Monday, August 17, 2015

Pushing the Blade-Life Envelope and Palm Stropping

The Eleventh Shave

I have written many times that I'm reluctant to cork or strop a coated blade -- especially when rather new -- because I'm concerned about the abrasive treatment doing more harm than good to the fragile edge of the blade.

But you may know that lately I've abandoned by practice of using a new DE blade every Sunday morning. Instead, I've been exploring areas closer to the limits of various blades' performance.

Yesterday, for example, was my tenth shave with a Dorco ST-301 blade. The shave was as close as I would normally expect, but I had noticeable irritation that lingered a bit after the shave. So today, in preparation for the eleventh shave with this blade, I palm stropped it making two strokes on each side of the edge; that is a total of eight palm-stropping strokes on the blade.

The result was a normally-close shave -- no degradation there -- but there was significantly less irritation than yesterday. Coincidence? Perhaps, but the palm-stropping certainly didn't reduce the quality of the shave -- at least not for this one use.

Effects of Edge Coatings and Stropping

Only based on what I have read including reports on microscopic studies done by others, the effects of shaving on the edge of a stainless-steel blade includes deformation of the fragile edge. It may experience some crushing or bending over, thus reducing the effective sharpness of the blade. By stropping a used blade, this may to some degree partially restore the edge by reducing the amount or degree of edge deformation.

I have also read about the purpose of various edge coatings. Metal coatings such as chromium and platinum have been reported to toughen the edge, to make it more durable. Polymeric coatings such as PTFE are intended to act as a lubricant making the edge pass more smoothly over skin. Yet these coatings -- especially the metallic ones -- are very thin, and stropping a newer blade with the intention of extending its useful life may actually remove the coating with resulting adverse effects.

Yet palm or arm stropping a very-used blade may bring very little risk of adversely affecting the blade edge. The coatings are likely gone or less effective, and the edge may be due for some mechanical improvement.

Another subtlety associated with stropping is whether it should be done before or after blade use. Post-shave stropping will certainly help to remove edge-degrading water and other contaminants. However, for those of us who gently clean and dry blades after the shave, stropping would add nothing in that regard. All it would do would be to potentially improve the evenness of the edge.

But there may be a benefit to waiting and stropping the blade only as part of the pre-shave routine.

A Bit About the Nature of Metals

At one point in my life I was a mechanical-engineering student. Part of that discipline is the study of materials science. One thing I remember from those days is about the molecular nature of metals. You ever bend a paper clip until it breaks? This happens because as one bends metal, it deforms the microscopic crystals of which it is composed. This deformation causes the metal to be both harder and more brittle. The more you bend the metal, the harder it gets, but also increasingly susceptible to fracture. 

This outcome of deforming metal is called strain hardening. This is why automobile manufacturers can take rather soft sheet metal and by stamping contours into it, can make it more rigid and suitable for use as auto-body panels.

However, an interesting property of metal is that over time, the atoms in the deformed crystals of metal tend to migrate into new alignment. This migration reduces the stiffness of the formerly-strain-hardened metal, rendering it softer, more pliable. That is why a ten-year-old car is not as crash worthy as a new car; the crystals in the metal have rearranged making the panels in the car more easily bent, less resistant to new deformation.

This applies to razor blades.... maybe.

The edge of a razor blade is so thin and fragile that the abuse of normal use certainly deforms the edge irrespective of coatings. So at some point in its useful life, the blade edge is subjected to strain hardening. Though it takes an automobile fender literally years for its microscopic crystals to naturally reverse strain hardening, because of the extremely small scale of the blade edge, it is possible (not guaranteed) that after a few hours the effects of the strain hardening from the previous shave may diminish. On the other hand, it may also be possible that the  strain hardening due to normal blade deformation may be minimal to begin with -- in which case there is no reason to delay blade stropping.


Without further research, all I can suggest is that, all other things equal, there is no harm but possible benefit in delaying the stropping of a used blade until just prior to the shave.

I can also say that there is likely some benefit to stropping a used blade. At the very least, stropping may actually further strain harden a shave-deformed edge, thus making it more resistant to further deformation.  However, how used it should be before one starts stropping depends on the blade's coatings. If it is uncoated (that is, plain stainless steel), then there is likely little risk to stropping early and often. However, stropping a coated blade too early in its useful life cycle may be counter productive due to premature removal of the applied edge coatings, which could reduce edge toughness, comfort, or both.

Certainly if I've got approaching ten shaves from a blade and it begins to feel less comfortable but otherwise seems sharp enough, then a little pre-shave stropping is likely to be included in my routine. I draw this conclusion because I assume that a degrading blade will first be irritating on my sensitive skin before it begins to cut hair less effectively. However, others with less sensitive skin may simply try experimenting with stropping a blade prior to the shave as it gets within a shave or two of its normal useful life; then by stropping these nearly-used-up blades, one may get some additional quality use from any given blade -- that is, if one is so inclined to go to that trouble. For me, it's more a question of academic interest rather than squeezing maximum useful life from my blades.

Happy shaving!

1 comment:

  1. I get the principle, it's just that it's so convenient to share a drop of oil with the blade by palm stropping, when moisturizing. I don't think the edge is really clean until it's palm stropped, either. It's like one has to choose between chemical or physical degradation.