When is a knife thin enough - my simple test for a pleasantly ground knife

[ethompson]

This all comes from my personal musings on what makes a knife perform well and an ongoing conversation I've been having with another member on what makes a grind good.

Disclaimer

Let's start this out with the obvious, all of the below in just my opinion and is designed to match my preferences. I prefer stone polished finishes and don't tolerate wedging. I also follow a bit of an unusual maintenance regimen. With each full sharpening, defined as resetting an apex with a stone 1k or coarser, I thin back to a zero edge and then sharpen at a slightly higher than typical angle - probably about 20* per side. Keep in mind these idiosyncrasies as you read this please.

On the other hand I think there are a few reasons my opinions are worth sharing. I've been lucky enough to own / try most of the high-end knives we talk about around here and have used most of them in a professional setting. Not saying that cooks know more about knives than the enthusiasts here (they don't), but it can definitely accelerate the learning curve given there is a much, much larger volume of product - cutting 25# each of onion, cucumber, tomato, and sweet potato back to back will quickly let you get to know a knife. I also spend a lot of time sharpening, polishing, etc. - usually upwards of 15 hours per week. Im certainly not the best at it, but I've spent enough time rubbing metal on rocks to have a decent idea of how the shape of a bevel impacts a knives performance.

The Rule

Any knife which carries convexity into the edge and easily passes the nail-flex test is thin enough to be an elite performer for kitchen tasks.

In Greater Detail...

I think this rule can apply equally to knives with all manner of geometries - wide bevel, short bevel, full convex, s-grind, forged geometry, etc. The obvious exception is wide bevel knives that are found on a hollow wheel resulting in concave bevels (eg - Birgersson, most of the Sakai wide bevel stuff, etc.) which I'm conveniently ignoring here since I generally can't be faffed with those given my stone polishing kink. So for this discussion let's just pretend those don't exist...

While the above (henceforth referred to just as the "Rule") is a seemingly simple statement, there is a lot that has to happen for these conditions to be met. First, to easily nail flex a knife needs to be extremely thin just behind the edge. Furthermore, for the edge not to immediately crumble with that type of geometry the steel has to be good. So at a minimum a knife that can hold this type of geometry has good steel and is thin enough to initiate cuts in a very pleasant "ghost-like" manner. To carry some amount of convexity into an edge like this comes with its own implications for the grind overall. If the grind from 5-15mm up from the edge is too thick, even a fully flat bevel without convexity can struggle to hit nail flex. To solve this issue you'd need to raise the shoulder, or in the case of a convex or compound grind the apex of convexity, enough to allow a geometry above the edge thin enough to convex into nail-flex behind the edge.* So to satisfy the Rule, a knife not only needs to be thin behind the edge but above the edge as well. This is indicative of a knife that not only starts the cut well, but will continue to plow through even dense product as well.

You may be asking why I'm bothered at all with convexity behind the edge, especially since it'll be rather subtle with geometry this thin. A couple reasons... First, I find that a subtle amount of convexity behind the edge allows me to run a near zero bevel without worrying about chipping or rolling the edge. I have had significantly more chipping issues with fully flat short bevels than subtly convex ones even with both at nail-flex thinness. And while this may be placebo, I do also feel like I have less issues with stiction during cutting and improved release with that subtle convexity as well. That convexity also allows for more subtle tweaking of behind and above the edge thickness across the grind without worrying about creating a foil edge. A small scale version of how hamaguri sharpening allows control over the edge and shinogi on a single bevel independently.

In summary, knives that conform to the Rule are thin enough to perform very well for all kitchen tasks. It's a simple thing to check without even cutting product that I find can tell me a lot about whether or not I will find the knife suitable. If either part of the Rule isn't met, I know I have work to do.

* allow me to define "behind the edge" as the area from the apex up 5mm and "above the edge" as the area from 5mm to 15mm for this discussion

Examples

There are several knives I've used that passed this test easily: Yanick, Milan (two thinner low-bevel examples), Birgersson (I stone finished that one), Kippington work-pony and laser-pony. Then there are knives that passed this test for me after a little work: Mazaki, Shigefusa, Kato, Takeda (iron clad nakiri), Z-Kramer (an early model that was ground rather thin from the factory). Some knives that pass the test so long as I diligently keep up with maintenance thinning: Halcyon Forge, Catchside, Raquin KT - these are all "workhorse" knives but follow the Rule, just. And then there have been a lot, including some from prominent very well regarded makers, that would have required serious alteration to satisfy the Rule....

Closing Thoughts

Thank you for reading my pedantic ramblings! This has been a very long way of elucidating my own rule of thumb for quickly judging a kitchen knife based on the grind behind and above the edge - using reflections and light and my thumbnail I bet this test takes me 10 seconds. I have yet to meet a knife that satisfied the Rule and wasn't fantastic fun to use. Likewise, I have yet to come across a blade that didn't satisfy one of the two conditions that I didn't think could be improved by some amount of alteration. Hopefully you notice from my examples that a wide range of styles can fulfill the Rule - from Kitchen Tractors to lasers. I'm not anti-workhorse, but I do find myself increasingly seeing knives that I doubt could pass my performance exam as makers pursue the current KKF zeitgeist of thicker and thicker knives.

 

[NickMinton]

Thanks as always for the knowledge @ethompson

 

[Heckel7302]

Can you tell me more about your polishing experience with Birgersson? I want to polish the bevels on mine, but I'm a little scared of it. It's outrageously thin BTE and appears to be slightly hollow in the grind. It might be a bit uneven in places, or it might just be the vertical grind marks playing tricks with the light. My idea is to start up at the shinogi and work gingerly down to the edge to avoid getting too thin and unstable.

 

[ethompson]

Can you tell me more about your polishing experience with Birgersson? I want to polish the bevels on mine, but I'm a little scared of it. It's outrageously thin BTE and appears to be slightly hollow in the grind. It might be a bit uneven in places, or it might just be the vertical grind marks playing tricks with the light. My idea is to start up at the shinogi and work gingerly down to the edge to avoid getting too thin and unstable.

Mine was definitely hollow in the bevels and a bit of a pain to convert to a convex bevel. You have the idea right to try and work primarily from the shinogi to avoid over-thinning the edge. The bevels on mine were not perfectly uniform but more uniform than other hollow bevels I've turned into convex. I would also start with thickening up the edge by sharpening on a 400 grit stone at a relatively high angle (like 45* per side) because no matter what you do you will end up eating away some height and core steel in the process. Might have to repeat this process a few times. It took me a loooong time though to get those bevels worked out. Going to be a project and a half for sure.

 

[NickMinton]

Mine was definitely hollow in the bevels and a bit of a pain to convert to a convex bevel. You have the idea right to try and work primarily from the shinogi to avoid over-thinning the edge. The bevels on mine were not perfectly uniform but more uniform than other hollow bevels I've turned into convex. I would also start with thickening up the edge by sharpening on a 400 grit stone at a relatively high angle (like 45* per side) because no matter what you do you will end up eating away some height and core steel in the process. Might have to repeat this process a few times. It took me a loooong time though to get those bevels worked out. Going to be a project and a half for sure.

Glad you spelled this out so I definitely don’t try this on my BB. If it was tough for you, it will be impossible for me

 

[ethompson]

Mine was definitely hollow in the bevels and a bit of a pain to convert to a convex bevel. You have the idea right to try and work primarily from the shinogi to avoid over-thinning the edge. The bevels on mine were not perfectly uniform but more uniform than other hollow bevels I've turned into convex. I would also start with thickening up the edge by sharpening on a 400 grit stone at a relatively high angle (like 45* per side) because no matter what you do you will end up eating away some height and core steel in the process. Might have to repeat this process a few times. It took me a loooong time though to get those bevels worked out. Going to be a project and a half for sure.

I'll also add this is totally unnecessary. Birgersson's blades flat out perform IME. Incredible steel and phenomenal through food. There is some absolutely bananas banding hiding in that mild steel cladding though

That said, you can also see along the shinogi where I didn't quite get a perfect crisp transition due to angle inconsistency when thinning at the shinogi while trying to not use the edge as an angle guide like you could with the thicker low-bevel grind. This will come out over time but is indicative of the fact that keeping the bevels even while thinning at the shinogi without hitting the edge being no small feat.

 

[esoo]

The Rule

Any knife which carries convexity into the edge and easily passes the nail-flex test is thin enough to be an elite performer for kitchen tasks.

So this is an interesting one - as a home chef, I prefer my knives to be just before the nail-flexing point. I know I give up pure performance for it, but I prefer to only grab one knife per session. As a result, I like a bit more resilience in my edge so I don't have to worry about it chipping out. Additionally, both the wife and daughter randomly grab a knife off the rack, so I need to be aware of the other users. Sometimes you have to choose practicallity over fun.

I also have to say that thickness at the behind the edge/above the edge range is crucial to performance. To my preferences, this area is where a knife can really fall down. S-grinds die right about here as they tend to get too thick too fast. Even subtle differences in thickness here start to show up - my Kono MM is thinner at this point than my Tanaka x Yohei and you can feel it when doing cuts in onions.

 

[Heckel7302]

I'll also add this is totally unnecessary. Birgersson's blades flat out perform IME. Incredible steel and phenomenal through food. There is some absolutely bananas banding hiding in that mild steel cladding though
View attachment 248439
That said, you can also see along the shinogi where I didn't quite get a perfect crisp transition due to angle inconsistency when thinning at the shinogi while trying to not use the edge as an angle guide like you could with the thicker low-bevel grind. This will come out over time but is indicative of the fact that keeping the bevels even while thinning at the shinogi without hitting the edge being no small feat.

Yeah, I can see hints of banding on mine even through the belt finish. Really makes me want to bite the bullet and lay it on stones.

 

[ethompson]

So this is an interesting one - as a home chef, I prefer my knives to be just before the nail-flexing point. I know I give up pure performance for it, but I prefer to only grab one knife per session. As a result, I like a bit more resilience in my edge so I don't have to worry about it chipping out. Additionally, both the wife and daughter randomly grab a knife off the rack, so I need to be aware of the other users. Sometimes you have to choose practicallity over fun.

If others are borrowing your knives then I can see that, but I've taken corn off the cob and removed avocado pits with nail flexing edges without chipping. Not to mention banging them into hard poly boards all day on occasion.

But... as I said in the disclaimer, this is just one man's ramblings

 

[esoo]

If others are borrowing your knives then I can see that, but I've taken corn off the cob and removed avocado pits with nail flexing edges without chipping. Not to mention banging them into hard poly boards all day on occasion.

But... as I said in the disclaimer, this is just one man's ramblings

I'd have mostly no problems with nail flexing. But I've come back from a weekend away to find a factory ground nail-flexing Yoshi with a chipped edge, so I don't trust the others in my kitchen. Mostly they stay to the knives on their half of the rack, but sometimes they grab mine. I even strategically place knives on the rack, so the ones of mine that are stouter are easier for them to reach.

 

[OkayMode]

I've spent enough time rubbing metal on rocks

This sentence instantly demolished the 'cool esoteric hobby' vibe I've been trying to cultivate for the last few years.

I now understand why my girlfriend calls me a goblin when she walks into the kitchen and i'm hunched over my sink bridge...

Also, great and informative post!

 

[KenjiF]

@ethompson this makes a lot of sense. I've 'accidentally' done this on a number of knives when thinning and convexing thinking that the close-to-zero grind was going to chip and found it outperforming my other knives.

 

[kpham12]

This all comes from my personal musings on what makes a knife perform well and an ongoing conversation I've been having with another member on what makes a grind good.

Disclaimer

Let's start this out with the obvious, all of the below in just my opinion and is designed to match my preferences. I prefer stone polished finishes and don't tolerate wedging. I also follow a bit of an unusual maintenance regimen. With each full sharpening, defined as resetting an apex with a stone 1k or coarser, I thin back to a zero edge and then sharpen at a slightly higher than typical angle - probably about 20* per side. Keep in mind these idiosyncrasies as you read this please.

On the other hand I think there are a few reasons my opinions are worth sharing. I've been lucky enough to own / try most of the high-end knives we talk about around here and have used most of them in a professional setting. Not saying that cooks know more about knives than the enthusiasts here (they don't), but it can definitely accelerate the learning curve given there is a much, much larger volume of product - cutting 25# each of onion, cucumber, tomato, and sweet potato back to back will quickly let you get to know a knife. I also spend a lot of time sharpening, polishing, etc. - usually upwards of 15 hours per week. Im certainly not the best at it, but I've spent enough time rubbing metal on rocks to have a decent idea of how the shape of a bevel impacts a knives performance.

The Rule

Any knife which carries convexity into the edge and easily passes the nail-flex test is thin enough to be an elite performer for kitchen tasks.

In Greater Detail...

I think this rule can apply equally to knives with all manner of geometries - wide bevel, short bevel, full convex, s-grind, forged geometry, etc. The obvious exception is wide bevel knives that are found on a hollow wheel resulting in concave bevels (eg - Birgersson, most of the Sakai wide bevel stuff, etc.) which I'm conveniently ignoring here since I generally can't be faffed with those given my stone polishing kink. So for this discussion let's just pretend those don't exist...

While the above (henceforth referred to just as the "Rule") is a seemingly simple statement, there is a lot that has to happen for these conditions to be met. First, to easily nail flex a knife needs to be extremely thin just behind the edge. Furthermore, for the edge not to immediately crumble with that type of geometry the steel has to be good. So at a minimum a knife that can hold this type of geometry has good steel and is thin enough to initiate cuts in a very pleasant "ghost-like" manner. To carry some amount of convexity into an edge like this comes with its own implications for the grind overall. If the grind from 5-15mm up from the edge is too thick, even a fully flat bevel without convexity can struggle to hit nail flex. To solve this issue you'd need to raise the shoulder, or in the case of a convex or compound grind the apex of convexity, enough to allow a geometry above the edge thin enough to convex into nail-flex behind the edge.* So to satisfy the Rule, a knife not only needs to be thin behind the edge but above the edge as well. This is indicative of a knife that not only starts the cut well, but will continue to plow through even dense product as well.

You may be asking why I'm bothered at all with convexity behind the edge, especially since it'll be rather subtle with geometry this thin. A couple reasons... First, I find that a subtle amount of convexity behind the edge allows me to run a near zero bevel without worrying about chipping or rolling the edge. I have had significantly more chipping issues with fully flat short bevels than subtly convex ones even with both at nail-flex thinness. And while this may be placebo, I do also feel like I have less issues with stiction during cutting and improved release with that subtle convexity as well. That convexity also allows for more subtle tweaking of behind and above the edge thickness across the grind without worrying about creating a foil edge. A small scale version of how hamaguri sharpening allows control over the edge and shinogi on a single bevel independently.

In summary, knives that conform to the Rule are thin enough to perform very well for all kitchen tasks. It's a simple thing to check without even cutting product that I find can tell me a lot about whether or not I will find the knife suitable. If either part of the Rule isn't met, I know I have work to do.

* allow me to define "behind the edge" as the area from the apex up 5mm and "above the edge" as the area from 5mm to 15mm for this discussion

Examples

There are several knives I've used that passed this test easily: Yanick, Milan (two thinner low-bevel examples), Birgersson (I stone finished that one), Kippington work-pony and laser-pony. Then there are knives that passed this test for me after a little work: Mazaki, Shigefusa, Kato, Takeda (iron clad nakiri), Z-Kramer (an early model that was ground rather thin from the factory). Some knives that pass the test so long as I diligently keep up with maintenance thinning: Halcyon Forge, Catchside, Raquin KT - these are all "workhorse" knives but follow the Rule, just. And then there have been a lot, including some from prominent very well regarded makers, that would have required serious alteration to satisfy the Rule....

Closing Thoughts

Thank you for reading my pedantic ramblings! This has been a very long way of elucidating my own rule of thumb for quickly judging a kitchen knife based on the grind behind and above the edge - using reflections and light and my thumbnail I bet this test takes me 10 seconds. I have yet to meet a knife that satisfied the Rule and wasn't fantastic fun to use. Likewise, I have yet to come across a blade that didn't satisfy one of the two conditions that I didn't think could be improved by some amount of alteration. Hopefully you notice from my examples that a wide range of styles can fulfill the Rule - from Kitchen Tractors to lasers. I'm not anti-workhorse, but I do find myself increasingly seeing knives that I doubt could pass my performance exam as makers pursue the current KKF zeitgeist of thicker and thicker knives.

Excellent write-up! The thinning+convex to a near-zero grind makes such a difference. It’s obviously more trouble/effort than just sharpening the edge and probably not practical to do with every sharpening, but even with a slightly more conservative microbevel, it gets you that extra level of sharpness and the “fresh off the stone”, ghosting through food feeling lasts longer.

After you establish an approximate “thinning bevel” on your knife to follow, you can actually do a reset pretty quickly. “Small scale hamaguri” is a great way to put it.

And the cutting 25# of a bunch of different product thing is so true too. Speed becomes much more essential and you really get to know the limits of a knife. Sometimes your hand gets used to moving at a certain speed/in a certain style with knife you’ve fine tuned and then when you go to another knife, your hand still moves in the same way, but the knife can’t keep up and you have to adjust and compensate.

My first knife to pass the test was a stainless clad Watanabe with a killer zero grind OOTB. I spent a lot of time wondering why none of my other knives could cut quite like that until way later when I learned to thin and sharpen enough to recreate that type of grind. Yoshikane and the thinner Wakuis also pass the test IME.

I remember I got to do a wide bevel sharpening in the Raquin nakiri from the passaround and at a near zero edge, it performed great but you could tell how performance would decrease without frequent wide bevel maintenance due to the type of geometry. Luckily, those bevels were super easy and fast to work on.

 

[dragjp]

Can you tell me more about your polishing experience with Birgersson? I want to polish the bevels on mine, but I'm a little scared of it. It's outrageously thin BTE and appears to be slightly hollow in the grind. It might be a bit uneven in places, or it might just be the vertical grind marks playing tricks with the light. My idea is to start up at the shinogi and work gingerly down to the edge to avoid getting too thin and unstable.

I recently did that with my 250 gyuto as well, the bevels are hollow and got low spots here and there. Took me 10+(maybe more) hours to sort both sides out, frustration and sore arms/fingers along the way but the steel is really rewarding afterwards!
So... join us?

 

[cotedupy]

I’ve only got half way through the OP so far, but this is easily one of the best sentences I’ve read on KKF. It’s probably actually in my Top 10 favourite sentences ever...

While the above (henceforth referred to just as the "Rule")

 

[ScharferMax]

I'll also add this is totally unnecessary. Birgersson's blades flat out perform IME. Incredible steel and phenomenal through food. There is some absolutely bananas banding hiding in that mild steel cladding though
View attachment 248439
That said, you can also see along the shinogi where I didn't quite get a perfect crisp transition due to angle inconsistency when thinning at the shinogi while trying to not use the edge as an angle guide like you could with the thicker low-bevel grind. This will come out over time but is indicative of the fact that keeping the bevels even while thinning at the shinogi without hitting the edge being no small feat.

I can only confirm what @ethompson has said! I have the privilege of being an owner of the a B. Birgersson gyuto too and must say, it's a clever mix between the qualities of Japanese cutlery and a more "western", innovative approach regarding the forging, grind choice, which particularly efficient in this case. There is no "choil shot" on Birgersson's product page, which is kinda surprising considering that this information "sells". In fact, while the length and height of the blade suggest a workhorse profile, the thinness of the blade is closer of that of a laser. And when I finally had the opportunity to get one, I was even more (pleasantly) surprised by its nimbleness and the characteristics of the grind, which is slightly concave/scandi-ish. More importantly, I was struck by its performance! It slays effortlessly through everything, and has a much better food release than most of blades I own. To sum up: it's a great tool that makes you want to cook even more!

 

[flow_the_pro]

I’m relatively new in the rabbit hole of nice knives and have not completely figured out why some knives are so amazing and others aren’t, so i find this post quite helpful! I’m just a hobby cook, so maybe things differ for a professional.
I recently got my first Toyama, 240 SS from JNS, and i am just amazed by it. Yet i would not call it nail flexing, so is there more to it? Or why do the Toyamas perform so well?

 

[deltaplex]

I’m relatively new in the rabbit hole of nice knives and have not completely figured out why some knives are so amazing and others aren’t, so i find this post quite helpful! I’m just a hobby cook, so maybe things differ for a professional.
I recently got my first Toyama, 240 SS from JNS, and i am just amazed by it. Yet i would not call it nail flexing, so is there more to it? Or why do the Toyamas perform so well?

Because Shuji gets all the aspects "right".

 

[ScharferMax]

I’m relatively new in the rabbit hole of nice knives and have not completely figured out why some knives are so amazing and others aren’t, so i find this post quite helpful! I’m just a hobby cook, so maybe things differ for a professional.
I recently got my first Toyama, 240 SS from JNS, and i am just amazed by it. Yet i would not call it nail flexing, so is there more to it? Or why do the Toyamas perform so well?

Hallo Flow! I'm a bit like you, and a single answer is almost impossible, since it depends on so many factors: how much/often do you cook? What type of cuisine? How much time do you have? Pro or amateur? Cutting techniques etc. I don't really get the "nail flexing" thing, and definitely don't think the thinness of a grind is enough to explain a knifes' performance. In fact, as you pointed out, the Toyama, despite not being super thin - performs outstandingly, which can be surprising when one handles it for the first time, since it may feel a bit bulky, heavy... I was used to thinner sakaï blades and they're great, super sharp and beautifully polished, but sometimes, you just want raw performance and this is where the Toyama shines. It's maybe not as thin, as light or pretty as others, but it feels soooo solid and has IMHO a perfect profile for chopping or rocking techniques...It's not a laser, nor a workhorse, but the handle/blade balance is spot on for many tasks, which is probably one of the reasons for its efficiency. It just works.