Eli5: Why do shaving razor blades become dull?

Question

I saw a video once of how a razor blade starts getting dull after just one shave. It doesn’t seem like it should be the case if it’s literally just cutting hair.

In: 254

Answer 1

You may have noticed your kitchen knives dull too, even after cutting through soft things like vegetables. That’s because to have a sharp edge you need a very thin edge. At a microscopic level, the thinner the steel becomes, the easier it is to bend. The edge rolls in on itself just slightly, but it’s enough to be misaligned and no longer cut properly. That’s what happens when you notice the knife doesn’t “bite in” anymore, you struggle to pierce even tomatoes and such.

The quick and usual remedy for this, of course, is to use another knife or a honing rod and grind the edge of the dull knife across it. You may have seen chefs or waiters use this in restaurants. That tool isn’t actually hard enough to shave metal and sharpen the knife, and neither is another knife. Instead, what that does is it helps bend the rolled edge of the blade into position. If the knife is of good enough quality, it should restore most of its sharpness.

Now, imagine a blade that’s many times smaller and thinner. This same process happens. But you have no way to individually realign the edges of blades that are this small. And no, harder steel wouldn’t help here, because the blade is too thin, it would just chip. A rolled edge still cuts, but a chipped edge wouldn’t, and would most likely scrape your skin or even cut you. It’s preferable to have a softer edge that rolls instead of chipping.

Add into this the fact that human hair is actually pretty strong itself. Keratin, the stuff hair is made from, is among the toughest non-mineral structures out there, alongside chitin. Pound for pound, I’m pretty sure it’s stronger than steel. That is, a human hair would be stronger than a steel string of the same thickness. You can see how on a microscopic level this is a tough problem to solve.

Even electric shavers need their blades replaced every 2 years or so because they dull out. And this despite having many more blades, and thicker, than regular razors.