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As per the question - how to I test that a knife is sharp enough?
And/or, how do you know when a knife needs sharpening?


Edit:

Yesterday I attempted (I feel quite successfully) to hone my knives for the first time ever (requiring me to purchase a honing steel), which immediately made a rather noticable difference (I shall be honing them regularly from now on).

Not sure whether additionally sharpening them as well would be a good idea?

The suggested tomato and paper tests make sense, and I shall try them in the near future and see..

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I'd sharpen them once to get them back to a factory edge at least. You should hone them every time you use one. –  hobodave Aug 2 '10 at 16:39
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Please note that honing a knife does not sharpen it. Rather, it realigns the edge, correcting for dulling causes such as a rolled edge. –  Steven Rumbalski Apr 20 '12 at 15:29

7 Answers 7

up vote 18 down vote accepted

I usually notice when cutting onions and tomatoes. With a very sharp knife cutting an onion doesn't cause much tears at all. As it dulls though it does more crushing than slicing which releases more gas into the air, which makes you cry more. Ripe tomatoes help because they are so tender. If it becomes difficult to slice a tomato without crushing it, your knife is too dull.

A well used, properly cared for knife should require sharpening every 6-18 months, depending on usage. Mine get sharpened yearly.

You may find it beneficial to read these related questions, and their answers:

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Wholeheartedly in agreement on the onion/tears part of this answer. –  stephennmcdonald Aug 2 '10 at 15:24

The most readily evident way of determining if a knife needs to be sharpened is when you notice that you're having to apply more force than normal.

When you start out with a sharp knife you will become accustomed to how it glides through food. Over time you're going to notice that you are having to apply more pressure than normal and that's when it's time to have it sharpened.

Factors that affect how frequently knives need to be sharpened include:

-The type of knife itself: Forged knives, if properly cared for will typically hold and edge longer than stamped knives.

-Care for the knife: Washing knives in dishwashers wears down the edge quicker. Storing loose in drawers without a blade guard will also cause them to dull quicker.

-The manner in which you use the knife: The "Whack" "Whack" noise that so many people associate with cutting is an audible clue that you're cutting incorrectly. The "Whacking" of the blade against the board is caused by pushing the blade downward rather than forward. Cutting straight down against the board dulls the blade through the blunt force pressure against the cutting surface and it also results in smashing and crushing the food instead of providing a clean cut.

-The surface that you're cutting on: The harder the surface the more damage it will do to your blade. Don't cut on surfaces composed of tempered glass, stone (natural or man-made products), solid surfaces such as Corian, metal, or hard plastic.

-The frequency of use: The more often a knife is used, the more frequent it will need to be sharpened.

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Noticing that you have to apply more force than "normal" is, I think, harder than expected because "normal" will constantly change as the knife slowly dulls over time (obviously I'm excluding unexpected damage as factor). I'll compare it to the handbrake in my car - I never notice it getting weaker/slacker over time, but in a service it sometimes gets tightened - and suddenly the handbrake is noticeably stronger. But I wouldn't have identified it as weak before the service, and it's that identification I'm looking for (with knifes, obviously, not handbrakes). –  DMA57361 Aug 1 '10 at 17:18

I usually hold up a single sheet of newspaper and poke the point of the knife through. If you can make a downward cut without the paper tearing, the knife is sharp.

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There's the old boy-scout test where you see if it will catch on your fingernail. It's not exactly sanitary for commercial kitchens, but if it tacks on your nail, you're golden!

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...and if it slices your finger clean in half, you should have gone with the silk scarf test... –  Shog9 Aug 3 '10 at 3:11
    
touche`, touche`. –  Ocaasi Aug 3 '10 at 5:15

Hold the knife straight out with the sharp-edge upwards.

Drop a silk scarf on it.

The silk scarf should be cut neatly in half as it falls to the ground.

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Somebody has watched too many samurai movies. ;-) –  Tim Gilbert Aug 2 '10 at 22:46
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lol. I have seen a sharp knife cut halfway through a piece of moderate weight paper dropped onto the blade. –  hobodave Aug 2 '10 at 22:51
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Joel Tarantino. –  BaffledCook Sep 1 '10 at 17:57
    
LOL! This was a great answer –  Josh Sep 10 '10 at 23:33
    
where do you get those cheap silk scarves? (My wife made me ask, I think she wants some). –  klypos Aug 31 '12 at 19:27

Hold the edge up to a light source. If the light glints off the edge, it isn't sharp enough. Back to the grind stone...

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If it grabs the hair on your arm and shaves it, then it is very sharp (almost too sharp for many uses). Like your ordinary utility work knife. I usually keep one blade this sharp in my pocket knife and the others sharp but not that sharp. A quick slip and you're off to the emergency room. I do find that I can only get it super sharp with honing steel rather than diamond steel.

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I think if I attempted that, I would shave off some skin too, knowing how clumsy I am. –  lemontwist Oct 25 '12 at 16:47

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