Checking Sharpness with Your Fingernail

Many chefs have likely been taught by their culinary school instructors or workplace mentors to check the sharpness of a knife by running it across their fingernail. The idea is that if the blade catches and stops on the nail, it is sharp, while if it slides smoothly, the edge isn’t properly honed.

This advice is half-true and half-false, which is what I want to explain in this article.

Why Does a Knife Catch on a Fingernail?

When a knife is sharpened with a coarse stone, the edge becomes serrated, similar to the teeth of a saw. These serrations are more pronounced and irregular with coarser grit stones, so the knife edge catches on the fibers of your fingernail, stopping the blade. The same principle applies to food; the serrations catch on the fibers of the food, allowing the knife to cut when moved back and forth.

Additionally, if the knife’s edge angle is large (obtuse), it won’t bite into the nail as easily, making it a useful indicator of the thinness of the blade edge. Therefore, the concept of “catching on a fingernail = sharp” isn’t entirely wrong and can be remembered as one method to check the condition of the blade edge.

Two Reasons Why a Knife Might Not Catch on a Fingernail

When working in a shop, you might see customers testing knives on their fingernails or checking knives we’ve resharpened using this method, often misunderstanding that not catching on a nail means the knife is dull. This belief is half-true. If the edge doesn’t catch at all, meaning it hasn’t been properly sharpened, the edge is still blunt, and of course, it will slide smoothly off the nail.

However, what if the knife has been finished with a very fine stone?

If a coarse stone creates a serrated edge like a saw, a fine finishing stone produces a much straighter edge, more like a cutter. Imagine trying to cut wood with a cutter; the blade would slide without biting into the material (although on a microscopic level, it is still cutting). But it cuts paper and cardboard very well.

This extreme example applies to knives as well. A finely honed edge might not catch on a fingernail, not because it’s dull, but because the edge is so fine that it doesn’t catch on the nail fibers. For beginners, it can be hard to judge whether a knife isn’t catching because it’s dull or because it’s finely honed. In such cases, trying a newspaper cut test is recommended.

The Newspaper Cut Test

By cutting newspaper, you can gain various insights from the feel and sound of the cut. With practice, you can gauge the condition of the edge. For example, if the blade stops while cutting newspaper, it indicates a nick or burr on the edge. A rough sound or vibration suggests a coarse edge, while a smooth, silent cut indicates a finely honed edge. These subtle differences become easier to recognize over time.

Abandon the Idea That “Not Catching on a Fingernail Means It’s Dull”

When a knife is sharpened to a serrated edge, it grips food well, making it feel very sharp to many people. On the other hand, for tasks involving fatty fish or meat, a too fine edge can become clogged with fat, quickly dulling the knife. Therefore, fishmongers and butchers often use a slightly coarser edge for efficiency. This approach enhances work efficiency but may damage food cells, affecting the taste of raw vegetables or sashimi.

Conversely, a knife finished with a fine stone has a straighter, more refined edge that minimizes damage to food, enhancing the taste. Personally, I believe cutting with minimal damage to the food is best, but work environments vary. For prep work, using a knife with an efficient edge and for final cuts, a knife with a fine edge can be a good strategy.

Knowing the characteristics of both types of edges is important to finding the best combination for your needs. Additionally, the material of the knife (stainless steel, carbon steel, etc.) also affects the taste of the food. Combining edge shapes and materials creates endless possibilities. The differences in sharpness and feel due to different steels will be covered in another article.

Critical Thinking in Knife Care

It’s essential to accept and learn from what teachers and mentors say, but their advice isn’t always correct. Questioning commonly accepted “truths” and constantly asking “why” helps develop critical thinking. Avoid becoming someone who mindlessly follows instructions; instead, research and think about what interests you, gradually increasing your understanding and skills.

By doing so, you can refine your ability to evaluate knife sharpness accurately and ensure you’re always working with the best tools for the job.