Making sense of J-Nat stone-seller's hardness ratings

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Rangen

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I like to sharpen on hard stones, apparently. But there seem to be a few systems out there for judging hardness, and I have no idea how to convert between them.

JNS: Softish stones are 2.5, hardish ones are 4.5 to 5+, never seen a 6
Watanabe: softish stones are maybe 30-40, hard stones are in the 60s-70s
AFramesTokyo: hard stones are 8.something, softish stones are 7.something

So, three different systems. What are they? How do I convert one to another?
 

Luftmensch

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So, three different systems. What are they?
There are some stone addicts here who have better first-hand experience than me. I have several stones and have looked at all these sellers - so I can shed some light on your question.

With the exception of Watanabe, they are arbitrary systems based on the vendors subjective judgement. So take their opinion with a grain of salt. Is it reasonable to expect a person (even an expert) to grade the difference between 8.8/10 and 8.9/10 hardness repeatably and without drift over the years? Probably not. Just accept those measures as some locally/internally consistent system vendors use for grading stock.

You cant really quantitatively convert from one vendor to another. If you are after hard stones, just look at the stones that are at the top of each vendors hardness scale! Easy, 😉. KKF members with good qualitative experience comparing the vendors may be able to give their opinion on a good numerical mapping relating the scales. I'll leave that for them!

As for Watanabe. He is using an objective hardness measure. In good faith, I assume this is generated by the appropriate machine and testing protocol (shame on me if I am wrong!). In which case, a negligibly small difference in HSD might be quantitatively true.... whether or not it is uniformly true across the stone is another issue! If another vendor used HSD, in theory you could productively compare stones on small hardness differences.
 
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nutmeg

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There is NO measure I know that can help predicting how one stone is going to perform compared to an other!!
Only serious pictures of the effect on the blade can help.
 
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nutmeg

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I think measuring the color and light reflection on the blade may be the most helpful but not sure it's doable.
After all, what we're looking for when we're grabbing a stone for a job is its effect only.
 
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nutmeg

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Hardness can be subjectively measured but often mixed up with grain size, density, thickness of the stone etc.
Ikarashi for example are not particularly softer than other stones but crumbly, low density and large grain size so people find them soft.
 

Luftmensch

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@nutmeg is just the sort of guy you should be listening to :)

And this observation is spot on:

Hardness can be subjectively measured but often mixed up with grain size, density, thickness of the stone etc.
Ikarashi for example are not particularly softer than other stones but crumbly, low density and large grain size so people find them soft.


There is no measure I know that can help predicting how a stone is going to perform. Only pictures of the effect on the blade can help.
I like to think pictures of the slurry tells me something. I guess it says whether it is a muddy stone or not? In reality, by thinking it is telling me something, it is probably leading me astray 😝
 

nutmeg

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Anyway, this is a delicate subject. Not sure we really want to know how stones are tested ;)

My advice: measurements are about as precise as S, M, L, XL.. for shoe size.
 
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