Thread: Thumb test -- how does it work?

I finally got up the guts to test my blade with a thumb test. On a sharp razor it definitely feels like it's sticking. On a dull razor it just glides. Amazing, just as described in the forums and no cuts.

But, how does this work? What is happening that makes the edge stick and what makes it glide?

I think the razor catches on the top layer of skin. If you look close at your thumb it should have a cut in the first layer of skin after "passing".

I assume you are running it against the blade, not down it?

What happens is that with a sharp blade, the blade actually penetrates the skin slightly (with next to zero pressure) thus you're basically scraping some cells off (like zesting a piece of citrus) hence, some drag. Sometimes it will leave a slice in a layer of skin, but with enough practice it won't. As even layers of skin have depth to them many (hundreds, thousands, I dunno) cells thick. Dull blades don't penetrate, so your only resistance is friction... which on the extremely small surface area of a razor's edge, is almost nonexistent. Of course on the other end of the spectrum, a jagged or dinged edge will have a LOT of friction... so a poorly maintained edge will drag too, but you'll be able to feel the difference (go to a grinders shop, feel the edge of a rough ground knife before he finishes it on finer stones... that's what a jagged edge feels like).

Originally Posted by IanS

I assume you are running it against the blade, not down it?

That's actually the way to TPT a kitchen knife.
A razor does get TPT-ed with gentle, very short slicing motions along the blade. The edge sinks a bit into the skin (without completely slicing through it) and you can feel how that adds a slight draw to the sensation.

I wouldn't pluck the edge of a razor sideways.

The next picture shows and edge that once accidentally got plucked by my thumb while closing the razor with one hand. It didn't cut me.

Congrats, first off

And yes, like AFDavis said, it's the razor slicing into the top layer of skin. Congrats on figuring the TPT out, it's very useful

Originally Posted by Bart

That's actually the way to TPT a kitchen knife.
A razor does get TPT-ed with gentle, very short slicing motions along the blade. The edge sinks a bit into the skin (without completely slicing through it) and you can feel how that adds a slight draw to the sensation.

I wouldn't pluck the edge of a razor sideways.

The next picture shows and edge that once accidentally got plucked by my thumb while closing the razor with one hand. It didn't cut me.

That's a great explanation, Bart. I'd wondered if it was along the edge or sideways (I've always just done it along the edge because I worried about damaging the edge by plucking). Now I know. Thanks!

s.

Ahh, good to know

Well, I kinda understand. I'm running my thumb along the blade, not plucking against it. All the videos show running your thumb "along" the edge. Still, I don't see any damage to my thumb. Maybe I'll have to dig out my scope.

Explain why I have to lick my thumb? What does the moisture do?

The Wiki describes the TPT differently.

Originally Posted by richmondesi

The Wiki describes the TPT differently.

Yes, I am aware of that.