Thread: Explaining the thumb test

Originally Posted by honedright

Joe,

Maybe we are talking about the same thing, and maybe not, but...

I have been using the thumb test for over 20 years, and when I run the ball of my thumb lightly along, parallel to, not across perpendicular to, a keen edge immediately after stropping, it does "stick" and that is exactly the sensation you should be looking for. Yes, it would slice my thumb were I to use more pressure and/or draw my thumb along the edge in a single movement.

Scott

And I'm saying it shouldn't along the length of a keen edge. In it's cutting direction, a blade will stall a lot if it has chips or a wire edge. A keen edge, however, should feel perfectly smooth, as smooth as the bevel. The termination of the striations forms the microserrations. I've never done it and I never will, so I can't tell you from experience, but I'm wondering what could grab grazing skin along a keen edge that wouldn't grab a whisker rather than cut it. When I move my thumb lightly along the bevel it feels smooth.

Where did you find your test? Is it described anywere?

Joe,

To do the thumb test correctly, you do run the thumb pad along the cutting edge, not the side of the edge, or bevel.

Any defects in the edge will be detected and corrected by doing the thumb nail test while honing, so nicks, wire edge, etc, shouldn't be a factor in doing the thumb test after stropping.

I'm not advocating that anyone do this if they feel uncomfortable with it, but this is the way it's done.

My source was and is, "Standard Textbook of Barbering", Associated Master Barbers and Beauticians of America, 1950, p. 25 (the thumb test was also demonstrated to me by a barber once, along with honing and stropping).

Scott

Outside of razors, I've sharpened knives for a whole bunch of years, starting when I was just a wee sapling. No telling how many of my dad's knives I ruined trying to learn. As I got older, I got more into the "theory" of blade sharpening, to learn why certain things with knives, stones, and other abrasives do what they do. Why, for example, an Arkansas stone cuts they way it does...why certain stones produce better edges faster, etc. The one thing I've learned is that if you take 50 knives, with 50 different people, all of whom were taught to sharpen by the same guy, you'll still get 50 different results and 50 different techniques. Same with the tests. Everyone adapts what they learn to their own style of doing things. Hence, in this example, there's about 50 different ways to skin the same cat. All the tests are in my arsenal, but the arm (now leg, since my right arm is bald ) hair test and shave are the most consistent for me. I'll use the others when a blade acts "funny" and doesn't give me the expected results, to focus my efforts and point me in the direction I oughta be going. Every test imaginable should be learned, because some day, that test will be the only thing than answers the question a particular blade asks. You might use it seldom or never, but it's there if you need it. The point is, use the "common" understanding of a test as a basis, marry it with experience, and you'll figure out how to make that test work for you to achieve the results you want.

honedright, Joe,
I have done the thumb test both ways, ie the passing across the edge way and the cutting into my thumb way.
The latter works imho because when you pull a blade with just its own weight over your thumb, the not-so-keen one will more or less slide along easily whereas the keen one will cut into the skin which produces a drag on the sides of the edge.
However, I had the feeling that this method is not sensitive enough to differentiate between keen and super-keen. Maybe I gave up too early. Also, while the cuts to the skin are fairly harmless, if you do this test a lot (eg honing multiple razors) your skin will get cut up and you will feel it when handling salt, alcohol, hot water etc.

The 'Joe' method is what I'm trying to learn right now. Again, I'm not too happy with my success rate in predicting the shaving performance of a particular blade.
I guess it's practice, practice, practice...

Redwoood

Originally Posted by Redwoood

The 'Joe' method is what I'm trying to learn right now. Again, I'm not too happy with my success rate in predicting the shaving performance of a particular blade.

Practice with a fresh single edged blade to calibrate your thumb. You'll notice that it not only grabs your thumb but you can actually feel a thin edge. That's the difference between sharp and keen, and you can feel it happen. When your last abrasive stops showing improvement you've done your best with that razor. Continuing may still give you some improvement but you'll start getting diminishing returns.

The danger of overhoning with your finest grit (assuming you go beyond 8K) is slim, but why waste your time.

What does an overhoned edge feel like?

Redwoood,

Back in the days when barbers were shaving customers regularly, they probably didn't use too many razors, maybe 2-3 at most. Also, the thumb test was used primarily after the first round of stropping after honing. Once the propper cutting edge was achieved, it didn't, and still doesn't, require more than a good repeat stropping to keep the edge keen for a long time. So I imagine they didn't cut up their thumbs too much, if at all.

As far as the feel, it is very subtle. It says in the barbering text book that it takes years to become proficient at the thumb test. There is a certain quality of the "drag" or "draw" that will tell an experienced operator whether or not the edge is ready. So it's not just a simple matter of the edge drawing or dragging on the skin, but also a specific feel, not just any feel, as it draws. It takes experience to differentiate what the feel should be.

Suggestion: Carefully take apart a disposable razor, and again carefully do the thumb test on the factory sharpened blade. This will give you an idea of what to shoot for.

Scott

Originally Posted by honedright

Back in the days when barbers were shaving customers regularly, they probably didn't use too many razors, maybe 2-3 at most. Also, the thumb test was used primarily after the first round of stropping after honing.

My barber had a lot more than that many razors. He had to be able to send some out for honing, and sometimes he switched razors during a shave or just put one away.

The one he put away would get refreshed at some point. He seemed to do a lot by feel, like knowing when to stop honing and how much to strop. Sometimes he stropped mid shave and I'm sure it had to do with the feel of the razor on the face. I got some shaves from him when I was old enough and his hand was rock steady despite his age. He hjad such a light touch that I never felt the razor. His one pass shave was as good as my 3.

I agree with what you say about the thumb test coming after stropping. He usually did it after changing razors. Maybe the difference in feel between the razors made him uncertain.

Frankly, I don't understand why it was done after stropping on a plain strop. The most pronounced effect of stropping is when you do it before shaving. If you don't the razor doesn't feel nearly as sharp. That I understand, because you're realiogning the microserrations, narrowing the edge. When you hone, you're narrowing the edge by removing material. It would seem to be especially effective if you honed first. But once the dge is narrowed by removing materal, it's not likely that stropping will narrow it any further. All it can do is polish and smooth the bevel. This is confirmed by the fact that we're told not to use linnen after honing. So, the benefit of stropping after honing is only polishing. This is confirmed by prof. Vanderhoeven's experiments. WHich showed that only stropping with paste affected sharpness.

The lesson this taught me is that if I strop with a very fine paste, like .25, plain leather stropping after that doesn't improve anything.

Originally Posted by Joe Lerch

[...]
This is confirmed by the fact that we're told not to use linnen after honing.

I never understood that. I like to think that the abrasive material from the hone and the metal particles that have been abraded away could still be present on the razor. And I don't want that stuff to get on my leather strop. So I strop on the linen side to 'clean' it. Of course those particles would now be embedded on the linen side, but I guess linen can be washed.

So what's the reasoning behind not using the linen side after honing?

BTW, I may get stoned for saying this, but the reason for testing the razor after stropping it may be because stropping aligns the burr. I don't think you can create a blade with such a thin/acute edge without creating a somewhat instable edge. What we try to do is minimize the burr when honing, but imho you can't eliminate it completely.
So just like steeling a kitchen knife, stropping aligns the small burr, which is what actually cuts through the whiskers. Now, this is not a huge thing, flip flopping back and forth, but it does get bent when it contacts the whiskers. Hence, we need to strop again to realign it and that's when you can only feel the real sharpness of your edge.

Redwoood

Originally Posted by honedright

My source was and is, "Standard Textbook of Barbering", Associated Master Barbers and Beauticians of America, 1950, p. 25

In the 1931 version it's page 27 and Fig. 24. If it was understandable, we wouldn't be having this discussion. Touching lightly along the edge (without specifying movement), which covers both approaches. It then says a dull edge will not stick or draw. Touching head on would cover neither and couldn't produce any kind of sticking. What does "draw" mean?

Classicshaving.com credits the barber manual and quotes the language, but then says:
"A razor that has the proper cutting edge tends to stick to the thumb and will not slide along it."
Again, if you're saying a keen edge sticks, it covers both.

My only other evidence is the now dead old time barber I mentioned above. THe guy would be well into his 90s by now. When I saw him do it I saw the blade move sideways, and I couldn't see the thumb slide along the edge.

Although it would be nice to have a definitive explanation, I'm content not to do movements that would cause my thumb to be sliced. Even with the sideways movement if you unconsciouslt angle the blade you can feel a slight cut being made.