Don’t Test Blade Sharpness With the Ball of Your Thumb!

Recently I was on a short trip with my friends from the university. I have shown you my traveling sharpening kit and said a bit about its evolution. I did not mention any details about what has happened on the trip.

In addition to a gratis sharpening of one blade per person, I have also offered a gratis lesson in sharpening and knife maintenance to anyone intersted. I did not expect that several parents will herd their children in (mostly, but not exclusively, boys) and that I shall have a complete class to teach. That caught me a bit unprepared, to be honest.

I have therefore included basic knife terminology and knife safety – do not carry a knife with the point upwards or forwards, do not cut towards yourself, that kind of stuff. One mother was afterward worried a bit that the children will try all that stuff I told them not to do just to test it. It was the same lesson I got when I was a kid and it never occurred to me to test whether a knife buries itself in my stomach or my hand if I do not heed my father’s advice (I cut myself plenty of times even so). There is one exception, however, and that is testing the knife sharpness with the ball of one’s thumb. That one thing is, to my bafflement, widespread and some of the boys already got into the habit of doing it before my lesson, and one of them did it on the just freshly sharpened knife after the lesson. For which I reprimanded him immediately.

“But I have never cut myself that way!” he replied indignantly, with his father watching in the background.

“That does not mean you will not cut yourself in the future if you keep doing it. I have just shown you that this knife is as sharp as a razor, it takes just a slight wrong move and you won’t even know you cut yourself until you have bled all over the floor!” was my reply, in a pretty pissed off tone of voice.

His father thanked me later, saying that the boy has picked up this habit somewhere and needed the reprimand from someone whom he recognizes as an authority when it comes to knives. Not the first time that I have ticked off an unruly child in the presence of their parent, and probably not the last time either (so far I have gotten away with it since all instances were about safety).

I do not know where people pick up this bad habit and why they keep doing it. It is completely useless for assessing the blade’s sharpness. Moving the ball of the thumb across the blade is kinda safe – it is the same movement used to shave hair, another method of testing – but with a sharp knife, a slight twitch of a muscle that flexes the thumb is all that is needed for things go wrong. A thing that I have seen happen. This can also easily result in non-bleeding cuts, those you do not know about until you wash your hands with soap – that is how my father got “cured” of this bad habit when he was young.

If you need to test a knife’s sharpness and you do not have a piece of paper or string to do so, you can put the blade on the fingernail of your thumb at an angle of approximately 45° and try to scrape it without exerting extra pressure. If the blade tends to dig into the fingernail with its own weight and resists movement, the knife is sharp. If it glides over the surface, the knife is blunt. It is completely safe and sufficient.

End of rant.


  1. Tethys says

    I’ve tested the sharpness of my knife against my thumb, but not deliberately. It bled all over the floor, and the flowers I was arranging, etc.

    I usually test for sharpness on fruit. Melons especially are much safer to cut with a nice sharp knife. Otherwise the blade tends to get stuck, mid slice.

  2. lochaber says

    I’ve never had much luck with the fingernail method.

    I usually lightly brush one of my finger tips perpendicular to the blade, so that I can “feel” the edge with the friction ridges/finger prints on my finger, and I’m able to get a general idea of the condition of the blade edge. Anything more than that, and I have to try slicing/shaving paper or similar.

    Most of my knives are for pretty general use, or even mild abuse, so I rarely need an edge much sharper than shaving sharp.

  3. says

    brushing fingertips across the blade is probably ever so slightly safer than brushing the ball of your thumb across it, still, it is not a method that I would encourage children to try. Funny that you had no success with the fingernail method. I have used it ever since I read about how to do it in a magazine for kids when I was young and I have used it ever since. The “lookandsee” method is, of course, also useful.
    I probably do have several knives that are “sharper than shaving sharp” but I am curious -- how does one about testing that? The bigger knife from the apple-root set springs to mind as probably the sharpest knife I have ever made. The blade goes completely invisible when looked at from a certain angle.

  4. lochaber says

    I’ve become pretty set in my ways, and generally sharpen most of my knives to “good enough”

    But, in the past when I used to be concerned with trying to achieve levels of “sharpness” beyond “shaving sharp”, I’d do a lot of the fingertip feeling I tried to describe, shaving arm hair, and occasionally cutting or even “slicing” of paper (I have no idea what the right term is, but I’d sorta flex or bend the paper, and try to use the blade to cut almost parallel to the plane of the sheet of paper. I don’t know. I just felt if I could shave a little sliver of paper off, without actually cutting through the paper or tearing, than that was somehow indicative of a pretty finely honed edge, at least much more so than what I needed to shave off arm hairs…

    Nothing against people who put effort towards achieving those really finely honed edges, (I respect the effort and accomplishment, congrats), but for the knives I use day-in and day-out, for my personal uses/experiences, I just have found maintaining a super-finely-honed edge to take far more effort than it was worth. again, personal preferences/situations/etc., but I’ve found approximately shaving sharp to be pretty easy to maintain, and covers 95%+ of my needs.

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