Fundamental differences between natural and synthetic whetstones

[Saul68]

Hi world,

I’m still building my kitchen knife sharpening skills and knowledge.
In the interim I’m using synthetic stones for my Stainless steel, VG-10, White #2 and AS stainless cladded knives.
Synthetic stone ranges I have are 220,800,1000,3000 and 6000 King and Suehiro.
Also using a flattening stone regularly (light touch after each session), and stroping for the last honing stage. I’m quite satisfied with the results but this is subjective of course.

Starting to look into the natural stones domain and the feedback I’m seeing as well as pricing points (!!) are intriguing.

Why are natural stones considered so much better for sharpening? What are the fundamental properties that makes a natural stone at estimated grit 6000-8000 so much better than an equivalent Suehiro, King or Naniwa?

And then for the return on investment aspect, would a x10 priced natural stone or even just x2 give similar performance ratio?

Thanks!
Saul

 

[Mike Pierson]

Saul, I only use natural JNATS and could not be more happy with my results. I do not have the specific answers to the stated questions but found a BF thread or 2 discussing those differences between ceramic & natural, plus synthetic stones-
https://www.bladeforums.com/threads...nthetic-vs-natural-sharpening-stones.1396165/
Hope this helps.
The straight razor members & threads talk a lot about JNATS & usage versus synthetic.

 

[Obsessed with Edges]

The main thing to know, about the difference between natural and synthetic stones, is that virtually all natural stones' grit is silica-based, and will be much less hard than synthetic stones of aluminum oxide, silicon carbide, etc. For this reason, even though natural stones can work very well for some steels not containing many hard carbides (1095, CV, low-alloy stainless like 420HC, 440A, and most of the typical kitchen stainless knives), they'll not be very useful for many other steels having more hard carbide content (chromium carbide, molybdenum carbide, tungsten carbide, vanadium carbide, etc). Such steels would include 440C, 154CM, VG-10 (all fairly heavy with chromium carbide, and some also containing molybdenum carbide) and even more wear-resistant steels like S30V/90V/110V, etc, all with fairly or very heavy content of vanadium carbides.

In Arkansas stones, for example, the natural abrasive grit is called 'novaculite' (based on silica, a.k.a., 'silicon dioxide'), with a Knoop hardness of ~ 825 or so. Compare that to the hardness of synthetic abrasives listed below:

Aluminum oxide ('corundum', 'alumina', Al2O3): Knoop ~ 2100
Silicon carbide ('SiC', sometimes called 'carborundum'): Knoop ~ 2600
Cubic Boron Nitride ('CBN'): Knoop ~ 4500
Diamond: Knoop 7000

And the carbides in wear-resistant steels presenting problems to sharpening with natural stones:

Chromium carbide: Knoop ~ 1700
Molybdenum carbide: Knoop ~ 1800
Tungsten carbide: Knoop ~ 1900
Vanadium carbide: Knoop ~ 2600 - 2800

You can see above, the least-hard of the carbides listed is still 2X as hard as the natural grit in novaculite or any other silica-based natural stone. This means any natural, silica-based grit is essentially unable to cut or shape those carbides.

There are some 'man-made' stones using natural grit like novaculite, which is quarried, ground up and then bound with synthetic binder material. The only advantage in making them this way is, they'll 'release' fresh grit as the stone wears, instead of glazing over like a purely natural, quarried and cut-to-size stone like an Arkansas stone. Otherwise, they'll have the same abrasive hardness limitation as the true natural stones in novaculite. So, to put it another way, even some stones that appear to be synthetic ('man-made') won't necessarily work like pure synthetic stones in aluminum oxide, SiC, etc.

 

[FortyTwoBlades]

The fundamental difference is that in addition to the issue of abrasive hardness, natural stones are inherently inconsistent in terms of their abrasive grain size, distribution, and hardness. They are generally quite slow-cutting compared to synthetics for a number of reasons. However, their hardness in terms of bond strength (not abrasive hardness) can make many of them quite nice for apexing low-alloy steels, as synthetics are difficult to make in very fine grits with a hard bond. Sintered ceramics are about the closest that we're able to produce as a synthetic equivalent, but they do not readily shed grit and need to be periodically reconditioned when the abrasive finally wears on the surface. There are many kind of natural stones that, while not as versatile in their abrasive properties, that are hard enough to form a good, crisp apex while still naturally shedding some small amount of grit. There's a local variety of black siltstone, for instance, that will form a very barely noticeable slurry in use, and puts a beautiful shine and a tree-topping edge on simple steels without ever glazing.

There's a lot of performance differences between different formulations of synthetics, alone. That variety of functional differences only gets greater with natural stones, but also introduces additional complications of material inconsistencies and reduced abrasive qualities, while also usually being much more expensive. But they can (and do) have their uses. There's just a reason why the invention of silicon carbide synthetic stones revolutionized the abrasives world.

 

[Wowbagger]

Why are natural stones considered so much better for sharpening?

Because God had no intention of it being used to sharpen modern alloy steel when he made the stuff.
It is really hard to come by because some body buried the dambed stuff in the ground under tons and tons of hard rock and water. Hence some of the price increase . . . that makes it all worth it and superior to man made stones that are made from readily available components, specifically combined and engineered to cut and polish specific modern alloys.

. . . well there you go . . . the natural stones are clearly better !
Right ?
. . . errrr . . .
I need to think about this.
I'll get back to you.

 

[ToddS]

...
Why are natural stones considered so much better for sharpening? What are the fundamental properties that makes a natural stone at estimated grit 6000-8000 so much better than an equivalent Suehiro, King or Naniwa?
...

First, you need to define "better". There is a pleasure in using natural stones that comes from the fact that they are found rather than manufactured. There is something pleasant about the earthy smell of natural stones. They are almost always more attractive than synthetic hones. There is a satisfaction that comes from using a natural stone simply because they are inferior to modern alternatives. I would suggest the analogy of hunting with a bow and arrows.

 

[Obsessed with Edges]

First, you need to define "better". There is a pleasure in using natural stones that comes from the fact that they are found rather than manufactured. There is something pleasant about the earthy smell of natural stones. They are almost always more attractive than synthetic hones. There is a satisfaction that comes from using a natural stone simply because they are inferior to modern alternatives. I would suggest the analogy of hunting with a bow and arrows.

That's the appeal for me as well, to natural stones, in instances where I know they'll work for me. I have a small soft/medium Ark stone that's a beautiful pink & white, and is a perfect match to blades in 1095 or CV in particular. Also works well for some low-alloy stainless blades, like 420HC. I keep wishing I had a benchstone-sized version that matched it's looks and performance exactly; but I've always hesitated in spending the money for such a stone without fully knowing if it'll be as good beforehand. Buying Ark stones untested is somewhat a gamble, per the variability in characteristics they exhibit due to their natural origins.

 

[FortyTwoBlades]

I suggest that folks who want to give naturals a try consider just looking at local geological deposits of sedimentary rocks. You may very well find you have something suitable locally and can make your own with nothing more than a masonry disc on an angle grinder to grind down the worst of irregularities (use it outside--lots of dust!) and a diamond plate for finish lapping. For most applications there's not a compelling argument for the expense of purchased natural stones vs. synthetics.

 

[eKretz]

Definitely. I've found a few that work quite well. Again, they're not super useful on the wonder steels but for basic carbon steels they work just fine.

 

[Wowbagger]

Hahaha
to be totally transparent and truthful . . .
the last stone I purchased arrived today . . . and are you ready . . .
is a Dan's Black Hard Arkansas oil stone.

After all I didn't say I wasn't attracted by them.
They do have a draw and fascination. I dare not even look at the Japanese naturals ($600).