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Thinning Knives

'Nothing tastes as good as skinny feels.'

- Kate Moss, 2009


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This post is going to look at hows and whys of thinness in knives. As well as touching on the reasons that, above and over anything else, you probably want to get a coarse-ish whetstone for kitchen knives, and the drawbacks of guided sharpening systems.

Even outside professional work I have to deal with this kind of thing on a weekly, sometimes daily basis:

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That's the knife my wife uses most, and it's got a load of small chips down the edge and has been tipped, again. I probably only sharpened this two weeks ago.

It also has another problem though... the kinda dark bits down near the edge there are the hardened core steel of the knife, which does the cutting, that's been slightly tarnished or oxidised making it dark. Which isn't itself an issue, but it does highlight that up near the tip there's a section where we can't see any of that core steel; the soft cladding goes all the way to the edge, and that's gonna screw with how it cuts. As I said - I touch up and remove chips from this knife on an almost weekly basis. And when you do that repeatedly over time; you removal metal and height from the knife, and that starts buggering up the geometry.

Below is a somewhat simplified drawing of a knife blade:

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Note that what is called the 'edge' of a knife is what would be called a 'bevel' on a SR. The main 'bevel' of a knife doesn't really exist on a razor, because it's hollow ground. And this bevel or main face of a knife blade is one of the reasons you can't sharpen most knives in the way you do a razor.

'A' represents our kitchen knife when new, 'B' is after it's been sharpened over time and subsequently lost some height as material gradually gets worn away. The edge angles on A and B are the same, because you've been sharpening at the same angle all the time, and if you used these knives to cut paper they'd probably do it just as well. But because a knife tapers down from a thick spine to a thin cutting edge, Knife B is thicker at 'shoulder' where the bevel meets the edge, marked by the green lines I've drawn, and this is going to change the way it cuts some other things.

I've written before explaining some of the more important factors that affect sharpness in kitchen knives vs razors, but that was slightly different from this issue of how thick the knife is just behind the edge. Whereas before we looked at factors affecting cutting into something tough like a tomato skin, now we'll look at how a knife goes through something harder. The classic test for this is carrots - if your Knife B is too thick it won't slice cleanly through a hard carrot, which is called 'wedging'.

This problem will eventually affect any knife that is constantly sharpened at the same angle. And my wife's knife above, which is san-mai construction, we can also see clearly a second problem arising from the same thing - that the core steel is no longer exposed at some parts of the edge.

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That was the theory, here's the practice, let's sort it...

We're going to be abrading a lot of metal here, all the way along the bevel to keep the profile of the knife, and I'm also going to have to repair the tipping. So we'll need coarse stones; in this instance King 300 on top, and then Suehiro Debado 600 below (this pic was actually taken at the end, as you can probably see).

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What we're going to do here is different from normal sharpening - we specifically don't want to be sharpening the edge of the knife. Instead we want to be sharpening at a more acute angle so that we thin the shoulders of the knife, behind the edge. After a bit of high pressure work on the 300 the tip of our knife is now looking like this. I've got rid of the tipping and exposed the core steel, which is now shiny, because it doesn't have a layer of oxidation on it like previously:

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During that, if you want, you can alter the profile of the knife. In the pic above it may look like it has a relative flat section to the edge, but it doesn't. My wife uses exclusively 'rock-chopping' motions so I've put a slight curve in all the way down:

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On to the Debado 600 to tidy it up. Same kind of thing you're doing here, but maybe with a little less pressure. Again - you're not really sharpening the very edge too much, you're trying to thin the whole main bevel of the knife so that the thickness and geometry of it mimics how it was originally.

This is how out knife looks by the end, it's nice and thin again behind the very edge, with some convexity still in the main bevel:

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I've also exposed a nice bit of shiny core steel all the way down, and the profile / general shape is quite good. Then we sharpen an edge onto it with the same 600 grit Suehiro Debado, et voila...

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A lot of this would be very difficult to do on a guided sharpening system, and some aspects completely impossible - you couldn't for example blend a very thin edge into a convexed bevel properly. Though I'm not here to criticise jig systems - they work very well for what they do.

What I am here to say is that everybody who sharpens knives should have a good coarse stone - it's more important than anything else. It doesn't need to be as low as 300, but something like the Debado 600, or in particular the awesome Shapton Glass 500, is all you'd ever need for kitchen knives.


 
Very nice tutorial, makes perfect sense.

I like thin knives for general food preparation work, love my Global's for that. The compromise is that you have to be gentle with them, and if there is any chopping of bones, even chicken, I reach for something sturdier.

I often get asked what is a good kitchen knife and for most the answer must be as cheap as possible. Why? Because if you can't keep a knife properly maintained and honed, an expensive knife is not going to be any better than a cheap one.
 
Wonderful explanations as always!

Except you're reminding me I need to thin my Hiromoto 240.

Ah cheers! Obviously this kind of thing can be a little difficult to explain in simple terms, without a massive overload of technical detail. Glad to hear I've got the balance right :).

You'll have to snap pic of your collection / roll sometime. Sounds like you've got some nice stuff in there...
 
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Ah cheers! Obviously this kind of thing can be a little difficult to explain in simple terms, with a massive overload of technical detail. Glad to hear I've got the balance right :).

You'll have to snap pic of your collection / roll sometime. Sounds like you've got some nice stuff in there...
Yay! I don't have to take more pictures. 😁
My miorashi deba and super cheap yanagiba I don't have current photos of. I don't need to use the deba that often. Work went to getting aides of fish over whole fish. I'm sad I don't get to use the knife but not upset over less work. It still get wicked sharp and acts as another sharp thing to slice fish with.
 
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Very nice tutorial, makes perfect sense.

I like thin knives for general food preparation work, love my Global's for that. The compromise is that you have to be gentle with them, and if there is any chopping of bones, even chicken, I reach for something sturdier.

I often get asked what is a good kitchen knife and for most the answer must be as cheap as possible. Why? Because if you can't keep a knife properly maintained and honed, an expensive knife is not going to be any better than a cheap one.

This is a good point too I think...

There are plenty of completely serviceable knives available for not much money, that will still very happily make your dinner, without ever really wanting or needing to do this kind of thing. And if you do need to, but don't have the inclination, then a cheap knife is a lot less to replace than a fancy one! I see a lot of Victorinox knives used in pro kitchens at all levels (especially in the UK), and that tends to be my recommendation to people who might not be after anything particularly posh.

Interested to hear what you and @ChefJohnBoy-ardee like as reasonable quality, not too expensive 'beaters'...?
 
This is a good point too I think...

There are plenty of completely serviceable knives available for not much money, that will still very happily make your dinner, without ever really wanting or needing to do this kind of thing. And if you do need to, but don't have the inclination, then a cheap knife is a lot less to replace than a fancy one! I see a lot of Victorinox knives used in pro kitchens at all levels (especially in the UK), and that tends to be my recommendation to people who might not be after anything particularly posh.

Interested to hear what you and @ChefJohnBoy-ardee like as reasonable quality, not too expensive 'beaters'...?
My brother studied to be a chef, and I remember him using a set of Victorinox while he was doing so.

In my kitchen I keep a few cheap stainless knives in the kitchen for the wife, and to be honest, I tend to reach for those more often than my carbon ones, because I can just leave them by the sink to be washed later, where as my carbon ones need to be properly washed and dried as soon as I am done with them. If I am just cutting up some fruit for the kid, the cheap knife (which I keep honed) works fine. It doesn't hold an edge as well as a good knife, but that is just an excuse to practice my honing more often.
 
There are plenty of completely serviceable knives available for not much money,
When asked, I would always recommend a stainless knife for general use.

I used to have a few Mora chef/butcher knives before moving to the UK. With their cast plastic handles, these knives are virtually indestructible, and they can have and keep a good edge for a reasonable time. Once honed, you can work for months with them, just touching up on a butcher's steel. Like you explained, this leads to a rounded edge, and every once in a while requires some thinning to bring it up to standard again.

Vitorinox - always a popular choice with an excellent reputation.

Just after arriving in the UK about 14 years ago, I spotted a bargain at Costco. It was a full set of Global knives (6), for less than £200. I bought them and after using them since, have truly come to appreciate them. Very thin blades, easy to hone and maintain, the only down side is that I would not risk any heavy chopping with them.

Being a person that appreciates knives, I have too many. I love the look and reputation of the layered Japanese blades, but my reality check has so far prevented me from spending a heap of cash on a knife that I will be reluctant to use on a daily basis.

The only high carbon knives I have are a few folders. That's different, because only I use them and there is no chance they will be left in a wet sink, or banged about in a cutlery drawer!
 
I get new line cooks saying they want to buy a knife and I point them to Victornox Fibrix line. Under $50 and will last a long time. I'll even teach them to sharpen and hone it.

I've given out a lot of King 600/1000 combo stones for graduation or promotion presents.

If they can take care of a house knife or their own starter knife and want to go down the Japanese knife route I'll loan them one once I can trust them. Once they go there and they still want a JKnife I'll help them shop. Even sold a few that I didn't love enough but they did.

I still have my Mercer Genesis knives from culinary school. They're excrement but work. The glue under the rubber epoxy handles eventually leaks out and makes them super sticky. I've had to stone off so much of the fat finger guard and thin the blade a lot after fours years of line cook work before getting something different.

I started with a Tojiro ITK. Just went from there.
 
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