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A Primer on Knives & Sharpness

Originally posted on my Facebook page, Refined Edge PH, on June 25, 2020.

Yoshikane White #2 Bunka & Whittled Hair

Let’s talk about sharp knives.

Nothing quite stands out about a knife’s functionality more than its sharpness. It always tends to be the first thing that people assume is a good indicator of quality, and some even think that it’s the single most important thing you have to think about when you first try out a knife out of the box. Though I and the wider knife enthusiast community don’t agree with this particular sentiment, it certainly still is a very important concept to consider. I hope this might be a helpful piece to explain sharpness and its implications for your knives.

A Brief Explanation

A knife is a mechanical tool, and a sharp knife is the tool itself kept in a good and functional condition. When a knife is sharp, it’s a joy to use in the kitchen because it allows you to easily accomplish various cutting tasks. It’s fairly common knowledge that everyone should have at least one sharp knife in their kitchen, but how do you actually define sharp?

Going back to the knife being a tool, at the end of the day, it simply functions as a thin wedge that pushes food apart. The thinness of the wedge, especially at the first point of contact, determines whether or not the food will allow itself to be pushed apart, while the rest of the wedge helps to continue the pushing motion.

Sharpness has to do with this first point of contact between the knife and the food, and so it’s most concerned with the quality of the edge of the knife. When put under a microscope, a well-sharpened edge looks like rows of aligned serrations just like you would find on a saw. The more refined the edge, the sharper the knife and the smaller the serrations will be. In contrast, a very dull knife under the microscope will reveal that the edge has actually become rounded, and few traces of the original serrations are left.

Most knives in use will fall somewhere in between these two extremes, but a knife doesn’t have to fall on either end of the spectrum to be considered sharp or dull. In all actuality, this would all boil down to your personal opinion on its condition, and for most people, it would be largely influenced by whether or not the knife can accomplish cutting tasks at an acceptable level.

Testing a Knife's Sharpness

Hair Split Test

There are also many different sharpness tests you can conduct on your knives such as the hair whittling test pictured on this post. The most popular tests for kitchen knives include cutting paper, shaving hair, and thinly slicing produce horizontally. Hair whittling is an extreme test and is very difficult to pass consistently. Meanwhile, the easiest test to pass is slicing paper, and success indicates a very workable level of sharpness.

Toilet Paper Cut Test

However, if your knife is not able to slice paper, it does not automatically mean that the knife is unusable in the kitchen. The most practical test for sharpness is still simply using the knife to prep food for a meal and to qualitatively judge how the knife makes initial cuts on a range of different ingredients. Passing different tests simply means that the edge is at least sufficiently sharp for normal kitchen use.

The Implications of Sharpness

1. Less Force When Cutting

Because a sharp knife has a thin edge and many small serrations, the force from the cutting motion is applied to a smaller surface area (i.e. just the tips of the serrations). This increases the pressure exerted on the food, thereby requiring much less force to make a successful cut. This is why you need to push much harder to force a blunt knife to cut a carrot or any other hard vegetable and why you only need to use light force with a sharp knife to make exactly the same cut. Especially with large cutting tasks at hand, a sharp knife will lessen the user’s fatigue, greatly speed up the prep, and prevent slipping accidents caused by exerting too much force.

2. How Your Knife is Sharpened Matters

Though one might be tempted to always go for very high levels of refinement, the optimal edge quality actually depends on the intended use of a particular knife. Higher levels of refinement will cause the knife to crush and rupture fewer cells in the food being cut, though very high levels of refinement, such as with sashimi knives, can be lost quickly especially when making hard contact with a plate or chopping board. In the case of cutting an onion, a more refined edge actually reduces the release of the substances that make you cry. Meanwhile, a rough edge can withstand significantly more abuse and can be useful on a camping trip or on a beater knife that you reserve for tasks like splitting a pumpkin, though it can leave a fairly coarse surface texture where the cut was made.

Basically, different levels of refinement on the edge serve different purposes so that a knife can effectively remain in service for as long as possible. This makes knowing the intended use of the knife paramount before determining an optimal sharpening progression and procedure. A knife not sharpened optimally likely won’t be bad, but you’ll just be missing out on unlocking the knife’s full potential.

Here are a few useful examples to demonstrate this point:

a. A Bone Cleaver benefits from a rough finish to give it more durability alongside the often convex geometry and wide sharpening angle (more on this in the future). The texture of the cut is much less important than the edge’s durability when used for such an abusive task.

b. A Yanagiba or Sashimi Knife requires extremely high levels of refinement to achieve an optimal smooth surface texture and to preserve the umami of the fish. The single-pull cut stroke is often performed in a way where the blade will only barely touch the cutting board at the end of the motion, and ideally, the cutting board is made of a softer wood or rubber, which are both gentle on the blade.

c. A Gyuto or Chef’s Knife might benefit from a hybrid style edge because of its multi-purpose nature. It will almost always be used on a chopping board, so the edge cannot be too refined. However, it may also be required to cleanly slice through proteins from time to time, so some refinement should be present in this situation.

Where do we go from here?

Test your knives, cook with them, and judge whether or not they’re still sharp enough for you. Again, making this conclusion is largely subjective, but in my opinion, if you find yourself needing to use lots of muscle or even your body weight to cut through a carrot, onion, or potato, then please seriously consider sharpening your knives with a whetstone or getting them professionally sharpened as soon as you can.

If you opt to go the professional sharpening route, make sure find out what sharpening process is used. Ideally, they use whetstones or a water cooled system for their work. If a belt grinder without cooling procedures or a bench grinder is used, these will cause irreversible damage to the heat treatment of a good knife. And well, if you do happen to live in Metro Manila, I’ve started professionally sharpening and repairing knives myself, so feel free to contact me through Messenger for any questions about that!

Alternatively, if you feel like the time is right, you may want to upgrade to a new and better knife. This can be talked about in extreme lengths, but that’ll be reserved for future posts. If you want to go down this road, I’m more than happy to help since I am a knife enthusiast, or you could check out the r/chefknives subreddit page and source lots of information from there. (You might even be able to find me there as well as a regular contributor. )

Thanks so much for spending your time reading this piece. I hope you’ve found it helpful!

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