes of these particles? some of the OGs probably talked about this in their writing. I'm yet to get to them

 

[Mr.Wizard]

Bless you for sticking with the vid. I need to skim or read between the lines to get the best value out of those. Since you mentioned not having used many naturals I want sure how far down the rabbit hole you were.

I have not used any naturals, other than mere curiosity to see if I could sharpen a knife on found objects. In terms of book-learning and interest in abrasives generally I would say I am fairly well down the rabbit hole. My practical experience is shallow however, and in the case of natural whetstones nonexistent. I neither wish to under-represent my interest and foundational knowledge lest I get the "dumbed down" version, nor over-represent my experience and mislead.

While it’s frustrating I think there’s a reason why many natural stone users shy away from generalizations, since the minute we do, someone shows us an exception.

I hope to have a useful discussion and exploration of if and how useful knowledge about natural whetstones can be communicated tersely within a chart like mine. I've gotten a tentatively optimistic comment from stringer ("I do think there's something valuable as a general ballpark/range ...") and ethompson certainly understands my motivation, so I don't think it was misguided to try to include these in the chart. I think a single-value representation is pretty dang poor however.

The notation for variance and range that I introduced in the last version at least feels like it would help if applied here. But is it actually good? Is it enough? In hindsight what information you would like to have found on a chart before you started your journey through naturals, and is effective grit range a significant part of it, or is it largely irrelevant?

 

[ethompson]

One major thing that has been intriguing me. There is a style of reflective and high contrast "eastern" finish that I love, with reflective dark tint on the cladding and a shiny core. I've mostly seen it done with eastern stones. This is of course achievable with alterations to technique like water content, slurry, etc., but I've seen Okudo Suita, Okudo Tomae, and some Kiitas do this readily.

Curious if this is due to higher concentration of certain elements or particular shapes of these particles? some of the OGs probably talked about this in their writing. I'm yet to get to them

I think you’re on the right track. I guess the relative concentration of Al2O3 vs SiO2 has something to do with it. If I’m remembering correctly, eastern mine stones tend (but not always) to have a slightly higher balance of Al to Si and an overall higher concentration of both relative to other components compared to western stones - we’re talking slight, not night and day. So the very fine eastern mine stuff can polish to a higher degree while avoid burnishing in a harder matrix.

 

[Mr.Wizard]

As mentioned previously - I don't like using grit ratings for JNats. The people I respect most in this space don't do so either. Particle size is a small, largely irrelevant part of the picture compared to binder makeup, abrasive composition / concentration, surface behavior / prep, etc when it comes to these stones. Jnats are best compares to other JNats, not so so much synthetic stones. However, without that broad experience to draw on, it's hard to contextualize so I dos see why assigning a grit rating is so desirable.

I did not see your post before penning the one above. Interesting that the phrase "largely irrelevant" is exactly how you describe it. You at least understand my motivation for trying to include these in my chart. Do you think I actually should, or would it be better not to? Can I convey information that is useful to a beginner, experienced user, both, or neither?

Would there be any value in creating a new chart dedicated to natural stones with room for more information than I could possibly fit in the existing chart, or is this simply not something anyone would find useful even if an agreeable representation could be found?

 

[ethompson]

@Censere, For me, I’ve found that for a given refinement level the faster the stone the more contrast I see. So those Eastern stones which are also blazingly quick for how fine they behave can maintain contrast while also giving clarity of polish. My ohira Habutae pulls off the same trick but is western mine (see rules are made to be broken!). Viewed from the flip side - typically softness correlates to speed, so softer gives more contrast a right? Common belief, something I’ve told people. But it doesn’t always… atagoyama are typically soft but also typically kinda slow. The finish is often low contrast. Good nakayama kiita are incredibly hard, but also very fast for the fine refinement. The finish can be quite high contrast. For my experience speed dictates how much contrast can be retained and much as the coarseness level or stone hardness.

 

[Censere]

@Mr.Wizard
When one get acquainted with what a type of stone typically does, it's much easier to talk about this, but not everyone has the luxury to. So for my money, especially when I was first into this
1. it would have been useful to reference the ballpark of different mines / layers
2. it would have been helpful to to refer to different types of finishes with "grit" as a shorthand in tandem with a descriptive sentence on the particular effect. Only the former would be a bit oversimplifying, only the latter could sometimes be hard to interpret for me when I hadn't seen it done in person.
3. The above mainly pertains to polishing. Sharpening effect is usually different from polishing grit.
4. A chart - I'm not entirely sure, maybe it's a structured way to see, but somehow feels like a looser way of organizing this info could be a bit more faithful to the nature of JNATs. Something like the general stereotypes that Edward talked about seems fitting.

 

[ethompson]

Interesting that the phrase "largely irrelevant" is exactly how you describe it.

As it relates to objective particle size, I do think that it's a pretty useless measure of how natural stones work. Pretty much every stone from the Kyoto area (which includes 90% of the Jnats people think about) have the same particle size. Same goes for cuticles, novaculite, etc. And yet there is HUGE variation in the sharpening behavior of these stones. So in terms of scientific, get out the microscope and measure particles... yeah, I think its not a good idea and perhaps misleading in terms of performance.

You at least understand my motivation for trying to include these in my chart. Do you think I actually should, or would it be better not to? Can I convey information that is useful to a beginner, experienced user, both, or neither?

Would there be any value in creating a new chart dedicated to natural stones with room for more information than I could possibly fit in the existing chart, or is this simply not something anyone would find useful even if an agreeable representation could be found?

I 100% see the motivation. As I said, I don't like assigning numbers to JNats, but it is also the thing I get asked about the most and unless you have a deep experience with these stones to draw on, it can be hard to compare. Having some frame of reference for what to expect is valuable.

While there are all sorts of caveats, there is value in having ballpark numbers, however rough. Knowing directionally what to expect from a high hardness eastern mine suita vs a soft western mine suita is helpful.

But the polishing bit complicates things too, which is more than half the point with Jnats in particular. So not sure how to get that info into a chart either...

 

[Censere]

@Censere, For me, I’ve found that for a given refinement level the faster the stone the more contrast I see. So those Eastern stones which are also blazingly quick for how fine they behave can maintain contrast while also giving clarity of polish. My ohira Habutae pulls off the same trick but is western mine (see rules are made to be broken!). Viewed from the flip side - typically softness correlates to speed, so softer gives more contrast a right? Common belief, something I’ve told people. But it doesn’t always… atagoyama are typically soft but also typically kinda slow. The finish is often low contrast. Good nakayama kiita are incredibly hard, but also very fast for the fine refinement. The finish can be quite high contrast. For my experience speed dictates how much contrast can be retained and much as the coarseness level or stone hardness.

That rings true to some of my experience, though yeah every rule has a ton of exceptions.
Oh for viewers, there's also the effect of acidity and inclusion of oxidized iron (which one I forgot..), which tend to make the clad darker. .

Strangely the majority of the Kiitas I've tried produced lighter colored clad without alterations to technique

 

[Mr Kooby Shemayrew]

What I would like to see is a kiridashi benchmarked on a full set of Naniwa, Shapton, or Suehiro and a picture of the scratch pattern taken at each level up to the finest stone. These scratch patterns would set a reference by which actual examples of natural stones could be aligned.

Then, using quality examples with high probability of the origin being known, use the kiridashi to find the scratch pattern and place that on the chart adjacent to the most similar synthetic scratch pattern. With enough stone examples, effective grit ranges for each type of stone could be developed.

(You could also get some of these stone hoarders to send you some nice stones to use and document.)

 

[Laplace]

What I would like to see is a kiridashi benchmarked on a full set of Naniwa, Shapton, or Suehiro and a picture of the scratch pattern taken at each level up to the finest stone. These scratch patterns would set a reference by which actual examples of natural stones could be aligned.

Then, using quality examples with high probability of the origin being known, use the kiridashi to find the scratch pattern and place that on the chart adjacent to the most similar synthetic scratch pattern. With enough stone examples, effective grit ranges for each type of stone could be developed.

(You could also get some of these stone hoarders to send you some nice stones to use and document.)

I was actually thinking about doing exactly this once I have the time. I'm about to order a kiridashi solely for experimenting on different jnats and recording my results to give a more visual representation on what effect each stone has.

 

[ethompson]

Can’t compare surface finish of natural stone to scratch pattern of a synthetic to determine grit. Doesn’t work like that - that’s why I described polish and sharpening “grit” separately in my chart. Look at the crazy burnish mirror that togishi can pull off a convex koma stone. That surface may seem like a 10k+ synthetic but the stone is actually much coarser. Or a soft stone finish that is “scratch free” kasumi even under hard light. It will seem finer even than 8 or 9k synthetic like Morihei hishiboshi but is actually coarser grain.

 

[cotedupy]

@cotedupy I am glad that you find my chart helpful and I welcome your help to improve it.

The majority of the chart is expressly about physical particle size rather than abrasive performance as explained in the README, wherein I note e.g. "Soft but sharp abrasive may work very fast on soft metal yet be ineffective on high hardness steel." In the case of natural stones however I don't think actual abrasive particle size is particularly useful, due to differences of hardness, shape, and structure. To that end I have accepted that natural stones will have to be charted based on some fairly arbitrary functional equivalences, dressed and used on steel as considered appropriate by an experienced user.

To pick one that I know you have experience with, in part 4 of the INFO 20M article the Coticule is described with "Grit between 800 and 16,000 depending on the slurry thickness." Do you generally agree with that? It is useful and informative to present it with such a range on the chart? Can it actually work practically throughout that range when applied to an appropriate steel?

I would say that I think it is useful trying to do something like this. I get asked about it the entire time; it's clearly something people want to know, and I think I'm quite good at giving JIS equivalents when I sell natural stones.

You've already noted some of the difficulties in trying to do it. The really big one is obviously hardness/friability, which has a massive impact on how natural stones finish, because softer silica burnishes far faster than AlOx and SiC. But I definitely don't think it's a futile exercise to try to make some general guidelines for people, I'll send a message with some thoughts later. Any particular types of stone you'd like me to include?

---

And FWIW - yes, 800 to 16k would be a pretty accurate range for yellow coticule (imo). I have couple that I would say work a little coarser than 800, and 16k would be only the very, very finest coticules, but that's a pretty accurate guesstimate range I'd say.

Though the problem with trying to make a generalisation like that with coticules is that they have as much, if not more, variability as all Jnats do. In his list above Ed has obviously included all sorts of different types of wildly different Japanese stones. There is at least the same level of variation in Belgian stones.

 

[M1k3]

What I would like to see is a kiridashi benchmarked on a full set of Naniwa, Shapton, or Suehiro and a picture of the scratch pattern taken at each level up to the finest stone. These scratch patterns would set a reference by which actual examples of natural stones could be aligned.

Then, using quality examples with high probability of the origin being known, use the kiridashi to find the scratch pattern and place that on the chart adjacent to the most similar synthetic scratch pattern. With enough stone examples, effective grit ranges for each type of stone could be developed.

(You could also get some of these stone hoarders to send you some nice stones to use and document.)

If you work the Super Stones enough, especially once they start loading up, the finish is much more polished than it's grit rating.

 

[Mr.Wizard]

Any particular types of stone you'd like me to include?

As I feel overwhelmed just reading through ethompson's list this is something I'll need guidance on, rather than providing it. Which stones are sufficiently common to be of general interest, and sufficiently consistent to have an estimated range apply to most examples of it?

 

[Mr Kooby Shemayrew]

If you work the Super Stones enough, especially once they start loading up, the finish is much more polished than it's grit rating.

Absolutely correct. Even when the variable of “grit” is locked in as with a synthetic stone, the how the stone is worked can vary the outcome greatly.

Now imagine when the grit is locked (synthetic) but the stones are produced with different methods: Shapton 1K, Sigma Power 1K, Sharp Pebble 1K, etc.

Now imagine throwing the variable of locked in grit out the window as with natural stones. It further expands the possible outcomes.

With all that said, if one hopes to chart natural stones, some form of a baseline has to be created. A single brand / line of synthetic stones, sharpening the same object, using the exact same technique is how that baseline could be established. This is solely on the aspect of cutting comparison of naturals to synthetic. The subject of polishing is kept totally separate.