What is sharp?

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abarbosa

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Lately I have been questioning seriously my sharpening skills. I started with a Kurosaki R2 and I learned how to get it sharp on my naniwa professional 3000. The knife not only cut well fresh out of the stones but stayed tomato skin cutting sharp for a week of home cooking at least.

I was then introduced to carbon steel knifes (O2) and 14c28n (stainless but fine grained). I quickly became amazed how easy was to sharpen and I never noticed edge retention problems. I got as well a kurosaki Aogami super petty that is almost as easy to sharpen. It’s a petty so edge retention is hard to compare since the contact with the board is minimal.

All this time my kurosaki r2 gyuto kept being my most used knife so I was super happy with the sharpness and edge retention of my knifes. I also bought a asahi cutting board that at first I didn’t think it made a difference but nowaday I think it does, even though a small one.

I then discovered that using a loaded strop was good to avoid going back to the stone and have a sharp knife for my prep. After a while I started noticing that the edge I got from the strop, at least on the O2 knifes, didn’t last long, like cutting one or two tomatos and was gone.

Recently I bought a Kisuke Manaka Aogami (Warikomi) and a custom spicy white knife 65 HRC.

Everything was great with the Nakiri until a certain day that I had just sharpened it and was making some food with some tomatoes. The knife started to not be able to cut the tomatoes. I tried the spicy white and something similar happened. I was in shock. I stopped using loaded strops, actually I almost don’t even use strop for deburring, I now try to do everything in the stones and just do 1/2 passes in the strop in case I am unhappy with the deburring.

Things got better, but I started to test more and more my carbons and analyse if I they are indeed sharp out of the stone. I check if they bite on my nails, if they can cut paper towel (even though not so cleanly because of the bite) and if they cut newspaper well.

My dilemma is that even though the carbon knifes perform well on these tests and cut food amazingly well, they loose sharpness sometimes too quick, not reaching the end of the prep basically.

So what is sharpness? If I can’t get to the end of the prep I would argue that my knife isn’t even sharp enough to begin with. I am hoping that you will point out that is my technique, sharpening angle or that is my food choices that include a lot of tomatoes and that ruins the edge.

I supect its certain acidic foods that are to blame, and if that is the case that makes me loose a big chunk of my love for carbon knives at least as all purpose knifes, since if they touch acidic food like tomatoes, the edge dulls enough to stop being able to cut tomatoes, I can’t call the knife all purpose any longer, at least not in my kitchen. I tested yesterday my kurosaki R2 and it cut in small pieces 2 kilos of tomatoes without any issue.

So please tell me what’s take on this because right now I am thinking that carbon steel is great but just for specialized knifes, but having an all-purpose knife in carbon it’s not a great idea because carbon steel is not an all-purpose steel.

When I say carbons I mean not stainless and in particular Aogami 2, spicy white and O2. Aogami super I can’t really fairly compare, but I suspect that it doesn’t suffer from these problems and could be suitable for my kitchen as all purpose.

I’m sorry for the long post, I could have perhaps just written: is it true that acidic foods damage the edge really fast of non stainless knives to a point that if one can’t even finish prep? That’s seems to be my conclusion at least, but perhaps you’ll tell me that R2 has better edge retention than any of the other knives I have, which may be true and disappointing as well because I like to sharpen, but if a knife doesn’t hold the edge through for home cook prep I say it’s not good enough.
 
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I’ve had knives in a wide variety of simple carbons (I’d lump AS into that category for what it’s worth) and all of them can hold an edge long enough to be useful through a full day of commercial prep in my experience. Sure, if I’m slicing a whole case of limes or dicing a case of tomatoes I can notice the edge degradation by the end, but it’s still sharp enough to get my work done without frustration. When I really do a good job on the stones I feel like I can get an edge that lasts for a month with only minor weekly touch ups (few swipes on a fishing stone). So in a home setting I imagine that edge would last for months… On the other hand, that just off the stone feeling tends to disappear real quick leaving behind something still very sharp, but not the super scary stuff you get those first few cuts.
 

blokey

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I had the same problem before, you can actually see my post in the similar threads. Personally I find it both a deburring issue and too fine a edge did not do well with waxy stuff, now I take my carbon knives to 3k Ouka max and deburring use the coarse side of the kitchen sponge, it worked well so far.

Edit: Said post Edge dull too fast?
 

cotedupy

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Haha... big question!

A very short answer would be: in my experience very sharp edges that don't hold are usually to do with the apex being too fragile, and steel that is slightly fatigued.

In a way this is kinda related to deburring properly, though it can be a little more nuanced - a knife can be fully deburred but still with an edge that is too fragile for heavy board work. And the definitions of 'burrs' and 'wire edges' change depending on how you're using something.

The first two of these potential solutions are fairly obvious, the last one less so; higher sharpening angle, microbevel, or a longer progression of stones spending less time on each and finishing quite fine.

The latter is an interesting solution to the problem because in theory if you finish an edge at a higher grit you might think it's finer, thinner, and more fragile. But actually what happens also is a gradual process of refinement and removal of fatigued steel that give the resulting edge more longevity. It's quite easy to make a knife absurdly sharp on a low grit stone. It's more difficult to get a sharp and durable edge off low grits. Longer progressions will help with this.

Tbh - a Naniwa Pro 3k should be high enough, but bear this thinking in mind I'd say.
 
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captaincaed

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If your edge doesn’t last through home prep, it’s not deburred properly, unless you’re prepping for multiple hours. I had one knife that would not hold an edge for ****, but it was one of ~60 I’ve ever used. If it’s happening on three knives … it’s you.

That’s a GOOD thing. You can always tune up your technique.

Q: What is sharp?
A: Two things, that’s it.

  1. It’s a refined apex with a small radius.
    1. It’s when you make two bevels meet an an apex. This apex is never perfectly acute, it always has a small radius (every edge is a little round if you magnify enough).
  2. It’s the qualityof the teeth at the apex.
    1. Different stones leave different types of teeth at the edge. Coarse stones generally leave bigger, longer-lasting teeth. We all chase fine stones that leave lots of teeth. The JKI 4k is the gold-standard synthetic.
    2. An 8/10k edge is refined, and lacks teeth, but isn’t quite refined enough to “fall through” vegetables (that’s more like 12k+). Useful teeth basically end at 3-4k, maybe 6k if you like that level of refinement (I don’t)
Q: How do you get there?
A: Make a decision.

  1. Do you want a refined edge for fine proteins, or a toothy edge for veggie prep?
Q: How do you sharpen?
A: Completely deburr.

  • You can deburr completely on a 400 grit stone. If you can do this cleanly, then you’re on a good path to make the most of your high-grit stones.
  • The burr is weak, and any remaining burr will tank your edge.
  • You may need to spend a bit longer weakening and refining the burr before you go to the last step, and try to remove it entirely.
  • I usually refine the burr until I can’t really feel anything any more, then keep going a little longer than I feel like I need to. My fingertips are far less sensitive than they need to be to tell when the burr is truly small enough to proceed to the deburring step.
Q: How do you deburr?
A: Try as hard as you can to find one after sharpening.


If you’re not being brutally honest with yourself, you’re going to leave some burr. Don’t just take a knife off a stone, cut some paper towel, and declare victory. A burr will cut paper towel no problem. Don’t even bother with printer paper, you can cut that with a pretty poor edge.

Draw the knife through the edge of your wood board, heel to tip. Does any black remain? That’s burr. You didn’t get rid of it all. Simulate some rock-chopping on the board. Now go cut that paper towel. A proper edge will do this cleanly. A deburred edge will survive board contact. If it snags, go do a couple more light edge leading strokes to clean the apex and give some new teeth. If a couple cycles of this leaves you with a clean edge, job done.

If not, you might have created a large burr during initial coarse sharpening, and left a fatigued, drawn-out edge at the apex. You might need to drop a grit, do some more edge leading strokes, and get a hard, clean apex. Then you can refine again on a finer stone.

Mostly, just be honest and try to screw up the edge (using normal cutting motions, no twisting or board bashing) before you do your test cuts. A properly deburred edge will survive this board contact, and then cut cleanly, heel to tip, through paper towel, without snagging or popping.

Also, ditch the strop for the moment. They mostly destroy the teeth you worked hard to create on the stone. Edge leading strokes at a slightly higher angle to the stone will deburr an edge, especially if you have a light touch, and go through a couple cycles of cutting into your board, then returning to the stone.

Also, R2 edges last a crazy long time judged against simple carbons, it’s not your imagination. It’s a pretty great steel, that balances sharpenability, stainless properties, and edge retention quite well. My last unicorn knife is R2.
 
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abarbosa

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I may sound like a broken record but this forum is really something special!! I just have to say it, because I always get extremely impressed and surprised with the quality and care that you all have on your replies.

This that you are telling me are great news. It means that I don't know how to sharpen yet, but I am ready to learn since I can now acknowledge that I don't know how to sharpen.

I'll try your suggestions.
Seems like sharpening is way harder than one may think, while being able to rub steel against a rock and successfully paper afterwards is surprisingly easy.
 

captaincaed

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You’re probably close; often the last 10% is the hardest, and the details aren’t explicitly talked about. Good luck, post progress!

I loved the post title, forgot to say. It’s an excellent, simple, but rarely asked question. There’s some nuance to find when you ask it.
 

milas555

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Only an edge refresh can be deceptive ...
As cotedupy mentioned: From time to time you have to get rid of the tired steel on the blade - add Chosera 800 to start with ...
 

cotedupy

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(If anyone wants a deep dive into what ‘sharp’ means, here’s something I wrote elsewhere. I don’t talk that much explicitly about edge retention, it’s more a look at how different kinds of things are sharp in different ways.)
 

PappaG

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Is it possible that you are reaching the stage where you need to thin your knives? Are they getting thick behind the edge?
 

abarbosa

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Is it possible that you are reaching the stage where you need to thin your knives? Are they getting thick behind the edge?

They are crazy thin still, almost as new, at least the ones that have been giving me problems. And my main source of complain is actually tomatoes, that's where I notice the lack of sharpness consistently, everywhere else is mostly fine. But that is definitely something that should be part of sharpness.

I think it should really be that I am not deburring properly mixed with not great technique, angles and all the stuff already mentioned. So I have been fooled by thinking I knew what is sharp.

What I would risk saying is that R2 is more forgiving to improper technique when sharpening. So the claim that simple carbons are easier to sharp should be complemented with the disclaimer that they are less forgiving to bad technique.
 

Benuser

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Microcorrosion does occur as well with some stainless. I have now no SG2/R2 at my disposal to check, but it would explain the eternal tomato problem.
 
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I have actually, not long ago was at that stage where I was losing sharpness quickly. I thought that I was doing everything right. But I've since changed my technique and am now getting stronger and sharper edges. Unless I bang the edge on something hard, only a strop seems to bring the razor back. I think time is key to learning. Knowing what you want to achieve and knowing how to accomplish it. For me 3 years and quite a few good knives damaged. But once you get there, it makes the whole journey worthwhile. Not saying I'm there yet 🙂
 

HansCaravan

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As others have mentioned, perhaps you are taking the carbon knives past the point of having some "toothiness" and they aren't getting the bite you expect. I've had to learn what each individual knife in my drawer can handle as far as a finishing stone - most stop at a 3K now, and I'm happy with how they perform. I also suspect that your R2 knife cuts well because of the carbide content; the edge retention would be better (all else being equal, of course).
 

ModRQC

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Tomatoes like teeth. Most people with typically poor knives at their disposal - unlike us - use serrated edge because nothing else they own will work.

Technique and deburring notwithstanding, the explanation is simple as to why you "think" R2 is forgiving and Carbons are not. It's abrasion resistance and carbide content. On your 3K, a steel like R2 starts to resist the abrasion while your Carbon knives still get cut. So with a very same amount of strokes - supposing all else correlates as well - your Carbon knives will have a finer edge than the R2. On the other hand, the carbides in SG2 makes very hard teeth prominent along the edge: these will resist longer and the knife hold on to a proper bite almost indefinitely.

We often say that Carbons are easier and more forgiving to learn on because they're so easily affected by abrasives that you get results fast - and less time on a stone reduces the error margin of bad technique and learning curve.

But the adverse effect is also true: you can kill a good edge really fast with some bad strokes on a stone.
 

abarbosa

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I have a small update. Taken your feedback into account I focused more in the deburring and I can say that to start with I think I get a better edge to begin with.

I have to say that this is how I started sharpening. For some reason having the strop at hand made me invest less time in deburring. I even had forgotten that some stropping on the stones ending with a lateral stroke like Jon teaches in one videos used to be able to bring an edge back to life. So I got too confident and sloppy in my technique. It’s part of the learning journey :)

I like and understand the explanation on why with bad technique I have been more successful with R2.
 

ModRQC

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I have a small update. Taken your feedback into account I focused more in the deburring and I can say that to start with I think I get a better edge to begin with.

I have to say that this is how I started sharpening. For some reason having the strop at hand made me invest less time in deburring. I even had forgotten that some stropping on the stones ending with a lateral stroke like Jon teaches in one videos used to be able to bring an edge back to life. So I got too confident and sloppy in my technique. It’s part of the learning journey :)

I like and understand the explanation on why with bad technique I have been more successful with R2.
Not necessarily bad technique. I mean, sharpening R2 implies more time on the stones, so a bigger margin of errors. But yeah if you're getting pretty good with maintaining angle, and not using much of pressure on the NP3K, you're likely just "stropping" that edge on the stones. If it was originally a nice edge OOTB, you've basically been maintaining it rather than sharpening it, and it seems you did good so keep to that.

The other thing is steel vs abrasive vs bite. We often see folks complain with their sharpening results and cutting tomatoes: half of the time it's a burr problem, half of the time is too fine an edge for the expectations they set to how the knife will cut tomatoes.

I don't want to name names but recently a nice member here posted a question very much like yours struggling with the same confusion as yours as to why his SG2 knife kept the working edge he liked for so long and his carbons seemed to lose it so fast. His main problem was with tomatoes also. His carbons or finer grained steel he probably refined too much for that kind of an edge. He also went on to try and improve deburring.

Good luck!

Edit: I myself STILL go on and try to improve deburring. It's not exactly something you get a final place with. Some steels they demand more work, some tricks work for you more than others, some stones you don't understand them as well as you should, and they don't all have the same effects neither. Pressure, type of strokes, etc... it's something you can always improve on or at least still find roundabouts that work better for you, for certain steels, technique, equipment.
 

captaincaed

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Tomatoes like teeth. Most people with typically poor knives at their disposal - unlike us - use serrated edge because nothing else they own will work.

Technique and deburring notwithstanding, the explanation is simple as to why you "think" R2 is forgiving and Carbons are not. It's abrasion resistance and carbide content. On your 3K, a steel like R2 starts to resist the abrasion while your Carbon knives still get cut. So with a very same amount of strokes - supposing all else correlates as well - your Carbon knives will have a finer edge than the R2. On the other hand, the carbides in SG2 makes very hard teeth prominent along the edge: these will resist longer and the knife hold on to a proper bite almost indefinitely.

We often say that Carbons are easier and more forgiving to learn on because they're so easily affected by abrasives that you get results fast - and less time on a stone reduces the error margin of bad technique and learning curve.

But the adverse effect is also true: you can kill a good edge really fast with some bad strokes on a stone.
Really nice post.
 

cotedupy

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I have a small update. Taken your feedback into account I focused more in the deburring and I can say that to start with I think I get a better edge to begin with.

I have to say that this is how I started sharpening. For some reason having the strop at hand made me invest less time in deburring. I even had forgotten that some stropping on the stones ending with a lateral stroke like Jon teaches in one videos used to be able to bring an edge back to life. So I got too confident and sloppy in my technique. It’s part of the learning journey :)

I like and understand the explanation on why with bad technique I have been more successful with R2.


Good to hear! And what CC said below is very valid I think...

I've been through cycles like this too. Some more recently than I'd like to admit. (Last week)


I reckon I'm a pretty competent sharpener too, but I certainly don't nail it every time. And for me it's always to do with deburring, especially if I'm sharpening a steel I don't know so well, or playing around with random stones. There are a lot of variables in how burrs or wire edges can feel off different stones, and sometimes it's only after using a knife that it becomes apparent. In fact only yesterday I noticed I rolled a burr on one of my own knives in use, after some whetstone experimentation.

(When sharpening for other people I tend to always use the same kinds of stones for this reason).
 

cotedupy

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Bravo!

I think we can all agree that the haiku is the single most effective medium when attempting to convey some of the subtler philosophical nuances pertaining to knife sharpening.
 
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