39

In order for knives to be useful, they need to be sharp. If I am sharpening one myself, how do I know when I am done?

What would be a simple easy test of whether a knife is sharp enough?

  • 12
    Note that after honing and using knives repeatedly in similar situations, you will start to learn when the knife is honed to the finest edge without testing. The way the knife feels against the steel tells you where it is in the honing process, once you've learned how that changes with sharpness. And also you can learn to feel the sharpness with a thumb, but it's hard to describe what it feels like - you just have to feel it and then try using the knife and then remember which feelings match with more sharpness. At home the ultimate test is a ripe tomato (IMHO). – Todd Wilcox Mar 18 at 21:22
  • 6
    "Sharpness" is a bit more complicated than you realise I think. As an extreme example compare a bread knife with a razor. They're both sharp. but you wouldn't use a bread knife to shave with or a razor to cut bread. When you sharpen a knife you typically have two wet stones, one rough one smooth. The rough one shapes the blade the smooth one smooths the edge. There are advantages to a rough knife edge in some situations and a smooth knife edge in others. So you test needs to reflect your need. The tomato test will be cut easier by a rough edge, the paper test a smooth edge works better. – user2766 Mar 20 at 9:11
41

Try to slice a piece of paper. A good sharp knife makes a clean cut. A dull knife makes either a ragged cut, or worse, just pushes the paper to the side. I like this article: https://www.fieldandstream.com/articles/hunting/2014/08/paper-cut-testing-blade-sharpness#page-4 and YouTube is full of videos of people showing off their knives via this test.

  • 14
    Generally speaking though paper dulls knives very quickly and should not be done often unless you like to sharpen often. – Nate W Mar 18 at 23:10
  • 12
    @NateW - I believe this to be a myth or old-wives-tale. Of course, cutting anything dulls a blade, but is paper any worse than wood, fabric, plastic packaging, ad nauseum? I have serious doubts. I am going to do a little research and even put it to this community. I don't have the time right now, but look for a new question here at TGO.SE later today or tomorrow. – cobaltduck Mar 19 at 11:46
  • 5
    @cobaltduck Paper contains clay, so it's plausible that it would be worse for a knife than the other substances you mention. I look forward to the results of your experiments. – David Richerby Mar 19 at 11:59
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    @DavidRicherby If paper was so bad, I'd need to sharpen my X-actos / craft knives constantly. Paper is very tame when it comes to dulling an edge compared to other fibrous materials. And the grade of paper has an impact. It's mostly coated paper that contains minerals. Regular white paper is fine. – Gabriel C. Mar 19 at 12:50
  • 5
    Here's the Follow-up: outdoors.stackexchange.com/q/21846/9109 – cobaltduck Mar 19 at 13:44
36

The back of a fingernail can be a good rough indicator while you're sharpening - see if it "catches" when you apply very gentle pressure at ~45 degree angle. This can also be a good way to check if the edge still has any dull spots.

  • 3
    +1 as this is the simplest and safest technique so far that can be done without any other resources (e.g. having paper or tomatoes to hand). – Ynneadwraith Mar 19 at 16:42
  • 1
    I second this method. It's what I was taught by more than one bushcrafter. Very similar to the 'brush thumb perpendicular against it' method, but maybe easier to get a feel for and maybe safer too (of course, use very little pressure or movement - a sharp knife will not take much to slice deep into a fingernail!) – cr0 Mar 19 at 20:43
  • 1
    This is also a good technique for figuring out if your fish hooks are sharp enough. If they don't dig into the nail at a 45 degree angle, they need sharpening. – Clinton Pierce Mar 20 at 17:57
20

The best method in my opinion is one that must be learned, and involves moving your thumb across the blade, perpendicular to the blade not down it! It's very difficult to describe the feeling but if you do it on enough dull blades and sharp blades you will begin to be able to tell the difference.

One other thing I will do if im sharpening a knife for a friend or coworker to display how sharp it is is shave a couple hairs off the back of my hand. Lick your thumb, moisten the patch on the back of your hand and shave as you would with a straight razor. (You may want to wipe the knife clean after this.) Sounds weird but i do it because your average joe will know its extrememly sharp if you can shave with it and be careful with it.

One other test could be to attempt to very thinly slice a tomato, with and exceptionally sharp knife you should be able to get nearly paper thin slices with a light sawing motion.

  • 2
    I agree with the thumb test. If the knife is sharp, you can feel tiny resistance from the ridges of your fingerprint on as you move your thumb across the blade (definitely not along the blade, or you'll cut yourself). Do it gently of course. – user85627 Mar 19 at 12:34
  • I use the shave test, but dry-shave the hairs on the back of my wrist. – Don Branson Mar 20 at 15:38
16

Generally I categorize sharpness into 3 levels:

  1. The tomato test - being able to slice through a tomato skin. Not use the pointy end to break the skin and then slice but directly slice the skin all the way through. This is sharp enough for cooking.

  2. The paper test - being able to slice thin phonebook paper or magazine paper or newsprint. For general carving knives I find some tearing after the knife has started a cut to be acceptable. If I really want sharp then it should be able to cut paper with zero tearing.

  3. The shaving test - you should be able to shave with it. This is literally razor sharp (otherwise known as "scary sharp"). I've only been able to do this on some knives and I wouldn't want all my knives to be razors anyway.

  • 2
    "literally razor sharp" exactly. my grandfather would sharpen his axe until he could shave the hairs off his arm with it. – aktivb Mar 20 at 15:49
15

Slice a tomato with very little pressure. This is almost always when I notice a knife needs sharpening, and the difference is profound.

Edit: I should have added ripe tomato; a sharp knife will slice it, whereas a dull one will smash it or rumple and tear its skin.

  • 2
    After struggling to sharpen kitchen knives for decades, I have found that some knives cut decidedly better on the "pull" stroke than on the "push" stroke. In fact, angling the tip of the blade downward - at as much as a 45-degree angle - before initiating the pull stroke also improves blade performance. Net: "apparent sharpness" may be affected by how you apply the knife to your cutting problem. – Brian K1LI Mar 19 at 13:22
8

Cheap, disposable sponges. Unlike paper, they can be tested on while wet, which is a big advantage when sharpening with waterstones. Don't use the scrubby side (some people like using it as a deburring aid, though)

Something dull will not be able to cut into a sponge. Something sharp will easily slice into it. Something d... sharp can push-cut into the corner of a sponge. Something b...y damn sharp can be laid on a sponge and pushed in (a razorblade easily will!).

7

If you're sharping the knife outdoors (where a tomato is not available), a good test is to wield it perpendicularly to a grass stalk and see if the knife cuts it off.

  • The brackets make me wonder whether more tomatoes grow indoors or outdoors worldwide. – Evargalo Mar 21 at 14:25
  • 1
    @Evargalo That depends. Do you count greenhouses as indoors or outdoors? :) – Dmitry Grigoryev Mar 25 at 11:51
  • exactly my point ! – Evargalo Mar 25 at 14:18
4

You could just cut of some hair.

No need to cut other resources and its safe because yeah its hair.

And for any type of baldness you can adapt the amount of hair you would like to test on.

  • My son tested scissors in school in this way, and was quite surprised we found it out at home. It was a small chunk, but it was still pretty visible on a haircut. – Dmitry Grigoryev Mar 25 at 11:55
  • So the test was successful. Congratulations! – Snickbrack Mar 28 at 9:59
  • Sure. My point was that unless you want other people to ask "Did you try to cut your own hair?", you shouldn't test knives in this way. Unless of course you don't have any haircut in the first place, so a few missing chunks won't matter. – Dmitry Grigoryev Mar 28 at 10:09

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