Thread: Explaining the thumb test

Originally Posted by honedright

The traditional thumb test to evaluate shave readiness is as follows:

Hold razor fully opened by the handle with point "pointing" towards the ceiling. Lightly place moistened thumb at heel of razor edge, first 2 fingers on spine, and lightly move the thumb, not the razor, move the thumb lightly along the edge towards the point. The thumb should "stick" or bind along the edge. This is because the razor is actually slightly cutting into the outter layer of your skin. When the skin binds, stop, lift thumb and replace higher up the edge and repeat until you cover the entire edge of the razor. If the thumb glides smoothly along the edge without a "sticky" feel, the edge is not cutting into the skin and is not yet sharp enough to shave.

I can't emphasize enough that this test is done with a light touch, or you may do more cutting than intended. The fingers on the spine help control pressure and movement of the thumb.

Scott

Scott,
I'm not suggesting that you are wrong, but if this is the correct way to do the thumb test, then I've never done it correctly...nor do I intend to. It seems like the perfect recipe for disaster (or stitches), at least for me. I test by drawing my thumb lightly across--not along--the edge.

YMMV,
Ed

Originally Posted by joshearl

Sorry for the bad ASCI art. It's my first attempt.

Unfortunately, HTML loses the formatting unless you put it in a code block (which was designed for displaying computer code without losing the indentation). The forum software treats code blocks wierdly, but it does at least still honor the formatting.

Code:

             ----------------->         (motion of thumb)
                     |                  (edge)
                     |
                    ||
                    |||                 (spine)

I think we're making a very simple proceedure into a complex subject. The thumb test is just a matter of running a your thumb pad at an oblique angle to the blade. The way you hold the razor is immaterial. As you run you thumb over the edge you will feel a roughness and thats good. If its smooth its not good. Its as simple as that.

Originally Posted by thebigspendur

As you run you thumb over the edge you will feel a roughness and thats good. If its smooth its not good. Its as simple as that.

Ok, now I'm confused, when I do this I never feel anything I would describe as roughness. A dull blade slides smoothly across my thumb, but a sharp edge has a kind of stickiness to it. Are we seeing the same thing and just describing it differently?

When you run the thumb along the edge, the micro-serrations slightly bite into the superficial layer of skin and cause a "sticky" feel. It should be done lightly and not in one movement (that could result in a cut), but tiny, delicate "touches" along the entire blade with the thumb.

Scott

honedright, I think you are desribing exactly thte same thing as thebigspendur, but I also think you are doing the test wrong. You can run your thumb along the edge and get feedback that way, but I wouldn't. I get great feedback from a moist (not wet) thumb pad and a very light touchacross the edge at a slight angle, like I might be shaving my thumb pad, but at too great an angle. All the while I hold the blade stright up. This is just a tactile test to see how the blade is progressing. I do it frequently while honing, every few strokes to guage the progress.

X

STOP! The thumb should never be moved along the edge unless you're trying to reduce its size.

The thumb is oved across the edge. A dull blade will permit it to slide across, while a keen one will grab the thumb and move with it.

The thumb tilting can be done by simply bending the thumb. I added it to weed out the butterknives. It's not part of the traditional thumbnail test.

You can find a thorough description of both tests and what the different results mean at classicshaving.com, in the "how to ... and why" section where honing and stropping are described.

Traditionally, the thumbnail test is done while you're honing to form the edge. It tells you when to leave the hone and begin stropping. The thumb test is then performed as you hone to determine when the razor is sharp enough. This assumes that you use one hone and a leather strop.

I modified the tests and combined them into a system. Prof. Vanderhoeven found that leather stropping after honing has little effect. I found that other than smoothing the edge, it does very little, but a litle smoothing doesn't hurt.

My approach is that when I pick up the razor, I do a thumb test, and I'll touch up if the razor is reasonably sharp. If it fails, I go to the thumbnail test to determine what I need to do.

If you're forming a new edge, you come off the 4K when the razor passes the thumbnail test. That test has several ways to fail, and each negative result indicates a different problem (see classicshaving.com for details).

When you come off the 4K, the blade will already do reasonably well on the thumb test. The idea is to get it keen. I'll work the blade on the 8K and test frequently. You can feel the grab increase, and you can feel an edge forming and getting thinner. When I see little improvemnt in successive test, I move on to the next finer grit and do the same, and I'll continue that way until I finish the finest abrasive. When you pass the thumb test on the 8K (no further improvement), the edge may already be keen. If so (or if you've finished your finest grit), move to the leather strop and do about 15 round trips to smooth the edge.

Originally Posted by honedright

When you run the thumb along the edge, the micro-serrations slightly bite into the superficial layer of skin and cause a "sticky" feel. It should be done lightly and not in one movement (that could result in a cut), but tiny, delicate "touches" along the entire blade with the thumb.

If you run your thumb ALONG a keen edge, there should be no drag no matter how lightly you touch the blade. In fact, the keen edge will slice your thumb, even with a very light touch. After all, it will cut a hair on contact with no sliding movement.

For the present purposes you can ignore the existence of the microserrations. The edge can be visualized as perfectly smooth. If you honed with an 8K stone, there are 8000 microserrations per inch. A hair will span 20 microserrations. I think it's pretty clear that they're not about to grab anything moving along the edge but will cut it very neatly.

If you lihtly slide your thumb across a sharp edge, like a fresh single edged razor blade (good for practice) you'll find that the edge won't let the finger slide but will grab and move with it. That's what we're looking for. But it has to be very pronounced, and you must feel an edge. A blade will grab some long before it's keen.

Originally Posted by mparker762

Ok, now I'm confused, when I do this I never feel anything I would describe as roughness. A dull blade slides smoothly across my thumb, but a sharp edge has a kind of stickiness to it.

That's it, exactly.

Joe,

I have done the thumb nail test recently a few times with razors that did not shave comfortably any more. They still pass the thumb nail test though not the HHT. I wonder why we should need a TNT other than to assess a razor we never used before or possibly to check for overhoning. What are your thoughts about this?