Forum: outdoors.stackexchange.com
Tags: <knives><knife-sharpening>
Title: How to test the sharpness of a knife?
Question: 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?

Answers:
sents: text: Try to slice a piece of paper.
text: A good sharp knife makes a clean cut.
text: A dull knife makes either a ragged cut, or worse, just pushes the paper to the side.
text: I like this article: https://www.fieldandstream.com/articles/hunting/2014/08/paper-cut-testing-blade-sharpness#page-4 and YouTube
text: is full of videos of people showing off their knives via this test.
answer_details: author: outdoors.stackexchange.com/users/9109/cobaltduck
score: 41
sents: text: 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.
text: This can also be a good way to check if the edge still has any dull spots.
answer_details: author: outdoors.stackexchange.com/users/16788/tempest_col
score: 36
sents: text: 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 !
text: It's very difficult to describe the feeling
text: but if you do it on enough dull blades and sharp blades you will begin to be able to tell the difference.
text: 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.
text: Lick your thumb, moisten the patch on the back of your hand and shave as you would with a straight razor.
text: (You may want to wipe the knife clean after this.)
text: Sounds weird
text: but i do it because your average joe will know its extrememly sharp if you can shave with it and be careful with it.
text: 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.
answer_details: author: outdoors.stackexchange.com/users/9054/Nate W
score: 20
sents: text: Slice a tomato with very little pressure.
text: This is almost always when I notice a knife needs sharpening, and the difference is profound.
text: 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.
answer_details: author: outdoors.stackexchange.com/users/17581/Oscar
score: 16
sents: text: Generally I categorize sharpness into 3 levels: The tomato test - being able to slice through a tomato skin.
text: Not use the pointy end to break the skin and then slice but directly slice the skin all the way through.
text: This is sharp enough for cooking.
text: The paper test - being able to slice thin phonebook paper or magazine paper or newsprint.
text: For general carving knives I find some tearing after the knife has started a cut to be acceptable.
text: If I really want sharp then it should be able to cut paper with zero tearing.
text: The shaving test - you should be able to shave with it.
text: This is literally razor sharp (otherwise known as "scary sharp").
text: I've only been able to do this on some knives and I wouldn't want all my knives to be razors anyway.
answer_details: author: outdoors.stackexchange.com/users/8914/slebetman
score: 16
sents: text: Cheap, disposable sponges.
text: Unlike paper, they can be tested on while wet, which is a big advantage when sharpening with waterstones.
text: 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.
text: Something sharp will easily slice into it.
text: Something d...
text: sharp can push-cut into the corner of a sponge.
text: Something b...
text: y
text: damn sharp can be laid on a sponge and pushed in (a razorblade easily will!).
answer_details: author: outdoors.stackexchange.com/users/8657/rackandboneman
score: 9
sents: text: 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.
answer_details: author: outdoors.stackexchange.com/users/7982/Dmitry Grigoryev
score: 7
sents: text: You could just cut of some hair.
text: No need to cut other resources and its safe because yeah its hair.
text: And for any type of baldness you can adapt the amount of hair you would like to test on.
answer_details: author: outdoors.stackexchange.com/users/17612/Snickbrack
score: 4

Summary of the answers:
The tomato test is very common. Other tests include trying to cut grass or a cheap kitchen sponge, shaving, cutting hair, scraping the back of a fingernail or slicing a piece of paper.
Try to slice a piece of paper, shave, cut some hair. You can use your fingernail or thumb. Slicing a tomato, using sponges and grass also work.