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Smoothness, sharpness and the HHT


Well-Known Member
I've noticed several posts questioning whether a razor that's properly honed on a coticule will be sharp enough to shave with, so I wanted to put down some thoughts I had on various aspects of edge quality.

A. Absolute sharpness
Absolute sharpness (as distinct from apparent sharpness) comes from getting as fine an edge as possible on your blade. This could be a global or local phenomenon. Ideally, you have the same sharpness over the length of your blade.

Absolute sharpness increases when you hone with finer grit abrasives. If you go from 12K to 16K to 30K honing surfaces (hones, pastes, or film), the edge will get progressively sharper. However, as Bart and several other members have mentioned, this doesn't mean that the progressively sharper edge is more ideal for the intended purpose (shaving whiskers on your face).

As Bart said elsewhere, if you finish your blade on 0.1 micron diamond film, the edge will be sharper than you can get on a coticule. The problem with honing on 0.1 micron diamond film, of course, is that the edge is so fine that it cannot stand up to shaving and will crumble - giving rise to a non-smooth edge that is uncomfortable to shave with (I speak from experience).

As a blade gets more absolutely sharp, it also gets more apparently sharp. However, when honing, we're blind to absolute sharpness (for the most part), and we test for apparent sharpness.....

B. Apparent sharpness (and smoothness).
The most talked-about test for sharpness is the HHT. Some say that the TPT in the hands of an experienced honer is better, but most people do the HHT.

The HHT is a test of apparent sharpness. Let me explain what I mean:

When cutting anything (a tomato, or your hair, for example) we're combining pressure and a sharp edge. Let's look at cutting a tomato in more detail.

As anyone who does any cooking knows: it really is necessary to have a sharp knife when you're slicing ripe tomatoes. Serrated blades are messy, and blunt blades are pure hell. So what you want is a very sharp, somewhat smooth edge. The sharp edge, combined with pressure, cuts through the skin of the tomato, without deforming (and squishing) it. Once the cut is started it's a lot easier to keep cutting. The trouble point is the initial breaking of the skin on each slice.

If you took even a relatively blunt needle and tried to stick it into a tomato, it would break the skin quite easily. This is because the pressure (force/unit area) exerted by the point of the needle is very high. When you use a knife that pressure is spread out over the length of the blade, and if your knife was blunt you'd have to use more force to get the skin broken, which would result in a squished tomato. ;)

The HHT (and shaving, to some extent) has the same factors influencing it. The hair takes the place of the tomato, and the blade is now a razor.

A blade with an uneven (micro-serrated) edge is essentially made up of an array of very tiny needle points. These points (teeth) find it easier to pierce a hair than a perfectly flat edge would because, at equal applied force, the pressure exerted at the point of each tooth is higher than that spread over an even edge. So, a toothed edge that's not ideally sharp could pass the HHT better than a sharper smooth edge.

NOTE: A sharp, toothed edge is most likely to 'catch-and-cut' a hair, rather than smoothly cut it. The point of the tooth catches the hair (like a needle piercing a tomato), and it then proceeds to rip the hair.

A razor with a micro-toothed edge is not ideal to shave with (even if it is sharp), as teeth are indiscriminate in what they cut (


What we ideally want is an edge that is sharp, sturdy, and smooth.

1. Sharp because it does have to cut hair, after all.
2. Sturdy because it has to cut several thousand hairs during a shave (over several passes) without deteriorating, and hopefully it should last for a lot of shaves.
3. Smooth because all that hair is on our skin (which we'd rather not have damaged).

This is where you try to balance these factors. A very sharp edge (say, off 0.1 micron diamond film) will pass the first criterion, but will fail Nos. 2 and 3. A blunt edge will pass Nos. 2 and 3, but fail at No. 1. A well-honed edge from badly-tempered steel will pass Nos. 1 and 3, but fail at No. 2.

There are many ways to get to the ideal end point where all three criteria are met. The coticule is one of them.

I've found that a coticule is one of the easiest hones on which to get all three criteria met repeatedly. Properly used, it produces edges that are sharp enough to cut hair easily, while it it next to impossible to over-hone the edges, and the edges are smooth - preferring to cut hair rather than skin.

If I want sharper edges, I can use my Japanese natural hones. These produce smooth, very sharp edges. However, you can over-hone on them. As the slurry breaks down on a japanese stone, it gets finer and finer, and can give rise to over-sharp edges.

NOTE TWO: All edges (even off a coticule) have micro-teeth. The point is to get to a stage at which they're so small that they don't matter.

The take-home message from this over-long post is this: What we're trying to do when honing a razor is to meet the three requirements of sharpness, sturdiness, and smoothness (the three S's). Exceeding these requirements is cool but not necessarily conducive to a great shave.

NOTE THREE: None of what I've said here is new. All these speculations are available elsewhere. :blush:


Well-Known Member
There's one obvious corollary to the whole 'effect of pressure on cutting hair' discussion:

The force applied on the blade while shaving is expended in applying pressure at the 'catch points' - the hairs that are encountered by the blade.

So, if you have a thin growth of stiff hair, you'd be able to cut the hairs easier than if you had a thick growth of stiff hair - because the force is distributed over more hair in the thicker growth, equating with lower pressure applied.

This explains why everyone has a harder time shaving around the chin (where your hair growth is denser).

Fine hairs give more, and therefore require a sharper blade.

Thick hairs, and very dense growth, benefit from a heavier blade because the mass of the blade helps maintain the force as you encounter the hairs.


Well-Known Member
That was a pleasure to read... I still think it's specious to say a heavier grind is better for thick beards because a Full Hollow can stand up to the thickest beards if it's honed well (and has good temper, steel, etc. :rolleyes: )

But the OP was excellent :thumbup:


Well-Known Member
richmondesi said:
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Actually, I completely agree with that. I was going to add a rider about how a well-tempered, very well honed full hollow would shave a heavy beard just as well as a heavier grind. Had to go shower and shave, though. :thumbup:

The reason I didn't add it originally was that I don't believe that there's been more than anecdotal evidence that a full hollow with those qualities can stand up to sustained shaving of a heavy beard (over a period of months). There may be such evidence out there; it's just that I'm not aware of it.

Still, the rationale holds anyway. :)

Actually, I just shaved with a 4/8" full-hollow, and I have thick, coarse facial hair. ;)


Well-Known Member
It's a great read. I always loved to hypothesize about the physics of sharpening.
You make many good points, but you need to be careful with making deductions too easily.
I don't claim to have any more answers that you, but there are a few issues I know not to be true, and other not necessarily to be true. I'm going to add a few remarks to your text, I hope you don't mind.
yohannrjm said:
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If I understand you correctly, you're talking about sharpness in the sense of the thickness of the line at which both bevel faces meet each other. I prefer to use the term "keenness" for that. I reserve the term "sharpness" for the perceived performance of an edge, what you call "apparent sharpness".
yohannrjm said:
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A good enough description for the practical approach of sharpening, but not good enough to serve as a base for a discussion about the finer physical points. Keenness (I founded the website, so I'm sticking with my terms.:p ) increases when the depth of the abrasion reduces. Using smaller abrasives is one contributing factor, but the penetration depth of the particles is at least equally important. Often overlooked, but it can make all the difference for the quality of an edge.

yohannrjm said:
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A very good point. And one that deserve elaboration. Not exactly my chosen field of action, but in the world of synthetic honing, there are several options, for improving the results, hardly explored. At least, not by amateur sharpeneers. The ultra-thin bevel apexes produced by the finest man-made abrasives are too weak to withstand the stresses they need to undergo during a shave. But what if we would considerably augment the bevel angle? Many disposable commercial razor blades carry quite steep secondary and even tertiary bevels. The manufacturers do apply science to their production methods.
yohannrjm said:
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My underlining. We should not waste or time with statements made with pubertal motives. Both test are valuable. If someone wants to say one is better than the other, that he comes up with verifiable arguments. "It's better because it's better" doesn't work for me. From a didactic viewpoint, I like the HHT, because I can explain how to standardize the test and calibrate the readings. It gives me a better clue what kind of advice to offer, then when someone would tell me "I honed my razor and the TPT felt like molted sugar, but not quite like my professionally honed Triple Goose, that feels like Maple syrup". But I don't doubt that the TPT works equally well on a personal level. It's just not as easy to communicate about.

yohannrjm said:
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A well chosen metaphor. :thumbup:
yohannrjm said:
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I have a Nakayama, and have had the opportunity to thoroughly test an Asagi. Your statements surprise me. I haven't been able to over hone on either of these hones. Also your statement about sharpness (keenness???) will probably have a hard time surviving a series of blind tests. I keep reading these claims about sharpness, the more expensive the hones usually the louder the claims, but so far, I have not yet been able to reproduce significant differences in keenness between a whole bunch of Coticules, both Japanese hones and an Escher I only briefly tested. On the same note, I think that if you would put, let's say a Norton 8K, into the hands of a diverse group of honers, and you would use a SEM to measure the apex widths on the edges theses gentlemen produce, I 'm willing to bet you would find a serious spread in final keennesses. I'm also willing to bet that someone who takes the time to really master his honing setup will produce outstanding edges, whether he uses a Coticule, a Japanese natural with different naguras, a setup of synthetic hones, the often disregarded Chinese hone, or whatever. I can put an edge on a razor with just a DMT-E, a bit of tape and some Chromium Oxide, that would offer a serious challenge to anyone, myself included, for improving this edge beyond the realm of the self-fulfilling prophecy.
Therefor, I believe, discussions about honing, must not loose track in ongoing comparisons about which hone cuts keenest.

yohannrjm said:
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See my point above.

yohannrjm said:
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That is an excellent conclusion.
In spite of my little rant at the end, I really like your thinking, and I believe we're certainly in the same book, albeit not always on the same page.:)

Kind regards,


Well-Known Member
Wonderful Thread with some excellent post gentlemen :thumbup:

the 1 thing that I have proved to myself time and time again, and Sir Bart makes this point elsewhere, is that the Coticule smoothness comes built in, as the sharpness (Keenness) improves the smoothness keeps up with it, I have never found that I can produce an edge thats scary sharp without being super smooth too, baring of course any chips etc. that have been overlooked.


Best regards
Ralfson (Dr)


Well-Known Member

Your points are well taken (and well-made, as well :thumbup: ). Thanks for pointing out several issues with my statements.

I usually start by mentioning that my observations in posts like these are not gospel, but rather a summary of my results in ongoing tests --- subject to change.

For some reason, I forgot to mention that this time.

The only point at which our results/opinions differ is at this one:

Bart said:
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Your last statement is completely correct. That was not the intention of my statements regarding the edges off my Japanese naturals.

I'll be the first to state that my experience with coticules doesn't match some of the people on this site. So perhaps it is possible to get more sharpness/keenness on a coticule than I can get. However, the edges I get off a coticule are perfect for shaving (for me at least). So there's no chauvinism on my part regarding my Japanese naturals.

I do have more experience honing razors on my Japanese naturals than on my coticules, so I'm simply stating fact that, in my case, I can get seemingly sharper/keener edges off my Asagi than I can get off my coticules. They're about the same as far as shaving efficacy. In some cases, I did get edge deterioration after the shave, so, from that observation (and the subsequent microscopic examination) I concluded that the edges were over-honed to start with. There was no burr to begin with, so that's out as a contributory factor.

It's been a long time since I've got an over-honed edge on any hone, but I've never been able to do that on a coticule (even when I started out)....which is why I think it's an ideal hone.

Your statement about the relationship between experience on a certain hone and the keenness/sharpness obtained is a good one, and is probably the contributing factor in this case.

Still, I can get a little more sharpness off my asagi than I get off my coticules - in my case this is fact. I just don't think that extra sharpness over the coticule is necessary, for the most part, but it's there if I want it. Is it a 'self fulfilling prophecy'? - Possibly, but I'll take that. :lol: