It realigns the edge from what I understand. Sort of smooths it out. Microscopically, the edge goes from this: ^^^^^^^^^^^^, to this: --------------
Close, but not exactly. If you look at the edge of a razor under a high-powered microscope you see that the edge of the blade actually looks kind of like a saw's edge. That is, a jagged row of teeth that are all aligned. These "teeth" are somewhat smaller than an individual beard hair. When you shave, the beard hair is so strong relative to the edge of the blade that they actually bend / throw the teeth out of alignment, creating an imperfect edge. This damaged edge is still able to cut hair, but if far, far less effective. Stropping on leather (or even your hand if you are stropping a DE blade) straightens the teeth so that your cutting edge is returned.
After many shave/strops however, stropping alone is not sufficient to repair the edge, so honing is required.
Lots of misinformation in this thread.
There are photos on these forums and elsewhere of razor edges at 800 - 10,000x magnification (electron microscope) that somehow fail to detect teeth in the edge of a properly honed razor. Low grit hones do leave a sawtooth edge, which is useful for knives but inapplicable to razors. At high resolution the edge of a properly honed razor looks like it's torn off - the steel is too thin to hold together under the tug of the hone, and rips off. A razor with a sawtooth edge has not spent enough time on the intermediate hones to remove the coarse scratches, which means that those intermediate and fine grit hones are only honing the high spots on the edge.
As for stropping, there is some evidence that the strop cleans off corrosion and removes oxidized steel. I've not seen any evidence that it aligns the edge. This is often claimed about strops based on a comparison to knife steels, but a leather/canvas strop is not made of steel and the comparison is inapt.
http://blog.modernmechanix.com/2008/06/02/wonder-photos-reveal-unsuspected-facts-about-razor-blades-and-shaving/ - 1937 article and optical photos of razor edges, showing what edges look like after use, and the effects of stropping a used edge.
http://www.mse.iastate.edu/fileadmin/www.mse.iastate.edu/static/files/verhoeven/KnifeShExps.pdf - paper by a Professor of Materials Science (he's a metallurgist) at Iowa State, on various aspects of honing. Not specifically about razors, but includes high-res shots of commercial razors and hand-honed straights. Also examines stropping, and found no evidence that it straightened the edges.
One caveat about the Verhoeven paper is that his methodology changes between chapters, so you can't compare results between chapters. e.g. You can't infer from his data that 0.5 micron diamond is better than 0.5 micron chrome oxide, because they were obtained in different chapters and were used differently.
As for stropping, there is some evidence that the strop cleans off corrosion and removes oxidized steel. I've not seen any evidence that it aligns the edge. This is often claimed about strops based on a comparison to knife steels, but a leather/canvas strop is not made of steel and the comparison is inapt.
Of the papers I've read from Dr. Verhoeven's studies the stropping was done on a spinning leather wheel and compared a loaded chromium wheel to plain leather. Thus, showing the plain leather added no refinement to the edge but the loaded wheel did. Thanks, Possum
Most anecdotes say to fill in strop nicks (if large enough) with a glue (CA?, what does CA stand for anyway?) because if the surface is left with an interuption, that interuption can transfer to the razors edge? Remember...I'm just asking but if we don't want our razor's edge to touch a divit caused by a nick, then there must be some association with that divit and edge mis-alignment
While Dr. Verhoeven's labratory findings are good, there is still much real world evidence that stropping on plain leather improves an edge to the point of realizing a decent shave. Theres just no denying this. How this actually is brought about I think we're hitting at some of it here. Possum
Linen is an interrupted surface, and is an important component to stropping. One of my favorite strops has a very coarse corrugated linen.
If you get a nick in your strop just cut away any flaps left hanging, and sand it down around the edges of the nick to smooth out the transition.
It isn't the divot in the leather that causes a problem, it's the edge of the nick that's the troublemaker. Leather tends to swell with humidity, and bathrooms tend to be humid places. The leather at the edge of the nick is exposed to the humid air on two sides whereas the rest of the strop is only exposed at one side, so the leather around the edge of the nick will swell more causing it stick up above the surface of the strop just a bit. The edge will bang against this as it goes over the strop, and has a minor negative effect on the edge. Sanding the edge of the nick to round it off a bit prevents this from happening.
I would not fill it in with CA (= cyanoacrylate = super glue). I have never heard of this recommendation myself, and it sounds like a good way to ruin a strop. CA will penetrate the leather and produce a hard spot on the strop which will not flex with the rest of the strop, so in effect it will also be a high spot on the strop, but even worse it will be a hard high spot.