For centuries, men have sharpened tools, knives, swords and razors. With the proliferation of disposable utility blades and cartridge razors, what was once a commonplace skill has become a lost art, practiced mostly by knife enthusiasts, hand tool woodworkers and men who shave with straight razors. Whether sharpening by hand with hones and abrasives can produce a finer edge than industrial scale, mechanized sharpening seems likely, but it is something we will investigate. Undoubtedly, the modern preference for mechanized and disposable sharpening is the choice of convenience rather than a confirmation of quality.
In principle, there are two general approaches to quantifying “sharp” or the keenness of a blade’s edge. The simplest and most common is through comparison and evaluation of use. Comparing the force to cut, the smoothness of the chiseled wood, the thinness of the sliced vegetable, or the closeness of the shave provides a relative quantification of sharp. Such comparisons are more than sufficient to allow a practitioner to develop and evaluate a honing procedure. To a Scientist, this phenomenological approach begs the questions of why? and how? and provides little insight into how the process can be improved.
My approach will be to use electron microscopy to physically observe the geometry and polish of the edge and to quantify the edge width and bevel angle. The goal is to provide an understanding of what is happening a the blade’s edge. The centuries old design of a straight razor provides the ideal system for scientific study of sharpening. Honing on a flat surface with the bevel and spine contacting the hone fixes the angle of the apex. The steel used in a straight razor is hardened and tempered to optimize the achievable keenness. In our experiments, the use of a straight razor will allow us to fix the honing angle at the value determined by the spine thickness and blade width.
scienceofsharp 
44 responses to “About”
This is great stuff. I am that knife enthusiast.
I don’t use a straight razor, but I sharpen many chef knives (Japanese and Western), fixed blades, folders, robot coupe blades, mandoline blades and anything else anyone will let me sharpen. I am still an amateur, I am so jazzed about this that I am sure to be great a couple of years and pretty good in less.
Reading these articles is awesome. I found them on Blade Forum. It’s is sometimes hard to find great explanations of the craft and what is truly happening in the microscopic scale. There is good stuff out there for sure, but great?…hardly.
I thank you wholeheartedly for this exchange of info. I will share wifh others, as I am sure many of my friends will enjoy on the same level as I.
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Hey Todd can you shoot me an email?
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What amazing pictures! Not only are they insightful but artistic as well. Beautiful. Your pictures show so much detail and relief and from angles that I didn’t think were possible. I also love the cross sections that you cut. I will definately be following your blog and look forward to more posts.
A lot of the advice and opinions I’ve read on knife forums and websites is contradictory and some of it illogical. The most thorough and objective treatise I have seen so far has been Professor Verhoeven’s paper and I’ve spent hours looking at the pictures that illustrate what I could only imagine. Unfortunately Verhoeven only looked at razor blades and his standard was a highly polished edge. He also didn’t look at blade wear under use.
I am not a razor person but I sharpen mostly kitchen and pocket knives for myself and for friends and family. I am interested not only in the sharpening process but also in how a knife becomes blunt from cutting through different foods and on different surfaces. I try to explain to people around me how to take care of their knives but I don’t have any photographic evidence to show them. I would also like to understand and develop sharpening strategies that would be best adapted to different kinds of cutting jobs.
In case you are looking for topics to investigate in the future, here are some of the questions I would like to find answer to:
• What is the effect on a sharpened knife blade of cutting through different foods: slicing through meat muscle, push cutting hard food like carrots or butternut squash. What if I cut meat and hit a bone or cut through a chicken breast that hasn’t completely defrosted. Does the knife become blunt by abrasion, deformation or chipping?
• In the same vein as the “Dulling on glass” series, what is the effect on a sharpened knife blade of cutting on different surfaces: end grain wood butcher’s block, bamboo chopping board, plastic, glass or ceramic?
• What does my knife blade look like before and after a cycle in the dishwasher?
• Molecular polishing is great for display knives but what degree of edge roughness/toothiness is useful in cutting food? Will a highly polished edge or a toothy edge cut for longer. Which kind of edge will chip, roll or deform the most from the food or the chopping board, and which will keep cutting despite the damage ?
• How do different sharpening/honing accessories improve or damage the cutting edge? Should I use a smooth steel, ridged steel, ceramic or diamond coated rod? How many passes are necessary? In which direction, edge leading or edge trailing? Do steels work by smeering the rolled edge or unrolling the rolled edge? Do smooth steels remove any metal? How much metal is removed by a diamond coated rod? Does a ceramic rod work by straightening the edge or by removing metal?
• There are various pull through sharpeners with carbide blades, crossed ceramic rods or diamond plates that “sharpen” parallel to the knife edge. How effective are they and how much damage do they actually do? Are there situations when these types of sharpener could actually be useful? What about the ceramic wheels with a V shaped groove set at an angle to the knife blade?
• When I sharpen a kitchen knife I have a selection of grits available. Mostly FEPA graded wet/dry sandpaper from an automotive shop on a glass backing for flat bevels or on a neoprene rubber backing for convex edges. I also use Chromium oxide paint pigment on leather for stropping. At what grit should I stop? P250 / P500 / P1000 / P1500 / P2000 sand paper? At what point will going finer bring no further advantage? From a cutting efficiency/edge retention perspective, what should my final grit be?
• If I finish a kitchen knife on say P250 or P500 sand paper, does stropping it afterwards do any benefit? Does the leather strop remove the burr or straiten the “teeth”. Do I need to go through a full progression up to P2500 paper before stropping becomes useful?
• How much metal is removed by different methods of sharpening/steeling/stropping? Which method is the most aggressive / respectful of the knife.
• From a cutting efficiency/edge retention perspective, what final bevel angle is optimum?
Lastly, that focused ion beam you use to cut cross sections is amazing. Could you use it to cut the ultimate edge?
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Liam,
Thank-you for taking the time to comment, your feedback is most appreciated.
You raise a number of interesting questions, and I will keep them in mind when designing future experiments and try to answer some of them in future posts.
Todd
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For the Myths Busted catagory, there is the myth that says you should let your razor rest for several days between shaves so that the edge will some how mysteriously return back to its original shape. Can there be any scientific reason behind this or was it all a marketing ploy to get men to buy packs of 7 razors (one for each day of the week) ?
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Well, there is no scientific reason to dismiss the idea; however, I am not aware of any evidence to support it either.
We know that the apex of a razor is much finer than the bulk grains of the steel, so we should not expect the familiar macro-scale behavior to apply.
We also know that the “fin” is flexible and is certainly deflects during shaving. Maybe the claim is that the deflection is elastic on a scale of a day or so. Maybe the idea is that the “fin” becomes embrittled with use and will break off unless allowed to anneal prior to stropping. Maybe rinsing the blade in hot water has the same annealing effect as 48 hours at room temperature – unlikely anyone shaved with hot running water in the 19th century.
Although I’m skeptical, I don’t see a simple experiment to provide compelling evidence either way.
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Sorry to drop in this discussion near 5 years post date, however: isn’t the simplest experiment to shave with a razor, make photos of the fin, and make the same photos 24hrs later? That at least should show if something has happend in the meantime?
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I have a question about slurries when using a jnat. I’ve read that a jnat will only polish if it has a slurry. This seems to be confirmed by the fact that I can’t seem to get my Asagi to sharpen because I don’t have a nagura hard even to raise a slurry.
However, this seems strange to me. The choseras seem to sharpen perfectly fine without a slurry and my elementary knowledge of mechanics suggest that a slurry shouldn’t be necessary.
Do you have any insight into this? Thanks
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I can’t speak to the specifics of your stone, only knowing the color, but I can make some general comments based on the handful of JNat stones I have analyzed.
The Chosera is designed to wear down and continuously expose fresh abrasive particles on the surface.
A “hard” natural stone will usually wear too slowly to expose a fresh surface when used without slurry. If there is no exposed abrasive, the stone can only burnish the metal.
You have several options; you can buy a nagura, you can cut a piece from your stone to use as a nagura, or you can use a diamond plate or card to raise a slurry and refresh the surface.
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Nice Site….you have the Bevels i have the Stones 😉
Would like to link your page, would that be ok ?
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Certainly.
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I’m new to this blog and to honing and or sharpening of knives and razors. I’ve just finished reading through all the blog posts and responses and I’m still trying to digest it. So to preface my question, and forgive me for my lack of knowledge and understanding. but which is more preferred Keenness or sharpness? I realize this is a seriously subjective question. the reason i ask is there is nothing i hate more that just putting and edge on a knife or razor and to feel like its crazy sharp or keen and then to have it the next day after cutting something like plastic feel like it needs to be completely re honed maybe I’m just suffering from a minor case of OCD. but i love my edged tools to be at the ready always…or maybe my edges just are not up to snuff. are there any other resources to learning sharpening and honing?
I learned how to hone and or sharpen from my grandfather but have never been able to get the edges he got on knives or razors…so i quite trying…now I want to learn how to do it proper and become proficient and good at a lost art. i have nice a nice vintage straight. looks like its never been used and i cant use it because of my lack of skill in the honing department. so the edge is not usable at all. don’t get me wrong its sharp just not shaving sharp or keen….I am definitely going to have to read trough all of this info again……
thanks for the info posted here!!
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In general, we want to match the blade and apex geometry to the cutting task.
For razors and shaving facial whiskers, keenness is critical. Perceived sharpness will obviously be affected by how quickly the blade deteriorates. In other words, a blade is “too sharp” if it fails during the first use. For example, there are straight razors available on Amazon with 12 degree (inclusive) bevel angles – these cannot be made to shave a coarse beard without convexing and increasing the near-apex angle to closer to 20 degrees, regardless of keenness.
If you are cutting hard plastic, zip ties for example, a razor-like knife will blunt rapidly. You can confirm this with a commercial utility blade. A straight razor, on the other hand, will be blunted by simply cutting printer paper.
There are many strategies for creating a “working” edge on a knife – I hope to discuss these in the future.
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Todd, I recently read a post by you on why a coarsely finished edge has better edge retention than a finely finished one. It had to do with micro-chipping occurring earlier on a coarsely finished edge, which keeps it toothy for a longer time. However, I cannot find the post back. Can you give me a link to that post?
Thanks!
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Mark, I’d be surprised if I wrote that, since it doesn’t make sense to me. I have briefly commented elsewhere on the role of carbides in a “coarse” edge (and their non-role in razor sharp edges). I will write a post or two here about that when I have time.
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Hi
Well, I graduated culinary school, became a hairdresser, then got my Barber license and now I am starting a shear and Razor sharpening business. My life has been surrounded by sharp blades… and I am struggling at creating that perfect edge. I have a few Shapton glass and king stones, I have just purchased some CBN emulsion to use on Kangaroo. I have green white and black polishing compounds not to mentions other stuff like Tormec paste. I have seen people draw the edge to remove the burr on shapton stones, wood, felt & Basla. I am spending large sums of money and as much as I enjoy the experimentation. I would love to find technic that would work well and make it my go to.
Based on what you have on this site what in your opinion would be a solid step by step procedure to creating a Strait Razor edge that would be consistent. From what I can tell from reading your site using shaptons to 16000 but finishing with 10 edge trailing so ther hopefully would not be the bur created by the 20 edge trailing. however at that point should I stope and if so should I use paste or CBN.
Sorry for babbling but would like to have your thoughts on a consistent step by step procedure.
Daniel
dan2kphoto@yahoo.com
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There are many ways to hone a straight razor. Some people prefer a keener blade than others. Beard toughness varies, skin sensitivity varies. It is difficult to compare the options unless you have a “bulletproof” approach to use a baseline. I normally offer the following approach, not claiming it will suit everyone, but because it is easy and consistent.
There are two parts to honing a razor; the first part is just sharpening to apex the blade (setting the bevel) , no different than sharpening a knife or plane blade. This can be challenging if the blade is warped, but assuming you can get the blade to that point, then you can “finish” as follows:
Make a strop from a strip of denim – I prefer 1.5 inches wide (you can just tape it to the edge of your bench) – and liberally apply any metal polish (Mother’s mag polish, or whatever you have). Rub it in well with a gloved finger and allow it to dry. When dry, work it with the spine of a razor or the shank of a screwdriver to push the larger particles into the weave. Strop the razor for 30 laps on this denim strop.
For maximum keenness, follow with a leather strop with 0.25 micron diamond or CBN, again about 30 laps. Finally, strop on your clean leather strop.
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Todd, thank you for publishing this excellent blog. Your articles are well written and so informative with your SEM pictures. Knife sharpening is only a hobby for me but your articles remind me of similar studies I am following at my job regarding grinding techniques for sintered materials using CBN and diamond superabrasives. Great stuff.
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Great place & posts. I am a member over at SRP but this is a nice change of pace!
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Great work! Thank you so much for using scientific method to unravel the mysteries of the razor sharpening arts.
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Hi Todd
I am new to straight razors I love your science based approach
Would you suggest at a 4K level on which synethtic stone would you recommend? From what I gather I can go from this to a coticule hone? What is the best way then to test the edge?
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I prefer a 4k Shapton glass stone, although there is nothing special about it’s performance, the ability to see the black swarf on it’s white surface is very helpful. If you follow with a Coticule, use a very thin slurry or none at all, and do not lap the stone. You would need to strop the razor before testing the edge. With experience, arm hair or a hanging hair can be a good predictor, but just shaving with it is more reliable.
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Sword polishing is something that fascinates me. Was wondering if there is a possibility of investigating the reason why a hadori polish appears to ‘whiten’ a blade using only JNATs ( komanagura, uchigumori, and hazuya stones) to achieve this effect.
Many blade smiths do a simple differential heat treating process and then use sandpaper, stones and then an acid etchant such as lemon juice or ferric chloride to bring out the hamon as well.
Seems both methods expose martensite crystals so white light is reflected.
I welcome anyone who help me better understand what’s happening with the stones or the etchants. Thanks in advance.
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The matte finish from JNats is usually a result of grit particles “rolling” between the stone and steel. These create a random texture to the surface of the metal and a diffuse reflection. The effectiveness of some JNats in achieving this finish seems to be due to the particular shape of the grit particles and their tendency to roll instead of plow. Understand that a series of parallel scratches producing the same microscopic roughness will appear shiny.
The matte finish also occurs when grit particles become embedded in the surface of the metal, essentially creating an anti-wear layer. This is very common with the matte finish produced by coarse, muddy stones.
Etching produces a rather different texture microscopically, although the reflection will still be diffuse. In general, etching will not only be selective to different materials (etching carbides at a slower rate to the matrix, say) but also materials etch preferentially in certain crystal directions, so that a different texture is produced on each grain surface. Again, the matte finish is the result of the random surface texture at micron scale.
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I want to try to sum up the other comment, with the chisel maker video (in the Index section of this blog).They said that tools that are made to hit something are not polished because polished surfaces tent to chip.https://www.youtube.com/watch?v=A5_bgQzOEvI
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Hi there,
I really like your work. Im in the business of sharp and have developed a few simple and quick tests to ensure my sharpness meets a minimum standard. I have been interested for a while now in correlating my tests with imaging to quantify sharpness. I have not decided wether to use your method, or scan a test cut made by a blade. A SEM is fantastic resolution, however, entirely impractical for productivity reasons in a commercial sense. So I have been considering utilising 3D optical microscopy. You did mentioned it is impossible to obtain the resolution required to quantify sharpness by using light. Do you consider that applies to optical microscopy? As it does seem that resolution is sub nano, scanning is incredibly quick with real time imaging and cost is 1/10th of a SEM. Not only that, analysis, ie slices, can be made of scans without needing to FIM an edge. Have you considered any potential use of this technology in quantifying sharpness?
kind regards
Stephen
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I’m not sure if you are also in the business of selling scientific equipment, but…
“3D optical microscopy” is essentially about building a composite image from individual optical micrographs. The resolution of these scans or “3D images” is still limited by the resolution of the optics.
(Keep in mind that a research-quality objective lens costs in the neighborhood of $10k).
The sales brochures of these systems will talk about “Z resolution” being sub-nanometer; however, the X-Y resolution is still about a half-micron for an ideal sample.
I don’t think it’s a reasonable argument to compare the purchase cost of these instruments, since SEM imaging can be outsourced for a tiny fraction of the cost of purchasing and maintaining an optical profiler.
As for studying the cut object at high magnification, I do agree that this can tell us a great deal about the blade performance.
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thanks for your reply. Im not selling scientific equipment…im looking at purchasing equipment to quantify the blades I make. I worked in marine biology and spent many years using microscopes. They are an essential quantification tool.
I agree 3d microscopy has limitations since light is split and recombined in the objective lens and requires scanning technolog. Thanks for helping me rule that out. Now Im left with holographic technology…or DHM…have you investigated thawasn’tffers sub nano z resolution with dynamic measurement over vertical ranges of nanos to 100s of microns…without scanning.
An auto stage would allow measuring a whole 100mm edge to 200 microns depth by stitching frames together. Also, holographic technology uses standard objectives…eliminating the need for expensive interferometric Michelson or Mirau objectives.
The other advantage is that holographic is 4d at upto 1000 fps. So technically we could observe and quantify effects of sharpening, cutting and degradation….in real time. Of course…making everything happen within 2mm of a lens presents challenges.
One other thing…I make edges from about 35 degrees to 65 degrees and they all have similar sharpness. So maybe keeness has little effect on sharpness? After looking at your images and my experience of sharp, I feel now, it has more to do with irregularities in the 3 micron radius of the edge…along the entire edge…regardless of angle. Maybe keeness has more to do with ability to utilise sharpness? I have made a 90 degree edge pass my sharper than razor blade test once.
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This is all really interesting. Thank you for taking the time to do so much research.
If you are looking for future subjects I’d be really interested to understand what is happening with a cabinet scraper when it is burnished.
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Hey Todd,
Have you ever contacted Roman Landes? He has a ton of old research papers on razor blades that he shared with me. Most of it is in German, but Larrin Thomas figured out a way to translate them with a smartphone and Google Translate. If you are interested, I can share the DropBox link with you, my email is jasonstone20@yahoo.com
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I could you comment on or recommend a site to address tree pruning tools?
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Hey Todd, could you send me an email?
Thank you.
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Hello Todd,
I stumbled over your articles 2 weeks ago. I printed them all out and read them all like a novel in 2 sessions over the WE. I am not knowledgeable enough to contribute with a meaningful comment or add something that would add any value. But I felt that the minimum I need to do, is to at least thank you for all the work you did and (more importantly) that you shared with the world, just like that! I wonder how much work this was/is and how much time you spent. Thanks a lot and very much for that!
Your articles sorted out the mess that was created in my head after listening for years to experts and reading experts’ statements in the internet. My favorite myth buster of your’s killed the stupid mantra that steeling does not remove metal, but simply re-aligns the edge, like helping an old lady up again, who stumbled. Booom! Myth gone.
Thank you very much again for all this and you know what they say: One guy, who writes is representing hundreds/thousands who did not.
All the best from Germany,
Niko
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Thank-you Niko. To be honest, this is just something I work on only very occasionally, but I suppose over 7 years it has added up to a fair amount of work.
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Great and interesting content. Thanks for doing this.
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Super interesting pictures and articles. Thumbs up!
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Hello,
visit your site regularly, great the scientific approach to a sharp knife. I work in the meat industry myself and the art of keeping the knife sharp all day long fascinates me. So much happens (right or wrong) when using a sharpening steel. Question Is it possible that if the knife still has a burr after sharpening and you use the sharpening steel you re-align the burr …
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Typically you would use a steel on a knife that has been dulled and if you use the steel at a higher angle than the bevel, the contact area is very small and therefore the pressure is high enough to remove steel via adhesive wear and form a micro-bevel.
If there is a burr that is thick enough to “roll” it isn’t usually possible to “stand up” that burr and “re-align” the edge in my experience – it will break instead. Maybe it can happen on softer steels, but that’s not something I’ve looked at.
If you have a thin burr, typical of what is created by sharpening a few passes on one side, and you steel at the bevel angle, then yes you can stand that burr back up and “re-align’
In general, steeling is very problematic with low angles and keen edges – because the angle is so low the apex can bend without breaking. What I would recommend is to cut into the stone to remove the apex/burr and then steel at a slightly higher angle to make a proper micro-bevel.
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I am a following this page since I started learning to hone my razor and I found all these informations extremely useful. Thumbs down to Keith Johnson from Tomo-Nagura for trashing this man and his work in his video Jnats for Dummies.
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Hey, can you try sharpening a blade with a focused ion beam?
maybe one regular razor steel then maybe maxamet or similar, and then try out the longevity vs stones.
just for fun. or for science.
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Yes, but the cost would be in the vicinity of $50k per blade, if you want to start a collection.
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Hi,
Any insight into how much of the finishing process applies to small knives(kitchen, woodworking, etc.) Do you think denim strops would be helpful before leather with edge tools less sharp and keen than a straight razor? From your images I am worried that by honing and using only a leather strop there would still be a burr. I will experiment with denim as well as certain number of edge trailing strokes at the end of the hone but curious if you had any insight yourself.
Thank you for all work, I have just recently found this website in the last few days and have been fascinated.
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It is a complex question because there are several different types of burrs – most cannot be removed by stropping on clean leather (or cutting into wood or felt), some can be broken off with leather stropping if they are folded over first. Metal polish on denim or cardboard seems to be a very easy and effective way to remove a burr. I would generally follow it with either a leather strop for keenness or a stone for more slicing aggression.
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Amazing work here. How in the world do you cut knife edges to see their cross sections? And how do you polish the cross sections? You must have some very special tools. Do they only work at micron scale or could you polish a whole knife edge the same way?
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