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

While I agree that the softness of a stone is not a direct indicator of particle size, it can have a direct effect on the edge produced by the particles. Basically, and i know I've said this many times, the abrasive particles in Japanese natural hones aren't that "fine," in comparison to what everyone expects-they average between 1 and 2 microns. However, harder stones keep those particles bound to the stone and reveal only small parts of them until released into slurry. They then break down by the action of hard steel on hard stone-thus getting finer.

Softer stones easily release particles, negating the breakdown and often resulting in an edge that is to as "fine" as that from a harder stone.

While it may be possible to get a god edge from a softer stone, I have yet to find one that can hit the top level of a harder stone, even with 0 pressure, due to the lesser breakdown.

This way, the abrasive particles are actually the binding material for smaller abrasive particles since they brake down in size. And, they don't abrade only the steel, but also the stone, thus surfacing new ones. Until they get fine enough while honing, the bigger sized ones (in the slurry) abrade the stone, and new big sized particles become part of the slurry and continues like a chain reaction. The reduction in size of those particles is possible if you use powdered stone on a piece of highly polished crystal diamond surface, but the hone is not hard enough not to be scratched by its own abrasive. If it can't be scratched, (meaning the stone is harder than its abrasive particles) then, the abrasives of the stone are not abrasives. I still can't understand the phrase "particles brake down in size while honing" and I find it very often, regarding always Japanese stones.

One point of confusion is the use of a nagura on
a very hard hone stone.

Think of it as loose abrasive on steel or glass
substrate. The a natural stone will have a bit
of tooth that abrades the rubbing stone that establishes
the effective grit. As the grit is pulverized the substrate
becomes a hard polishing base...

Other natural stones are rubbed to release abrasive bits
in the hone material itself like a coticle.

Another turn of the rock is the Arkansas hones. All
are the same mineralogy with an abrasive hardness
that is the same as quartz. The abrasive hardness
never changes but the hardness as measured by density
does. The denser/ harder Arkansas is considered the
finer hone even when the grain size is almost constant.

Yes, novaculite is an exception. The binding material has the same composition with the abrasive. If you use a nagura on a hard stone so that the particles of the nagura will brake down, why spend a few thousands $ for something that performs in that sense like a piece of glass shaped like a hone? and, why do you need a nagura, since just powdered stone does the same job? There must be something more than that about those stones. Regarding the "density" of the stones; The binding material could be cinnabar. Pretty soft, usually found near quartz veins, but weights 4x more than clay or CaCO3 or any binding material. And the stone has much bigger density. Or it could be any other soft salt of heavy metals. SiO2, AlOx, garnet, SiC etc have all of them pretty low density and they are all very close to the density of the binding materials. This density rule is not absolute.

Quote:

---

Originally Posted by Vasilis

Yes, novaculite is an exception
.....snip...
This density rule is not absolute.

---

Yes....

My point is that each hone type/ family has different care and feeding rules.
By understanding your hone you can best use the tool.

I am sure I will miss things that are important and I misunderstand
other bits but here are some thoughts.

There are two hardness components: 1) scratch hardness
and 2) binding strength.

Density has two contexts: 1)specific gravity and 2) percentage
of the abrasive component.

Uniform grain size is most important when the abrasive grains release and
tumble on the surface. Less so when held tightly so only so only
small portions are exposed as cutting tools.

As far as rubbing stones go, some contribute grit, some clean swarf,
some reduce matrix and expose grit, some condition the surface not
unlike cutting teeth in a file some pulverize grit or remove grit tops
some are multi-purpose. The pulverizing bit is also critical on man made
hones.

The binder is also critical... as thin and fine as a razor edge is a largish
chunk of soft released matrix/binder can dull the edge. This dulling can
be inferred in some posts based on "over-honing, lapping and slurry"
discussions. This is why some hones seem to have a "hard to damage"
matrix and might be called a hard hone in a family.

Yep, I am sure I missed things that are important and I misunderstand
other bits.

The percentage of abrasive component as I read in a previous thread does not means that the stone is faster. It usually does, but for most of the novaculite family stones, their abrasive component percentage is 100% (without counting possible air filled pores). But they are the slowest stones I know. Regarding the qualities of the slurry from the slurry stones, my opinion is, there is no reason to over think about it. For me (without judging the way you see it of course), their grit, hardness and shape of the crystals are enough to explain the way they work. The binder is of course critical, but if the stone is so soft that brakes like a cookie, exposing those bigger pieces, then it's not suitable for honing, regardless the shape of the abrasive crystals, or the percentage etc. and it's easy to overhone on those conditions. When a stone is prone to these things, the pressure we apply can change dramatically the shape of our razors edge. And, the same thing could happen if you have a hard stone, and you apply heavy pressure.

Quote:

---

Originally Posted by Vasilis

Yes, novaculite is an exception. The binding material has the same composition with the abrasive. If you use a nagura on a hard stone so that the particles of the nagura will brake down, why spend a few thousands $ for something that performs in that sense like a piece of glass shaped like a hone? and, why do you need a nagura, since just powdered stone does the same job? There must be something more than that about those stones. Regarding the "density" of the stones; The binding material could be cinnabar. Pretty soft, usually found near quartz veins, but weights 4x more than clay or CaCO3 or any binding material. And the stone has much bigger density. Or it could be any other soft salt of heavy metals. SiO2, AlOx, garnet, SiC etc have all of them pretty low density and they are all very close to the density of the binding materials. This density rule is not absolute.

---

Vasilis excellent point. This is something I have been thinking for a lot
of time. Personally I think honing on a super hard stone is just more
attractive that honing on a piece of glass. The stone is something
special for the owner whereas a piece of glass is not.

I think the best finishing stones are those that release super fine
grit(20k or more) and allow that grit to breakdown somewhat before
new grit is released, so you end up getting a 30-40K edge. I think
these are the stones that are really high quality. The super
hard stones that don't release any grit and therefore should be used
with a nagura slurry, are probably much less worth. As Vasilis wrote,
you could just use a piece of glass with powdered stone.

Now a super hard stone, very uniform, that releases super fine grit(30K+)
at a pace that makes polishing a quick job and isn't scratchy,I guess would be
the ultimate polishing stone and probably very expensive. I don't know if such a
stone exists. Maybe Alex can comment as he has many high quality stones.

sharpman

I think I've said before, this is a very complex subject. When you write down all the physical characteristics of a rock or mineral and you start considering the varying parameters as these characteristics change you begin to appreciate why the best honing stones in the world are so rare and expensive. You could write a scholarly thesis about this stuff.

Any rock will act as a hone to one degree or another. You could experiment with anything and everything and one change in one parameter could trump everything.

Now I've got this chunk of Zoisite (a Calcium Aluminum Silicate) with tiny ruby xtls in it and if I wanted to sacrifice this and grind it down it might make a great hone but it's rare and I won't.

Quote:

---

Originally Posted by thebigspendur

I think I've said before, this is a very complex subject. When you write down all the physical characteristics of a rock or mineral and you start considering the varying parameters as these characteristics change you begin to appreciate why the best honing stones in the world are so rare and expensive. You could write a scholarly thesis about this stuff.

Any rock will act as a hone to one degree or another. You could experiment with anything and everything and one change in one parameter could trump everything.

Now I've got this chunk of Zoisite (a Calcium Aluminum Silicate) with tiny ruby xtls in it and if I wanted to sacrifice this and grind it down it might make a great hone but it's rare and I won't.

---

Absolutely. You could think of sandpaper as a stone, and it wouldn't be a hard stone. Yet the particles it contains could be various hardnesses. Or the particles in various different shapes, all of which affect cutting speed and performance. Coticule is actually the sandpaper of natural stones, with an almost soft clay matrix containing very hard particles on the soft fastcutting examples. So you have a soft stone that uses hard particles to cut. Is the stone then soft or hard? Likewise, density doesn't matter either because the matrix breaks down in a slurry very quickly so the stone has one of the lowest densities available. Yet the garnets are very hard and dense, but we have no way to measure them - only the results of their breaking free of their matrix to cut and polish.

The arkansas stones work differently so hardness and density play a substantial role. On other stones, hardness and density have little or no bearing. I think this is where theory breaks down and one simply has to go by results of using different stones. But then, that's what all the fun is about anyway.

I think sandpaper works differently from the stones. It's just a mix of particles, all of them abrasive, and the grit of the sandpaper written behind gives us the size of the biggest ones. Using it, those big ones are the the first ones that get loose, and the smaller ones remain on the paper. That's why it gets finer after use. The same happens on the diamond plates. Neither of them has another layer of homogenous particles beneath its cutting surface, that's why they get finer and finer. I have been able to sharpen a knife with a 800 grit sandpaper sheet to the point of hair shaving, it was just popping hairs (and I was really surprised), but not fine enough for using it on my face. This would be impossible with an 800 grit stone.