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

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

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

checking for hardness do the water test put one drop of water on a few of your hardest stones which ever stone holds the water the longest is the hardest

Jimmy,

That stone is amazing, you really are lucky to find one that NOS. The one I have from you is amazing, I love it, I hug it in my sleep at night.

Originally Posted by eleblu05

checking for hardness do the water test put one drop of water on a few of your hardest stones which ever stone holds the water the longest is the hardest

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.

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 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.