Thread: What actually is Hardness in natural stones, and how it affects sharpening

This is a very complex subject. How well the stone cuts is dependent on so many things. The minerals the rock is comprised of, the kind of rock, the density, the particle size and shape, the mix of minerals in the rock and how they are mixed, and a host of others. Remember that running water is the best cutting force there is. It formed the Grand Canyon and can polish a rock to a mirror effect.

But as it stands almost all posts comment in some way about how hard this or that stone is

As far as I could make out the statement of hardness always stands in comparison to other stones of its kind.
"My Coticule is fairly hard" could mean "For a coticule mine is fairly hard"

I don`t think people actually take their Ohzukus into consideration when deciding if their Coticule is a soft one or not.
They compare it to others, or express their opinions. Hardly ever has there been a cross referencing.


I believe hardness is an information that can not stand alone. It is much more interesting to know, and tells a bit about the feel the stone should provide than about the performance

And if sharpening is the goal, the blade is less distorted with the fewer strokes.

Well, that depends! If your technique is good, you can do 100 or 200 laps on a slow Chinese without disorting the edge.
It is not recommended to do much more than necessary

to me the imparitave is the cutting qualities of a stone

The cutting quality of a stone is highly dependend on the very nature of the stone.
That´s why you can find hard and coarse, soft and fine, quick and fine, quick and hard stones across the universe.
Hardness is of no value unless it stands in bulk with the rest of the information.
It is a bit like saying the weight of a Car defines how fast it will go.

Well that may be true in some cases, but obvioulsy there is way more to take into consideration

For razors we ussealy talk about polishing stones and only final polish, not realy sharpening !!
Thats why i like coticules a lot they sharpen and polish very greate and i use them a lot my self, but if you take Ozuku or Thuri they are ONLY for final polish and coticule can not get that kind of polished edge !
You can get some prepolishers Jnats in same grit range as coti, but they are bit slow for razors, and some very fast ones is super expencive.

Originally Posted by thebigspendur

This is a very complex subject. How well the stone cuts is dependent on so many things. The minerals the rock is comprised of, the kind of rock, the density, the particle size and shape, the mix of minerals in the rock and how they are mixed, and a host of others. Remember that running water is the best cutting force there is. It formed the Grand Canyon and can polish a rock to a mirror effect.

true.... and a blade of grass can crack the strongest concrete...

Originally Posted by alx

If you an scratch a stone with a tool this is a crude type of proof of the hardness, the stone has in fact shedded grit particles. Incorporating a given force upon a tool with given and known characteristics, this simple test on a stone could be useful in rating stones as having a certain hardness. The closest entity we have on this forum to conduct this test are the various descriptions of hand pressure, and various qualities of tool steel in the from or shaving razors. This is a start. Alx

Maintaining a flat bevel geometry while sharpening for me is not as crucial with a razor as say a gouge or a plane blade. From discussions I have had here coupled with the fact that so many great shaves are accomplished from so many methods where flatness in not considered makes me think. An accomplished sharpener knows from experience that a harder stone will not deform and round the bevel. With the light pressure used in finishing up a razor, I wonder how stone hardness applies?
I have accomplished great edges happen on stones like the Nakayama from you which happen to be hard. Eschers also give me great results and as Jimmy noted they are considerd soft? Good food for thought, thanks

Originally Posted by Kingfish

I have accomplished great edges happen on stones like the Nakayama from you which happen to be hard. Eschers also give me great results and as Jimmy noted they are considerd soft? Good food for thought, thanks

I used to fish a lot & there were always people arriving for the run in tide & those for the run out tide. They all caught fish but disagreed with each others beliefs about when to fish
When you're talking about 10-15 finishing strokes with no pressure will an Escher give a better edge than an Asagi or vice versa ??? I think there's a lot more to it than hard vs soft.

Originally Posted by Kingfish

Maintaining a flat bevel geometry while sharpening for me is not as crucial with a razor as say a gouge or a plane blade. From discussions I have had here coupled with the fact that so many great shaves are accomplished from so many methods where flatness in not considered makes me think. An accomplished sharpener knows from experience that a harder stone will not deform and round the bevel. With the light pressure used in finishing up a razor, I wonder how stone hardness applies?
I have accomplished great edges happen on stones like the Nakayama from you which happen to be hard. Eschers also give me great results and as Jimmy noted they are considerd soft? Good food for thought, thanks

Hi Kingfish
I am on the same bevel with you as far as plane blades are concerned which traditionally are based on flat bevel configuration. Razors are also requiring flat bevels, but currently there are custom and finely tuned bevels that go beyond what most tool users require or consider. Razors edges are immediately critisized by their users as they tug on our skin, edges tug because of improperly formed bevels which converge poorly or intermittently. Shaved wood boards do not talk back with swear words.


Granted, hard stones will not so easily round a blade in part because the stone itself dishes more slower, but a faster cutting softer stone will dish to the same small degree if the same amount of actual sharpening (bevel formation) is done with fewer strokes. Here is a scheme I worked out a few years ago to compare stone hardness. I used the same blade to test each stone using the same amount of hand pressure, length of stroke and water.

So you can see by the black swarf in the water and slurry that with a faster cutting stone using fewer strokes you can remove the same or equal amount of steel. Now this is useful if you are actually sharpening a blade. If you are just polishing a blade to make it shiny then any hard stone or maybe even glass or would work.

www.thejapanblade.com/hard203.jpg

Originally Posted by alx

Hi Kingfish

Granted, hard stones will not so easily round a blade in part because the stone itself dishes more slower, but a faster cutting softer stone will dish to the same small degree if the same amount of actual sharpening (bevel formation) is done with fewer strokes. Here is a scheme I worked out a few years ago to compare stone hardness. I used the same blade to test each stone using the same amount of hand pressure, length of stroke and water.

So you can see by the black swarf in the water and slurry that with a faster cutting stone using fewer strokes you can remove the same or equal amount of steel. Now this is useful if you are actually sharpening a blade. If you are just polishing a blade to make it shiny then any hard stone or maybe even glass or would work.

www.thejapanblade.com/hard203.jpg

I definitely see what you are saying from the pics. Softer stones from the selection you have get the job done much quicker. Of course that is contingent on many other factors being equal in terms of quality, etc.

I never would be happy if a Jnat could only polish. I expect way more than that from a Jnat, that is why I kept the stones you sent me!

Wow this is one fine thread!

One way to explore this is to try and
sharpen butter and cheese.

Some tools shave off curls some tools
smear the surface or gather a lump.

Steel like butter can be brittle (when cold)
or soft and malleable (warmer)....
Compare and contrast a soft cheese
with a hard cheese like Parmesan and
all the ones in between.

Then explore hardness tests... take
a marble and "measure" how it dents
the cheese with pressure.

The hard Parmesan cheese is interesting because
it shows "micro chipping" and also has salt
crystals.

The obvious thing missing
in my cheesy analogy is work
hardening. Work hardening can be key
to the shave-ability of many steels and has
applicability to final stropping.

I was just reading this thread again, very interesting points.

I only have 4 natural stones. A takashima karasu, an okuda suita, a nakayama
namito(I guess,got it from mainaman) and a chinese guangxi hone. When I tap
all stones:

-the okudo doesn't really give a high pitched voice, the others do.
The okudo also feels softer and produces easier slurry.

-The karasu, namito and guangxi are close in hardness. All three give high pitched
sound. They give their slurry very slowly.

Then I tried scratching one stone with another.

1)The karasu did not really create any scratch on the chinese guangxi hone(CGH) and
vice versa. My conclusion, both stones are about the same hardness.

2)Used the namito to scratch the karasu and it did. The karasu could not
scratch the namito. The namito also scratched the CGH.

Conclusion, the namito is the hardest. This is hardness based on the scratch
test, feel of the stones and the sound they give when tapping the stones.
A clear caveat is of course that the sound given after tapping has to do with
the size of the stone.

The scratch test is confusing for me because I would guess the sharpening particles
in all japanese stones have the same hardness. It's just the how tough the binder
is and how much compression there is that AFAIK defines hardness in these stones.

Sharpman