Testing Blade Sharpness


I did not manage to photograph the newly made knives. Thus I will write this week a bit about testing knife sharpness.

For reasons unfathomable to me, a huge amount of people have a habit of testing blade sharpness by holding the knife in their dominant hand and running the ball of the thumb of the same hand perpendicularly across the cutting edge.

Beware that if you have done this in front of me, I have suppressed an urge to give you an educational ding behind the ear. And what I did not suppress was an urge to explain why and how what you just did is daft.

Running one's finger across the edge does convey information about its sharpness, no doubt. However, when doing it with the same hand in which you hold the knife, it is possible – and when done frequently, practically inevitable – that when doing it, you flex the thumb just a tiny bit too, thus performing a short move along the edge. And when the knife is freshly sharpened, you notice it in the best case only when later washing your hands with soap and it stings in the shallow cut you just got on your thumb. In the worst case, you will only notice something is wrong when you start leaving bloody fingerprints on everything. With properly sharpened knife it is possible to cut yourself without even noticing it.

If you want to test blade sharpness with your thumb, it is much safer to hold the knife in one hand and test it with the other. I still do not recommend it, but it is safer.

Even better is to hold the knife lightly in your dominant hand with the blade level, cutting edge down, putting the cutting edge on the thumbnail of the other hand at approximately 45° angle, and try to lift the blade with the thumb. A sharp blade will catch on the nail with its own weight sufficiently enough to lift the knife. A blunted edge will just slide.

Visual control is the safest method. When holding the blade level with cutting edge up in good light it is possible to see whether it is properly sharpened on the whole length or whether it has blunt spots. In the picture, you can see on the left side one of the freshly made and sharpened blades and on the right side one a really old one that I am using for all kinds of things. As you can see, with the sharp knife there is visible just a very thin dark line, where the bevels reflect the surroundings. With the blunted old knife there are shiny spots, where the blade is blunted.

Cutting freely hanging paper is one of the most known tests, but when it comes to that, not all papers are equal. Printer paper will test the knife, but not particularly well, because it is relatively stiff. Newspaper is better, but not many people buy it anymore. I found out that unused voting ballots serve excellently in this role, and with the frequency of various elections around here, I have not run out yet.

Properly and evenly sharpened knives will perform a clean cut with the whole blade length without hiccups. A very well-sharpened knife will be able to cut thin slivers of paper and/or change the direction of the cut and cut arcs and curves. Only a truly excellently sharpened knife will be able to cut into the edge of a freely held paper towel.

A knife this sharp will in all probability pass even the most difficult test – shaving forearm hair. I do not do this very often in real life though, except for demonstration purposes when teaching someone sharpening.

I will endeavor to write something each week, although I do not know right now, what I will write about next time. Maybe about photographing. But it is early autumn and I have an inordinate amount of work in the garden.