Sciencing Sharpness – Part 5 – My Sharpening Kit

I have found a blunt-ish knife, one that I use in the cellar to open wooden briquette packages. It was not exactly blunt but it was slightly blunt and I have run it over a quartz stone to make it blunt. It is a knife from high-quality stainless steel but it has been sharpened so much that it has lost about 30% of its width. I have tested with it my traveling sharpening kit. I have done completely freehand sharpening, holding the stone in my left hand and knife in the right hand, no angle measuring, not even angle estimating, just putting the blade to the stone in a way that “feels right” and going on from there like I used to before I got the machinery to be precise. I tried to look up on the internet the grits of the stones that I use but it ain’t easy because one of them is a no-name generic grey carborundum whetstone and as you will see, it does not appear like I found the right ones. I have also used all of the orange thread that I have reinforced with PVA glue and from now on I will use the nylon thread since I have already bought it.

Here is the picture that’s worth a thousand words:

© Charly, all rights reserved. Click for full size

And here are less than a thousand words explaining what’s in the picture:

It is a bit strange that the fine grey layer of the cheap whetstone does not perform much better than the coarse layer and even performs worse than the beige layer on the colored stone which should have slightly coarser grit. There are several possible explanations or a combination thereof – I got the grey stone grits wrong, I done did do a bad job with this layer, or the fact that this stone uses a weak binder and sheds grains very easily plays a role, or I messed up the measurements. Nevertheless, the knife was capable of cutting freely hanging printing paper at this point, although not very easily, and it did bite into a fingernail.

Anyhoo, the second stone is much harder and I actually know the exact grits from the manufacturer. And once I got to the red layer with “just” 500 grit, the knife was shaving-sharp. The leather strops might have burnished the edge a bit but there is no statistically significant difference anymore from the 500 grit stone. It looks like there might be one, a very minor one if I had performed more measurements or had a more precise method.

I would say that it is pretty convincing for my argument that a two-layer whetstone and a strop are all that is needed to get and keep knives sharp.

It also appears like 500 grit stone is sufficient to maintain a knife edge shaving sharp without any further ado. But I would say that shaving hair with the stropped knife feels slightly “smoother” on the arm than with one that went just over the stone and I feel inclined to trust my skin sensors (they are much more sensitive than the kitchen scale after all) on this issue so I’m not convinced that the strop is completely useless. Measuring as fine differences as these might be is not a task that can be done with a rigged-up kitchen scale.


  1. flex says

    Still following along, and still interested in the results. I’m certain a lot of other people are too even if we haven’t commented on the last couple posts.

  2. Tethys says

    I’m happy to hear that the drugstore carried nylon thread so you can keep testing your sharpness.

    I don’t think I quite follow all the details of the measurements so have no useful commentary, but I too have been reading all of your posts on sciencing sharpness.

  3. tuatara says

    I have found this series very interesting due to my own interests in sharp tools.
    I don’t quite know how to put my thoughts about your excellent posts in a sensible or scientific way, but I have some ideas about the method and results that I would like to share.

    Part 4.
    1) the comparison between razor and knife, with the knife sharpened at 10 degrees each side at I presume your 500 grit then stopped.
    You say yourself that the N690 steel is prone to chipping. At 500 grit I imagine it will be heavily chipped and serrated compared to a razor, which should be a highly polished edge in comparison. While your knife may be ‘shaving sharp’ I would certainly not use such a rough edge against my face.

    2) the razor/nylon appeared to need about 18gf and the knife/nylon about 55gf. So the razor required about 1/3 the force to cut the nylon than the knife. We do not know the grit size used to polish the razor, but I have heard that a good razor should be polished to around 10,000 grit.
    If 10,000, compared to 500 we are looking at a 20-fold reduction in grit size.

    In the results posted in part 5, the nylon required what appears to be about 180gf to be cut at 100 grit, then at 500 grit only required about 60gf, or again about 1/3 the force for a five-fold reduction in grit.

    All this appears to me to be plotting a nice hyperbolic curve (I hope that is the correct term) which is exactly what I would expect in this type of experiment.

    It certainly confirms your hypothesis that there is a reducing effectiveness of finer grits. But I still don’t consider a 500 grit edge to be sharp. It would cut most things easily because of the chips and serrations in the edge, but sharp? Not really.
    Given the right treatment with fine enough abrasives and a sound polishing method, I bet that even the N690 steel could take a very fine edge that will last well if treated well.

    I mean no offence. Your post have been very interesting and well presented. It is just that your ‘sharp’ and my ‘sharp’ are obviously quite different.

  4. says

    What has to be taken into account with these posts -- one cannot simply take values measured one day for post 3 and compare them with values taken the next day for post 4. I have performed some measurements in my workshop (the first two) and some indoors, where there is a significant difference in temperature and humidity. I am not sure how big a difference that makes, but to be on the safe side, all experiments should be taken as separate units. They each test different things in different ways.

    I also do not show the exact results of the statistical tests that I have done because I do not think very many people would understand them and I also do not think they are super conclusive due to the extreme impreciseness of the measurement method. All the results need to be taken with a grain of salt because of the measurement method. They are not worthless, but they also are not very exact and the confidence intervals for means are usually in tens of gf or more

    @tuatara, for the knife with the 10 degrees sharpening I still went all the way to 2500 grit because that is where I ended up with sharpening before evaluating the data in the second experiment but the blade did have a few chinks and blunt spots from the comparison measurements and another tomfoolery I subjected it to. So does the razor btw. They both still shave but not with what I would call a “smooth feel”. Not all “shaving sharp” sharpnesses are equal, and indeed in this post, I do mention precisely that. Smooth shaving does require an extremely sharp blade, but it is also one that blunts extremely fast, otherwise, there would not be a huge industry of single-use razors. Also important about this test is that it was not designed to compare the razor to the knife, for that I would re-polish the knife because I tried to also cut wood and leather with it prior to that test so it was ever so slightly blunted at that point (note that previous day it cut at about the same strength as the razor, although the first caveat applies here). But the purpose of the test was to compare the threads, not the blades.

    But even ignoring that, a reduction in cutting force down to 1/3 may sound impressive, but is it really worth it if you take into consideration that you are going from the barely measurable 60 gf to 20 gf with about the same amount of work that it took to take the blade from 400 gf down to 60 gf? Most people -- and I know this from my previous job for a fact -- won’t be able to recognize a difference between 60 and 20g with a spread +-10 either way, but they most definitively will recognize a difference between 400 and 60 g. It might be worth it for someone who has a favorite blade/s and who enjoys sharpening and to tinker with the blade to give it that extra bit of zing. I have no problem with that, I do that. But that is not the issue here!

    The issue is not about what is “my sharp” or “your sharp”, neither is it to find out how to best and most effectively sharpen a razor blade for shaving faces. The point is how to best and most effectively sharpen an ordinary kitchen knife and whether going higher in grits or being anal-retentive about angles is really practical for an average knife user. To both of those questions, the answer is a resounding “NO”.

    Charly-made knives do not require very frequent sharpening at all. I usually need to sharpen them once to twice a year when they no longer cut tomatoes very easily. In the meantime, my mother uses her knives to chop whole chickens in half, bones and all. 90% of people I know would still consider them “scary sharp” at the point she starts to complain a bit. Indeed I have friends who refuse to use my knives when I bring them along on a trip because they are afraid of them for some reason :). I do not pester my customers with questions but the one who bought the cherry-handled santoku told me on his own that it is his wife’s favorite knife and that it is still sharp after a whole year and all of the knives out there that I know of serve their owners well. And I did not notice any bettering/worsening of knife edges in our kitchen after I switched from the cheapo whetstones to my belt grinder. The bevels look nicer from the belt grinder, but the knives cut the same now as they did before. I will continue to grind and polish nice bevels on handmade knives all the way I can, but when someone brings me a box of heavily used and abused knives from a supermarket to re-sharpen, I won’t bother.

    And I mean you no offense either, but I think you might be a case of “knifecebo effect”. I would love to have a get-together where I would give you a bunch of identical knives sharpened with different grits/methods and let you sort them into groups. That is unfortunately something that I simply cannot test on my own.

  5. tuatara says

    I would love to come and check out your knives. Not to sort them in a any way*, just to admire your lovely work.

    * maybe the ones I would really like to own.

  6. dangerousbeans says

    Interesting. It would be interesting to test placebo effects on perceived knife sharpness, but that’ll be very fiddly

    Re shaving sharp: something i have noticed is that it’s easier to shave my leg hair than my arm hair, my arm hair being notably finer than my leg hair.
    Not that i think it matters here, this does support the conclusion that a relatively coarse grit is fine for non-nerdy users.
    What i’ve heard from wood turners supports this too, a lot of them are only sharpening at 240 to 500 grit

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