Advanced Knife Sharpening: Thinning Your Blade, and the 'Taste of Sharp'

Sharpening a knife is simple and easy: just grind the metal off from the edge at the same angle across the blade. But sharpening is also difficult — because there is so much more to it than that. If you just want to slice tomatoes easily, the job of sharpening is very quick. If you want to shave your beard with your razor, sharpening takes time and patience. If you want to sharpen your axe to split firewood, you are looking at a different kind of sharpness, same with scissors for silk, paper or hair. Different types of blades need different edges and depending on what you want to do with the tool, you need to sharpen them differently.

Even kitchen knives vary. The edge you would give your knife for all-purpose use v.s. the edge used specifically for fish filleting or deboning are going to be quite different, and they should be sharpened differently. There is a Japanese term for sharpness: “kire-aji” which translates to "sharp taste". It describes the nearly in-describable, how a knife feels and functions in regards to the job it is designed for.

If you sharpen your splitting axe until it can shave your arm, you will probably find the edge gets damaged too easily when you chop wood, and will begin to feel like it is not as sharp as it should be. In Japanese you would say it has bad kire-aji.

As you might have guessed, this blog is about nerd-level sharpening.

Our sharpening expert Naoto Fujimoto-san, testing natural stones from Masashi Fujiwara-san to bring back to Canada for Knifewear.

Dress for the job you want

Each knife is designed and shaped to do certain things. A chef’s knife has a long enough blade to dice, mince, slice and chop with a tall enough blade for knuckle clearance. A paring knife or petty knife is a small knife with a skinny profile, which allows you to peel, core, slice small stuff. Each knife is designed well and the blade shape have been made the same way for decades, if not centuries. 

Another example: Japanese specific shapes like the deba (fish filleting), usuba (vegetable slicing) and yanagiba (sashimi slicing) were born around the 1800s. Even with technological advancements, the shape of these knives has not gone through major changes or modifications. Many blacksmiths agree that these shapes are perfect for their use and any modification on the blade shape will ruin the functionality. Therefore, when you are sharpening them, you want to retain the same shape as much as possible.

Each sharpening will take some material off, causing the knife to become shorter and narrower. This is normal! The ultimate goal here is to maintain the same profile and same bevel height throughout multiple knife sharpenings.

Polishing the bevel of a knife on a natural stone to test the finish.

Blade geometry

Most knives have a similar profile when you look down their spine. They are usually thicker at the handle and taper down towards the tip. Japanese knives in particular sometimes have what we called “primary bevel”. This primary bevel is also known as kireha (切れ刃). It is important to thin the kireha section every time you sharpen your knife, because it is important for a good kire-aji. Imagine trying to cut a potato or yam. Have you ever felt like you are trying too hard, and the vegetable is splitting like wood? This is what happens when you do not thin the kireha part of your knife, so thinning the knife is vital.

When you sharpen the kireha at the same angle all the way along, the heel section will get thicker or the tip of the knife will be much narrower than the rest of the knife. This is caused by the blade taper that was described above. This is most prominent when you sharpen an Usuba or Nakiri since the blade profile is flat, but tapers towards the tip.

In order to avoid an uneven bevel or kireha, you want to sharpen the kireha at a different angle on different parts of the knife. You want to sharpen at the shallower angle towards the tip and a higher angle at the heel. This advanced method of sharpening will give improve the function of your knives: by having thinner kireha at the tip, you can use the tip to easily and smoothly score and pierce food. Having a thicker bevel at the heel will make that part stronger for rough chopping jobs.

This also effects how you use the knife. An usuba is usually used with a push cut, because the thinner bevel at the tip allows the knife to glide into food nicely. As you press the knife toward the heel, the thicker bevel at the heel allows the knife to separate the food better. A deba, which is a fish filleting knife, needs that thick, strong heel to break bones, crab legs etc. You can sharpen your differently for the type of fish you are cutting, and sharpen certain sections of the knife for certain jobs. If you are a chef who is preparing aged fish, you should make your kireha as mirror polished as you can. This will allow the knife to glide very smoothly. If you are like me and prefer the freshest fish, then finish your knife with a kasumi, or "cloudy, finish. 

So, you can see there are many ways to sharpen your knives. Sharpening knives can be as simple or complex as you want to make it. Whatever you want to do, we’re here to answer your questions and help you get the supplies you need. Happy sharpening!