How sharp is sharp?

Oct 17, 1998
I've got the bug. I realized that I wanted to collect these sharp metal things about a year ago. Recently bought my first sharpener, a Spyderco Sharpmaker.
My question is - how do YOU know a knife is sharp? What tests do you use? I hear about people shaving hair on their arms, but what is your preferred test for sharpness? Teach me Obi Wan...

In order to save my arms I've learned to push the blade gently onto a thumbnail at an angle. I do this twice at opposite angles so I can feel the edge from both directions. That's all it takes for me.

It takes very little time to learn to judge the sharpness of an edge this way. Take care.

Knife Outlet
Rob - lightly touch the edge of the blade to the hair on the back of your head. Almost as though you were going to comb your hair with the knife. If the edge of the blade "catches" (remember lightly) your hair, then it is sharp enough to shave hair.
I use paper. The thinner you can cut the sharper you edge. If you can cut sigaret paper easy you will have a very sharp edge IMHO.

Jan Dirk Wijbenga

Knife philosopher
I don't generally shave with my knives and have noticed that some edges that can shave hair well don't do particularly well cutting various mediums that are most representative of what I typically cut. The most expedient method of edge testing that I've yet found is to test the edge with my thumbprints. NO, I'm not suggesting running your fingers up and down the edge resulting in blood all over, just lightly testing the edge in a perpendicular very light controlled manner to see if the blade tries to grab into the skin or simply slides off it.

Believe it or not a lot of highly polished edges that will shave hair will not grab into the outer layer of callus very well at all.
Testing the edge lightly on the surface of the thumbnail or fingernail is also good, but I like to feel the uniformity of edge.

Keep well in mind the words light touch and perpendicular to the edge, *never* parallel, or you'll be sending slightly more nerve ending feedback to your CPU than is pleasurable. Actually, you'll probably wind up needing a few bandaids early on anyway, but that's the only way I know of to really get a knowledge of what constitutes a good edge. After a nick or two you'll learn not only what you can get away with, but you'll have a much better feel for edges than if you shave your whole body. ;->

That's not necessarily how I'd test a microtome blade, but it seems to work for me for work and kitchen knives better than catching hair.

One other word of caution is beware of the wire edge. The wire edge or burr is a natural and necessary component of sharpening a knife but you don't want to leave the burr on else it will feel very, very sharp at first, but will break off the first time you use the blade and you'll be left with a much less sharp edge to use. The wire edge can easily shave, catch fingernails and cut calluses and can be quite deceptive until you know to look for it. It's probably most easily simply felt with the fingers. Gently, gently, and never towards the edge.

The *best* method of testing sharpness is of course to use the knife on the medium that you're attempting to cut, but that's not always possible or desireable.

All above are good tests and advice. I also use a Sharpmaker a lot and find it works well for me.

I use the test Sal mentioned above as my usual basic sharp test. Then, I use a steel to get the edge just a little sharper. Normally the steel restores the sharpness without having to go back to the Sharpmaker.

Be sure to read the Sharpening FAQs. They are pretty complete.
I saw a method of evaluating sharpness many years ago in an article in the American Rifleman on Knife Sharpness. You need a magazine with smooth shiny paper. Fold up the edge (about 1/4") and without touching or holding the paper, try to cut the vertical edge. I have found only the sharpest edges pass this test without crumpling the paper.
I must confess that all the knife testing I've been doing lately has led me to be a little nutty. After I sharpen a knife, I first do the usual shave-hair-on-arm test. My goal here if I am polishing the edge, is to be able to shave so easily I can barely notice it. If I'm leaving the edge more coarse, then I still expect it to shave well, just with a little tugging.

Then I typically head to the garage, and try some whittling, and slicing some rope. These tests tell me if I'm really done. I've done these tests so consistently with so many knives, I kinda just know if this particular knife is performing as well as it should.

The whittling test seems to tell you that your knife has a thin enough edge, and is sharp. Slicing rope tells you if your edge is coarse enough for slicing duties, and is sharp. Between them, I have a good idea if the edge has the 3 attributes I want: thinness, sharpness, coarseness. Where "thinness" and "coarseness" are relative terms, depending on what the knife is meant to do.

I will cut the first person that laugh at my story. Last weekend I did not like the edge on my Kershaw Random Task so I decided to make the edge really thin and sharp. At the end I put on a really thin and convex edge and off to the sharpness test. The blade will easily shave hair longer than 1/16 inch in both direction but was having problem with the stubbles from previous sharpening job. I ran out of places on my body where I can safely reach and test the blade so I asked my wife if I can test the knife on her. She agreed to let me try it on her arm.

Her arm's hair is so fine that I was having problem shaving it without really scraping her soft skin. I suggested that may be I should try it where the hair is more coarse. Now, y'all know that there are only a couple of places on a woman body where the hair is coarse and I will not tell you where I picked to do the test. To make a long story short, I nicked her due to my testostorone imbalance and now I am in the dog house until Monday. Uh!Uh!Uh! I said no laughing.

The moral of this story is sometimes it just not worth it to get it razor sharp.
For a shaving test I shave the razor stubble off of my face. This hair is very course and strong, and you face is very sensetive. This will tell you just how sharp your edge is very easyly.
I mainly use the same thumb methode as MPS, but this takes some time to learn.

I also use the thumb method posted by mps. Sometimes I will follow that up by slicing paper held vertically.


NamViet Vo, might I suggest that you're gonna be even more in the dog house when the stuble starts to grow back out and itches and chafes? ;-> Get a pair of round tip barber shears.

BTW- I'm simply guffawing, but I'm not the first to laugh so don't cut me. ;-> ;->

Three cheers to Joe for posting about using wood and rope.

Another good test is one I got from a Rob Simonich post: ...styrofoam! I've been experimenting with various kinds of fine and coarse styrofoam as a test medium and seeing how it compares to wood, rope and cardboard. It's still too early to be definitive, but so far it looks like edges that will grab into and cleanly sever coarse beaded styrofoam packing material without shredding it or creating little static charged particles are good on rope, wood, meat and veggies. In coffee cups and styrofoam packing there's several grades of the size of the component beads. I'd recomend using the coarsest grade for edge testing since it seems to be the shoddiest made, (and therefore the first to shred). The fine grade used for egg cartons doesn't tell one quite as much, quite as early since it doesn't break up into component beads and can be simply torn.

NamViet Vo,
It's supprising your wife agreed to let you get near her with that knife! My wife is quite fed up with my cutting up everything in the house. I now have to get approval on what is junk mail and what is not. Good story though. I tried not to laugh, but could not help myself towards the end.
NamViet Vo

Thanks man, I needed that.

I prefer to use a Gillette Sensor Excel on'm.

Ron Knight

Yeah I'm crazy, but what do you want me to do about it
I use my thumb as the first test. I hold the blade steady in my right hand and rub my left thumb ACROSS the blade. In both directions. If it is sharper in one direction you have a wire edge. I have seen people run their finger down the blade and cut themselves. If I am about done I use the knife to push cut little curls from my thumb nail. You can also use wood. The idea is to see how fine a curl you can make. I have tried Sal's test, but so far I don't have enough experience to tell much. By the way, I often finish by stropping backward on a piece of cardboard.

The problem with the "combing your hair with a knife test" is that it doesn't take into account the variables of human hair. That test was very likely initiated by a people with straight, fine black hair. It doesn't allow for those of us with curly red hair, or kinky black hair or fine blonde hair etc. I read somewhere that different hairs have different thicknesses, and certainly diffenrent hairs have vastly varying degrees of ability to withstand cutting or breaking strength. Not to mention what all goop some folks put on their hair. What of moussed hair? Is your blade supremo baddo if it can reach out and grab moussed or gelled hair? Anybody wanna do a Brylcream vs. "just washed with Prell" test? Doesn't PH say it all? Hmmm . ;-)

;-> ;-> ;->

Styrofoam, what a great idea! Love it.

Also, you're probably right about the hair thing. Looks like YMMV rules here.

Just think though if we use Rem oil up top, the blades could be tested and coated all at the same time.
A good way to test for a subtle wire-edge is to run your fingernail across the flat of the blade, like so:

/ |
/ |
|) |
| |

where ) indicates one's nail. If there is any sort of wire edge on the blade, it will grab right onto your fingernail, without you having to worry about cutting anything sensitive.
Thanks all for the replies. I, once again, have a wealth of information to use.

Stay sharp,

[This message has been edited by Robert Monteux (edited 02 March 1999).]
NamViet Vo -- not laughing, but smiling so widely that my wife just asked me what I'm doing.

mps -- great point about different materials having different optimum cutting edges. IMO, it's a detail that's frequently overlooked.

dfhoward -- my favorite portable strop is a 2" wide X 6" long piece of leather dressed on the flesh (i.e., rough) side of leather with jeweler's rouge. I like steels for quickly bringing an edge back but have found that this simple strop often works faster for finer edges.


[This message has been edited by bcaffrey (edited 02 March 1999).]

[This message has been edited by bcaffrey (edited 02 March 1999).]