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Micro-convexity and stone hardness

How important is the hardness of the stone?

I have looked at and compared a few edges off stones i would consider to be in the medium hardness range. They all seem to create more or less the same micro convexity with edge leading strokes. The grit rating on these stones vary from 6-9k.

I have a JNAT that is rated at 5+. This stone seem to create more or less the same micro convexity as my 8k stones, which i consider to be in the medium hardness range. Although you can not assign a grit rating to these stones, judging by the scratch pattern this stone is finer then 8k. You can also get an outstanding shave off this stone. I am not able to do that with the 6-8k synthetic.
If i use a much harder JNAT, it does not seem to leave a much finer scratch pattern, but it creates less micro convexity then the softer stone, and a sharper edge. Even with a coarser nagura this stone creates less micro convexity then the other stone.
This is probably not new information. To me it just seems like the hardness of the stone plays a bigger role then i though, especially during the finishing phase, if you count these mid-range stones as part of the finishing phase.

It just seems like the finishing potential of the stones rely more on the hardness then the particle size, even if light pressure is used.

Another interesting observation is how fine the edge is at 8k, and how small the difference is (optically) compared to a 12k + edge. Still the performance difference is significant.
 
Do you think this is a function of the softer stone dishing slightly to produce the convexity in the edge?
 
I don't know so much about micro convexity, but I've looked into the hardness thing quite a lot. This is is absolutely true:

To me it just seems like the hardness of the stone plays a bigger role then i though, especially during the finishing phase


As you get into the kind of level of stones that will finish a razor then hardness and other aspects of the composition start to have a much larger impact on the finish level than the grain size does. Overwhelmingly so in many cases.

Have a look at your Maruo under a scope and compare to a razor finishing jnat. The MSS will likely be at least as fine in terms of particle size, if not finer.
 
Do you think this is a function of the softer stone dishing slightly to produce the convexity in the edge?
I do not think dishing is causing this. On the higher grit stones i use light pressure, and generally do not spend allot of time on the stone. It might be a contributing factor on a lower grit stone, like a king 1200, where you need to spend some time on the stone, and generally use higher pressure.
 
I don't know so much about micro convexity, but I've looked into the hardness thing quite a lot. This is is absolutely true:




As you get into the kind of level of stones that will finish a razor then hardness and other aspects of the composition start to have a much larger impact on the finish level than the grain size does. Overwhelmingly so in many cases.

Have a look at your Maruo under a scope and compare to a razor finishing jnat. The MSS will likely be at least as fine in terms of particle size, if not finer.
I have actually done that comparison. My Shiro Suita seems to be just as fine as my harder Nakayama Jnat in terms of particle size (maybe finer), but introduce allot more convexity. This is a much softer stone, so this was to be expected.
What surprised me was how much convexity i saw off my Nakayama Asagi, which is rated at 5+. This is a hard stone (razor grade), but the edge looked surprisingly similar to what i saw off the Morihei 9k. I think they are quite comparable in terms off hardness. I do not get a good shave off the Morihei 9k compared to my JNAT. So there is clearly something the jnat add to the table.
This JNAT falls in the category hard enough, and creates a little more comfortable edges, which i personally prefer.
It also looks like, and feel like honing on melted chocolate. The best analogy i could come up with:) I know it is just a stone:) I prefer this stone over my harder one, even if the harder one i have can give me sharper edges.

I was considering doing a comparison between a Naniwa 12k super to a kouseki 10k. The particle size should be quite close, but the Naniwa is much softer.

For me it really seems like most steels start to approach their limit at around a 12k synthetic stone (1 micron). The Arkansas stone (or some other natural stone) really seem to follow as a "post finisher" really well.
 
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So…. Some people love the edge off a hard stone like a trans Ark.

Some love the edge off a soft stone like a Escher…


Is one “better” than the other?
To me "better" is really not a term i like to use. I would just call them different.

Some people just want to find something that works for them consistently. I like variety, but i do wonder why i do not just stick to one routine.

Sometimes i like to live on the edge:) Those diamond balsa edges sure are sharp, but if my kids run into the bathroom it is much safer for me to use something else daily.
Most of the time i just need to shave. A coticule edge might do just fine. I do not need to worry about slicing my ear off.

Sometimes you just want to find out how sharp you can the edge, only to find out that this is not safe in your hands.

The achievement of getting a good edge from something simple like a rock formed millions of years ago might be a motivation for some (someone like me). It not that important that it is better then anything else.
 
I find this an interesting read after using my very hard (to me) Cnat. The Cnat has been giving me edges that are about comparable to what I get off my diamond pasted balsa strops but sometimes just a little more comfortable.
It has been interesting to read about your Cnat journey. Are you using it with slurry or just with plain water? As you probably know, the slurry also seem to create some convexity, but also a level of comfort in some cases.
Is this a slate type of stone?
 
I feel that sharp and comfortable are often not the same thing, and are somewhat subjectivity measured.

I was talking about this with a member a while ago, and he was saying that stone Y was not as good as stone X. But he was primarily using the HHT as his metric. And I asked, have you shaved with both edges? And when he did, the outcome was no longer so clear.

When people think “sharper” is automatically better, that is usually a red flag of inexperience, IMO.
 
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I feel that sharp and comfortable are often not the same thing, and are somewhat subjectivity measured.

I was talking about this with a member a while ago, and he was saying that stone Y was not as good as stone X. But he was primarily using the HHT as his metric. And I asked, have you shaved with both edges? And when he did, the outcome was no longer so clear.

When people think “sharper” is automatically better, that is usually a red flag of inexperience, IMO.
That is why e.g. the coticule edge (and stone) is so interesting. Some coticule edges look like a mess under magnification, but shave smooth, and cut surprisingly well. Other edges from a different stone might look grate but give a really bad experience.
Rolling the edge under a bright overhead light source seem to give a better indication of the edge condition then HHT results for me at least. Maybe i need to get some thicker hair. I have nothing to spare on my head:)
 
That is why e.g. the coticule edge (and stone) is so interesting. Some coticule edges look like a mess under magnification, but shave smooth, and cut surprisingly well. Other edges from a different stone might look grate but give a really bad experience.
Rolling the edge under a bright overhead light source seem to give a better indication of the edge condition then HHT results for me at least. Maybe i need to get some thicker hair. I have nothing to spare on my head:)
Finer hair is the best for testing. I’m about to start harvesting my daughters, when mum is not watching.
 
Ah ha! No, you need to standardise. Laying around hair is no good. You need to clip a lock, preferably freshly washed, so it is all the same, and put it in a baggy.

Hence, don’t let mum know. kids think it is fun. They can wear a hat in winter.
I will wait for them to get to sleep. Steal some hair and leave some money from the hair 🧚‍♂️
 
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It is difficult to compare synthetics and naturals because we do not know fully the composition of naturals individually.
Hardness is a function of the binder. Take a known grit stone, and compare a soft binder to a hard binder, say a 1k grit stone.

The soft stone, binder releases around the grit exposing more of the grit, the hard binder holds the grit and the grit wears away at the top.

The soft binder stone will cut deeper, the grit is 1k deep and 1k wide, the hard binder only a fraction of the 1k grit is exposed and in use it changes shape to a flatter surface, (burnished). It is still 1k but can produce a higher finish.

A simple test is to hone on 1k with heavy slurry, look at the random stria pattern on the bevel. Now clean the stone and quick lap it. Now do a few laps on just a clean, wet stone. Note the shallower more uniform stria pattern using the same stone and grit.(A good way to perfect an edge at each grit in the progression).

This is why 8k Fuji or Snow White stones can near mirror polish, where other 8k cannot, and 8k’s can produce different shaving edges.

Micro convexing is more common on soft stones, as soft stones self slurry. Picture a layer of ball bearings or marbles. It does not take a lot of pressure to cause the edge to dive into the balls to reach the flat hard stone and the back of the bevel is supported by the grit/balls. The edge part of the bevel is polished more than the back of the bevel creating a micro-convex bevel. Remember we are talking Micro here.

Alex Gilmores micro-graphs in his videos bevel setting on a 1k and jumping straight to a Jnat with Diamond slurry, show very clearly where the leading edge of the bevels are polished, the base still have deep 1k stria.

It does not really matter that the back of a bevel is not fully polished or that 1k stria is not removed, by the time the hair reaches mid bevel it has long been cut.

So, is harder better? It depends on the stone, grit composition, the steel you are honing and technique. Generally, for polishing, yes. But YMMV.

Sharp is easy, Sharp and Comfortable, is in that ethereal world.
 
This is allot more complicated then I thought. I did a comparison between a kouseki 10k and a Naniwa 12k. I am really impressed by the Naniwa. The hard kouseki seem to give a little less convex apex, but after a round on the strop they are quite similar. The kouseki seem to give a little deeper and coarser striations.
The bevel is concave, so only part of the bevel is active on the stone.

IPC_2022-06-22.17.31.14.8730.jpg

Jnat
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kouseki 10k
IPC_2022-06-23.18.20.29.4230.jpg

Naniwa 12k
IPC_2022-06-23.18.20.19.9770.jpg

12k upper part of the bevel
 
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rbscebu

Girls call me Makaluod
It has been interesting to read about your Cnat journey. Are you using it with slurry or just with plain water? As you probably know, the slurry also seem to create some convexity, but also a level of comfort in some cases.
Is this a slate type of stone?
My Cnat is metamorphic but does not appear to show any signs of foliation so I wouldn't call it a slate. It is more like a very fine-grained quartzite.
 
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Slash McCoy

I freehand dog rockets
Keep in mind that results with a Jnat vary wildly, depending on the slurry and the nagura used to raise it. Jnat experts are welcome to refute, if their experience says otherwise. I only own one and it is the only one I have ever owned.
 
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