What’s the Sharpest Knife?
Shoshui Takeda-san firmly believes “the sharpest thing in the world is nothingness.” He’s on a mission to forge the thinnest knives he possibly can, and if you’ve ever held a Takeda, you’ll see he’s not far off from his goal of creating paper-thin knives. There’s a lot more to it than that, though. The ultimate, sharpest knife you could ever find would be impossibly hard and thin: likely made of diamond that’s no more than a micron thick. You might be thinking that would be wildly impractical for use in the kitchen, and you’d be right!
Heads up, we’re about to get REAL nerdy. If you’re just looking for a sharp kitchen knife, we can help you find one here.
SO WHAT’S SHARP?
Sharpness is relative, and there are a couple of factors that make up “sharp”: the material the blade is made from, the geometry of the knife, and the polish of the primary and secondary bevel. This little triangle is going to fluctuate a lot depending on what you’re doing, and “sharp” for cutting wood is far different from “sharp” for cutting fish. Obsidian, for example, made excellent arrowheads because of its hardness. it could be chipped away into an extremely fine edge, but that hardness came with brittleness (like glass), which means you’ve got a more delicate tool for limited use.
An axe can have a mirror-polished edge, and that still won’t mean it’ll slice fish for you like a yanagiba. Today, though, we’re narrowing our scope to kitchen knives only. We’re living in reality here, so we’re making some assumptions before we craft our perfect knife:
- We want a reasonable, workable material, so we’re not making knives out of diamonds today.
- We’re using our knife to cut regular food responsibly: meat and veg only. No swords going into airplane propellers here (looking at you, Forged in Fire).
WHAT’S IT MADE OF?
Let’s start with material. If you’ve poked around our site for long enough, you’ve probably heard this already: Japanese steel is much harder than conventional knife steel, which is why it takes and holds an edge so well. It’s still tough, absolutely, but it’s hard enough that you want to make sure you never cut through anything you wouldn’t bite with your own teeth, because that hardness means your knife can chip. Or, more simply, if it seems like it might be a bad idea, it probably is.
To mitigate some of the brittleness of hard steel, most Japanese knives are made in a sanmai style, which means that there are three layers: a harder steel in the middle, clad in two layers of less-hard steel, one on either side, to protect the super-hardcore. Think of it like a steel sandwich making up the blade of your knife.
Harder on its own doesn’t necessarily mean sharper, but it does mean your knife can stay sharp longer, and well-made steel is going to offer enough hardness without being a monumental pain to sharpen. So let’s aim for something like Aogami Super or Shirogami 1. Maybe even a HAP40 if we’re feeling zesty.
HOW’S IT SHAPED?
Next, what constitutes a sharp edge is the angle and smoothness of a primary bevel, the steel that tapers down behind the edge, and the quality of the edge itself. The theory goes that the thinner the apex of your edge, the more force gets concentrated into that point, which is why you don’t need to push at all with a sharp knife: the weight of the knife is enough! So we want a knife that has a thin, tall bevel sharpened at a narrow angle, but not so thin that the core steel is overexposed, which can make a knife too delicate.
DOES THE FINISH MATTER?
A tall bevel means our knife is very thin behind the edge, and now that we’ve spent all this time making this tall primary bevel, we want to make sure it’s polished smoothly so that it doesn’t drag through our food. It’s not the most important factor of a sharp knife, but it’s still part of the overall puzzle. A mirror polish will glide through food better than a non-polished primary bevel simply because there’s less for the food to grab onto.
FINALLY. THE EDGE.
Alright. We’ve got a sanmai knife with a very hard core steel, impeccable edge geometry, and a polished primary bevel. We have finally arrived at the cutting edge, AKA the microbevel. To me, the sharpest-feeling edge is something called differential grit sharpening. One side of the edge is sharpened at a rough grit like #400, and the other side to a fine grit (I like #4000). 
Why is this ideal? If you sharpen your knife on a coarse stone, like that #400, you’ve got a ‘toothy’ feeling that bites into food easily. Think slippery tomato skins, or all the muscle fibers in meat. The downside is that your rough edge is going to tear through super delicate foods like fish, which is where you’d want a much smoother edge in the #4000 range. The advantage of a differential grit edge is that you get the best of both worlds! It creates micro-serrations like microscopic serrations with even tinier serrations on them - much like shark’s teeth. A differential edge is all about the feel - the subtle toothiness bites into tough foods, the smoothness glides. These microscopic teeth also help the edge stay sharp longer as they are a little tougher than a super-fine edge!
IN CONCLUSION
Absolutes are fun. They’re reassuring and sexy in a weird way, and I would love to tell you definitively that there is exactly one legendary knife out there, tested against millions of others, that takes the proverbial cake. Maybe there’s a gal out there who sharpened her knife to 800,000 grit and stropped on unicorn leather loaded with .25 micron diamond paste. That would be the fun answer, but the honest truth is that the sharpest knife will most often depend on the job it’s used for. And for this question, assuming that we want the best knife for home use for someone who reasonably knows how to use a knife, the sharpest functional knife is going to be a HRC65:66 with a mirror polished high Shinogi (primary bevel) and a differentially sharpened edge, preferably with Naoto’s 4D sharpening technique for practicality. And if that means absolutely nothing to you, that’s totally fine… but in retrospect, that looks an awful lot like Maruyama-san’s knives. 
