Hanging Hair Test
There are frequent posts asking about the hanging hair test (HHT), and frequent responses discouraging its use to determine whether an edge is shave-ready. An HHT is best used as a personally developed measure; one shaver's results will not translate to the same results for another shaver. This is because we have different pelts, different honing styles, different hairs to use in the HHT. Some of us do not actually grow hair that is useful for HHTs, and must steal it from other people's hairbrushes or beg it from acquaintances or strangers.
Here are the steps I used to make my own HHT meaningful to myself:
1) Identify suitable hairs. I use my chest hairs--in particular, the white ones. Dark pigment toughens hairs; that's why you see it at the tips of wild animals' hairs, for abrasion resistance. For your best internal replicability, always do your HHT with hairs that have been recently washed, so they'll have consistent hydration, sebum load, etc. from test to test. Some people test with the hair root held between their fingers, others test with the hair tip between their fingers. In actual use the razor encounters hairs in the root-to-tip direction, so I hold my test hair the same way, by the root end.
2) Select a razor that shaved well for you last time you used it. Before your preshave stropping, do your HHT and note how it performs.
3) Do you linen-side stropping passes, the normal number. Repeat HHT and note any change.
4) Now do your normal count of leather-side stropping passes, repeat your HHT, and note any change. If your linen strop imparts any toothiness to the edge, you might actually notice a decrease in HHT performance from the linen stropping to the leather stropping. This is because, given the same keenness, a toothy edge will be better at initially piercing of the hair's cuticle than will a smoother edge. I'd wager, though, that the linen-only edge would give you a harsher shave than the linen-then-leather edge.
5) Perform your normal shave then rinse and dry, but don't yet strop, your blade. Repeat the HHT and note how much wear and tear your edge picks up during normal use.
6) NOW do your normal postshave stropping (30 or so laps on leather is what most folks write about using), and repeat your HHT. Note how much of the wear and tear is corrected by this stropping.
7) Do your normal preshave stropping routine, then another HHT to see how much improvement that stropping routine brings. If you still have a good sense of how your HHT performed back in Step 2, then you can appreciate how much a little oxidation affects your edge between uses (that is, if you don't oil your blade before putting it away).
What all these steps do is give you a sense of your HHT's sensitivity, and also an appreciation of how much a shave degrades your edge. Everyone's experience with this will be extremely personal, given that some of our beards are much harder on the blades than others, that the hairs we each use for our HHTs are very different from one anothers, that our stropping results differ, etc. There is no sense discussing the HHT, or even an HHT; the only sense comes after you've calibrated your HHT.
It's not even quite accurate to use the term "calibrate" here, since calibration implies a common standard. I'll reiterate: You're just demonstrating to yourself your HHT's sensitivity to what your blade experiences during normal use and storage. That experience will make your HHT more informative to you, but will not enable you to make any better comparisons to other peoples' HHT results. It is a personal and relative test, not an absolute one nor one replicable by other shavers.
And, as is always emphasized in discussions of this topic, the only way to determine if an edge will shave well is by shaving with it. Over time, you'll learn that your HHT has some predictive value, but again, it's unlikely that your prediction points will be the same for your shaves as another person's prediction points are for his shaves. In addition, wonderful shaving edges from some hones will perform very poorly at HHTs. I've had this experience with an Asagi edge another member put on a blade I bought from him. It was a dazzling shaving edge, extremely nonirritating and, though lethal to hairs, extremely reluctant to sever skin. I once made a bad pass with that blade that should have cost me a decent fraction of my lower lip, but escaped unscathed. That edge would not pass my HHT very well. For that reason, your HHT is not a fair evaluation of an edge you've recieved from another honer. Only a full shave will provide a fair evaluation.