12.2.4 Sharpness (2024)

12.2.4 Sharpness

Defining and Measuring Sharpness Your sword or knife that's just lying there is either sharp or blunt. So sharpness is a static property. Well, yes, but there are properties closely related to sharpness that are more dynamic:

• Retaining (or loosing) sharpness while using the blade.
• Reconstituting sharpness after it was lost.

And now I have opened a rather large can of exceedingly squiggly worms! Let me make one thing very clear right away: All I can give you is a little "theory" of sharpness and retaining same. But that is not overly helpful for sharpening a blade. It is a bit like playing the piano (or any other musical instruments): Knowing all about the theory of musical notation and how that transfers into hitting the right key the right way at the right time, will not a piano player make. And the top players (who do certainly know the theory) don't know exactly why they are somewhat better at it then the second (still very good) tier of players.
Some top experts can sharpen your sword better than "normal" experts but nobody knows what, exactly, they do differently. That's why sharpening a blade by hand is still an art. Sharpening blades by machines is different. The razor blades you buy are all extremely sharp (even so there are some differences between brands) and come straight from a machine. For reasons not all that clear to me, the concept of sharpness did not receive much scientific attention until quite recently. References 1 and 2 (freely available in the Net) give examples of recent papers dedicated to the subject; their literature lists will lead you on if you like scientific fights and heavy math. What I learned from perusing some more publications is that there is no general agreement on how to define and measure sharpness. Greatly simplified, two basic ways of defining the sharpness of a given blade by a number are pursued:

1. Sharpness relates to the geometry of the blade, in the most simple case it relates to the inverse of the radius of curvature of the edge.
2. Sharpness relates to the performance of the blade, e.g. how deep it cuts into a standard substrate for a given force pushing it down.

The picture below gives an idea of how one could relate sharpness to the radius of curvature of a blade:

Sharpness demands a small radius of curvature at the blade edge

Sharpness sort of begins at a radius of curvature of a few micrometers (µm). If you want "razor-sharp", you need to do better: A radius of 0.01 µm (=10 nm) is a good number then. The limit, of course, is the size of an atom (imagine the circle in the picture to be an atom), giving a radius of about 0.0001 µm or 0.1 nm. That would be a more than 10.000 fold improvement on sharpness relative to a 1 µm radius.
I'm not sure if anybody has made a length of blade "atomically" sharp. But one-atom tips are common goods in "scanning tunneling microscopy" or STM. But is it only the radius of curvature that determines sharpness? Of course not, consider the next two pictures:

Blades with identical radius of curvature but different shapes

Ideal and real edge

Not much needs to be said. The upper picture shows blades with the same radius of curvature but different blade geometries, Would they all be of identical perceived sharpness? Probably not - but it always depends of what you have in mind. Cutting hairs close to the skin without cutting the flesh certainly would profit from an optimized blade geometry like the one on the left. A meat cleaver wouldn't do so well with this shape, though.
A more severe problem, however, results from the fact that most likely the geometry changes as you move along the blade. The radius of curvatures will not be the same at every point, the edge is not perfectly straight, and so on. My drawing skills cannot do justice to that but you get the idea. Irregularities along the blade are probably not so good for cutting straight into something by only pressing the blade down but might give better results compared to the "ideal" blade if you start "sawing". Saws do not have teeth just for looks, after all. To conclude:

1. The (average) radius of curvature of your blade is not a unique and precise measure of the sharpness of your blade. But the trend is clear: A smaller radius of curvature will tend to increase the sharpness.
2. The (average) radius of curvature of your blade is not a convenient indication for the sharpness because it is difficult to measure. Cut your blade and look at the cross-section in a light microscope? Won't work, you need far higher resolution than what a light microscope has to offer. You need a (scanning) electron microscope! Sharpness is nanoscience!
3. Getting numbers for the radius of curvature thus is possible but not convenient.

Indeed, if you look for high-magnification pictures of blade cross-sections in the Net, you won't find many, if any - as long as you do not hit on the pages of "scienceofsharp". This site features many excellent pictures that were taken in a "scanning electron microscope" (SEM); here are a few:

Rather good edge (left), and a somewhat crumbly one (right)

Source (for all SEM pictures here): from the scienceofsharp web page Whoever you are (the site doesn't reveal the maker), thanks a lot for sharing!

About as sharp as it can get

It's not easy to obtain an edge like that! Don't ask me how to do it! Consult the page I mentioned.

Retaining Sharpness It's difficult to produce a sharp edge but it is impossible to retain a sharp edge if you use your blade frequently. What causes an edge to blunt, and how does that happen in detail? Rather tough questions, in particular the second one.
If you want answers to the detailed mechanisms of blunting, you need to look at the blunted blade with a high-powered electron microscope once more. That's not for everybody to do, and if you want pictures I must refer you to the the scienceofsharp site once more. Or even better, the article of our old acquaintance, John D. Verhoeven ³⁾ who has written an extensive article with many (SEM) pictures about the subject. The first question is easier to tackle - at least up to a point. All of us know one sure way of blunting a blade: Use it on something harder than the edge of the blade. Hit a decent stone with most blades and they are now definitely dull - if not fractured, dent and bend What happens is quite simple in principle. During impact (slow or fast) stress builds up on the blade edge and on the regions of the target that is hit by the edge. Hardness essentially measures the stress needed to induce plastic deformation (the yield stress) or, more loosely speaking, the onset of local cracking, and the softer material will "give" first, deforming in some way and thus blunting itself.
Here are a few pictures showing what could happen:

Razor edge dulled by pulling it "sideways" over glass

No surprise here. We just bend the edge by plastic deformation. This can be reversed to some extent by "stropping" because the sharp edge is still there. You "only" need to bend it back.

Edge dulled by drawing it across the lip of a glass beaker

Here we have a bit of bending but mostly deformation by compression and "filing" or abrasion, resulting in a blunt edge. Glass is just quite a bit harder than most steels and thus acts as the file; the softer steel will be the filée The hardness of a material is a reasonable well defined property, I have gone through that. Below are some old examples:

Metals	Vickers Hardness		Ceramics	Vickers Hardness
Tin (Sn)	5		Limestone	250
Aluminum (Al)	25		Magnesia (MgO)	500
Gold (Au)	35		Window glas	550
Copper (Cu)	40		Granite	850
Pure iron (Fe)	80		Quartz (SiO2)	1200
Good tin bronze (Cu + 10% Sn)	220		"China" (Mostly Al2O3)	2500
Mild steel	140		Tungstencarbide (WC)	2500
Hardened steel (extreme)	900
Polymers
Polypropylene	7		Polyvinylchloride (PVC)	16
Polycarbonate	14		Epoxy	45

This table makes clear why our ancient forebears were reluctant to embrace early iron technology, considering that they had marvellous bronze blades that were generally superior to blades made from wrought iron or mild steel. It also makes clear why case-hardening the edge of a steel blade by quenching makes all the difference. You might end up with an edge that could, in principle, cut glass or granite! However, the first law of economics still applies! You pay dearly because there are plenty of problems, too:

1. You need good and hom*ogeneous carbon steel to start from.
2. You can re-sharpen your edge only a few times (if at all) because you quickly wear off the thin layer of hard martensite.
3. Your edge is rather brittle and chips easily.

The Japanese sword demonstrates what it takes to make the best out of extreme edge hardening while not yet in possession of superior modern steel that was liquid once and can be cast. Now to the trickier points of blunting a blade. All of us know that our kitchen knifes will eventually become dull even if we never ever try to cut anything hard! One of the key words hear is "wear" and with that you open the door to hell. I'm not going through it. I'll just show two pictures demonstrating what can happen:

Formerly sharp (and hard) knife blade after cutting about 7 m of heavy (but soft) cardboard

Razor edge after cutting a few cm of bond paper

Paper is normally considered to be much softer than hard steel. But "steter Tropfen höhlt den Stein" (constant dripping wears the stone) as the Germans know, and the wear of the steel cylinders of rotary presses (used, e.g. for your newspaper) caused by their exposure to "soft" paper is a major issue in technology. If you want to know more than that, you are best of by reading the article of Verhoeven and colleagues about wear of steel blades 4). Here is the abstract:
.

A study is presented on the relative wear rates of two carbon steels, a Damascus (wootz) steel and a stainless steel, using the Cutlery and Allied Trades Research Association (CATRA) of Sheffield England cutting test machine. The carbon steels and stainless steel were heat treated to produce a fine array of carbides in a martensite matrix. Tests were done at hardness values of HRC=41 and 61. At HRC=61 the stainless steel had slightly superior cutting performance over the carbon steels, while at HRC=41 the Damascus steel had slightly superior cutting performance.

¹⁾ C: T. McCarhty, M.Hussey, and M. D.Gilchrist: "On the sharpness of straight edge blades in cutting soft solids: Part I - indentation experiments", Engineering Fracture Mechanics, Vol 74 (2007) p. 2205 -2224
Available in the Net ²⁾ P. Stahle, A. Spagnoli, and M. Terzano: "On the fracture process of cutting", Procedia Structural Integrity, Vol. 3 (2017) P. 468 - 476 ³⁾ John D. Verhoeven: Experiments on Knife Sharpening
Directly published in the Net ⁴⁾ John D. Verhoeven, Alfred H. Pendray, Howard F. Clark: "Wear tests of steel knife blades"Wear, 265 (2008) pp 1093 – 1099