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

Originally Posted by JimmyHAD

Wouldn't holding water longer be a matter of density ?

BTW, my posting the Escher seen above, within this thread, was due to the label "guaranteed soft." I think the relevance to this thread is demonstrated by Escher finding guaranteeing softness for a razor hone, which is what Eschers were/are, an important enough selling point to print and apply a special label noting that characteristic.

yes the denser the stone the harder the stone is. if the water gets absorbed into the stone that means the stone is porous not as dense as the other stone holding the water on top of the stone

Originally Posted by JimmyHAD

Wouldn't holding water longer be a matter of density ?

BTW, my posting the Escher seen above, within this thread, was due to the label "guaranteed soft." I think the relevance to this thread is demonstrated by Escher finding guaranteeing softness for a razor hone, which is what Eschers were/are, an important enough selling point to print and apply a special label noting that characteristic.

Jimmy I am with you here.
I have been thinking about it for a while and I am not sure the water drop test is actually that indicative.
Example Escher, it is considered soft but it holds no water at all, it is just the binder is not hard compared to some Jnats.
On another hand I feel like the harder the stone the more compact it will be and thus the less water will it absorb. In this light water drop test will show that the hardest stone releases water the fastest. If anyone knows how to explain the test please do so.

the denser the stone the less water it will absorb density = hardness the porous the stone the more water it will absorb

It seems to me that hardness would have more to do with the mineral composition of a stone than the density.

Maybe I'm not thinking of hardness the right way though.

That is why I repeat: there is no definition of hardness of a stone. Why should there be?
What could be the benefit of a unified hardness system for naturals?

BTW I had very hard and very quick coticules.
It remains difficult to find a definite consens for naturals

Originally Posted by Lesslemming

That is why I repeat: there is no definition of hardness of a stone. Why should there be?
What could be the benefit of a unified hardness system for naturals?

BTW I had very hard and very quick coticules.
It remains difficult to find a definite consens for naturals

I too have a coticule that is extremely hard (a La Verte), but also is fairly fast on water. It's speed on slurry is on the slow side, but on water it catches up quick. Interestingly it is also the only coticule I have that leaves an edge that I can get weepers with, similar to a thuringian edge if I'm not careful, yet my thuringian I would describe as soft.

Trying to relate hardness of a stone to the edge it gives for naturals I think would be in vain, as there really is no uniformity IME.

Originally Posted by SharpMan

Alex, thnx for the post.

Could you explain why some very hard stones
still have very good cutting abilities whereas
other hard stones are very slow? I know you
wrote that depends on grit per pound, but
what affects this?

The scratch test makes sense.

Basically what I would do, is I would hold the stone
in my hand to check its weight/density. Then I would
knock on the stone to listen what kind of sound it makes.
I assume a soft stone would make a duller,lower sound
whereas harder stones a more ''ping'', higer pitched sound.
Next thing I would do is sharpen on the stone, if I feel that
the surface of the stone is very hard(indentation wise, no
scratching ability), then I would think ''man this thing is like
glass!'' and conclude this is a hard/very hard stone.

Sharpman

Hi Sharpman
Without actually trying out the particular "hard stone" I would venture to guess that some hard stones do cut very fast at first because they are loaded with grit. But if the swarf from the steel blade is not carried away in some manner, the stone will become loaded with compressed cleaved grit particles that are essencially welded into the voids of the stones surface along with ground off steel. This glazed stone will at this stage stop being a hone but more like a burnishing plate. Here again some people think that a bright shiny blade must be sharper, but I suggest that the metallurgical changes necessary to polish a blade a blade by burnishing will have mixed consequences including destruction of the temper of the steel maybe by a few points, and an ultra thin but very fragile edge that will only last a few shaves at best.

My Shapton 30k on glass becomes black when I use it, the grit is so sharp and the action is so fast and the stone so hard that the black metal filings just have no where to go but to fill up the surface voids of the stone. Do I just continue to use a stone that was pure white but is just becoming blacker and blacker? Or should I lap the stone to clean off the swarf and in doing so reveal fresh grit? When does the 30k start acting like a burnishing plate?

Also I like your idea of testing the hardness, totally sensual and empiric and based on observation. And the ping of a stone will deduce consistancy and reveal any imperfectons. The water test has some cautions with japanese stone. Some miners coated the top surface of freshly surfaced stones ready for market with an eggwash to prevent the soiling of the surface with grime and dirt. And also I have found that Jnats will over time from a natural oxidized film that is microns thin, both of these issues will imped water. So unless the stone is lapped, but with a used stone the water could be uses as an indicator. I have to agree that a dense stone will be more compact and will shed water more easily than a porous (more voids between physical particles) but also a dense stone may be less porous because of a high concentration of binders.

Originally Posted by Lesslemming

That is why I repeat: there is no definition of hardness of a stone. Why should there be?
What could be the benefit of a unified hardness system for naturals?

BTW I had very hard and very quick coticules.
It remains difficult to find a definite consens for naturals

Lesslemming. Maybe we do not need a hardness test or scale. If you could arrange and enforce the use of the word "hard" on this forum, then we would not. But as it stands almost all posts comment in some way about how hard this or that stone is. I myself do not feel that the hardness of a stone should be forefront, to me the imparitave is the cutting qualities of a stone. 20 strokes on a slightly softer stone does more for me than 100 strokes on a harder stone if the grit particles sizes are equal. And if sharpening is the goal, the blade is less distorted with the fewer strokes.

Also I think that Dr. Naka uncovered a unique answer as to why some edges are more comfortable than others. His description of Iwasaki-sans using his hands to describe the profile shape of a finished blade, all finger tips touching and both palms together, with fingers straight or with fingers bowed. The bowed being more comfortable for softer skinned beards. A slightly rounded blade profile. The perfectly straight flat profile may be sharper but usually harsher. A softer stone will aid with its free roaming grit to provide this splayed profile, the harder stone will provide a flat profile. Alx

P.S. I did not mean to use the term grinding as in coarse grinding or gross removal, but instead to mean just the mechanical act of steel removal at all or any grit levels. a

Yep i think too that there are different prefferents on the edge, coticule is a lot coarser then a Jnat but still shave very nice and sometime better, the hybrid coticule that i have is a lot harder then my Ozuku but stil bit coarse but shave is super nice like any jnat !

For hardnes there are a lot of different stones and hardnes and you cant prove them all i have a Okudo suita that is lv 5 hardnes but lv 2 fines, so you cant realy comper hardnes on Jnats or others natural stones you just have to try them and give approx value on fines and hardnes and what they have to be used for there is not realy right answer here !! Becouse they are all different !!

Originally Posted by maxim207

there is not really a right answer here !! Because they are all different !!

Well said Max. I was just thinking about this today. I don't get hung up on one particular stone, because they're all good for something. Variety...

Hence, HAD....