What does stropping do?

Stropping a blade on a clean (without abrasive) substrate achieves FOUR results:

1) REALIGNING THE EDGE

Although a straight razor is made of hardened steel, the edge is flexible and malleable. Below is an example of a relatively large ‘ding’ in the edge.  I received this blade from a custom razor maker with this defect and can only speculate as to the cause.  The blade was stropped 50 laps on clean linen, 100 on clean horse leather and then 10 on clean linen (to remove residue from the leather).  The same location on the blade was imaged, clearly showing the blades’ edge has been realigned.

misalign

A misaligned region of the edge.

realign1k

The misaligned region following 100 laps on a linen strop.

realign3k

The misaligned region following 100 laps on a linen strop at 3kX magnification.

Realignment more commonly occurs at much finer scale; the above series of images was chosen only for the clarity of showing the same location on the edge before and after stropping.

2) BURNISHING

Burnishing refers to the movement of metal, distinct from abrasion and the removal of metal.

as_honed_20k

A straight razor edge, as-honed on a Belgian Coticule stone.

Straight razor edge following 100laps on leather, immediately following honing on a Belgian Coticule.

Straight razor edge following 100laps on leather, immediately following honing on a Belgian Coticule.

stropped_100linen

Straight razor edge following 100laps on linen, immediately following honing on a Belgian Coticule.  Keenness is REDUCED, due to micro-chipping of the edge.  This could also be called “foil-edge removal.”  The broken edge is partially reformed, at this point, due to burnishing (see below).

burnish stropped

The burnishing effect is evident on the linen stropped edge, particularly near the top of the image where metal is observed to wrap up and around the broken edge.

The burnishing effect is most clearly observed at an area of the edge that was previously chipped.   This leaves a near square corner, and the easing of this corner occurs with metal pulled up towards the edge.  Burnishing also occurs on the bevel face, smoothing asperities by plastic flow (this can be observed in the series of images shown below, where both abrasion and burnishing occur).

3) ABRASION

Abrasion (the removal of metal) occurs by two processes; nano-scale abrasion and edge chipping.   The series of images presented below were taken from the same location on a  razor (as witnessed by the x-shaped pattern in the top right of the images).   The softening and loss of scratch definition occurs primarily by surface abrasion in the last 2 or 3 microns of the edge, increasing both keenness and sharpness.

ashoned

As-honed on 3-micron lapping film.

20

20 laps on a clean leather strop.

50

50 laps on a clean leather strop.

100

100 laps on a clean leather strop.

200

200 laps on a clean leather strop.

500

500 laps on a clean leather strop.

Abrasion by micro-chipping is manifested primarily through the removal of weak or damaged metal at the edge and occurs more rapidly with stropping on linen than leather.

4) COATING

Manufactured razor blades are typically coated with a lubricating coating to reduce the force-to-cut and improve the apparent “sharpness.”

Fusion_10kx

Cross-section of a Gillette Fusion blade showing the thin layer of fluoro-polymer (Teflon) that coats the blade. Most of this coating is removed with the first use.

For SEM imaging, this coating blocks the view of the metal edge and I normally remove it prior to imaging stropped edges (as, for example, in the images above).   It is, in fact, extremely difficult to remove.  (In an optical microscope, these coating is transparent, although they may impart color due to interference effects).  The effectiveness of this coating in lubricating the passage of the blade and thereby reducing the force-to-cut is yet to be determined.

strop_coating8

A straight razor after stropping on leather.   The ‘black’ material is an organic (oil/wax) coating the steel of the blade, out to the apex.

 

strop_coating_5

A straight razor after stropping on leather.  The (black) coating shown here is what remains following very aggressive solvent cleaning. Without removal of this coating, it is not possible to view the morphology of the steel with low energy SEM imaging.

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18 responses to “What does stropping do?

  1. I just found this blog and it’s great! You also managed to create a great setup for making photographs. (Are they SEM images?)

    Your blog examines some issues I’ve also tried to examine on my own blog. I love the clear way in which the pictures demonstrate the various effects of stropping. However… I still don’t see how the pictures illustrate burnishing. The last picture in the burnishing section seems to show a much rougher edge than the pictures before.

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    • Thank-you Mark, I have been following and appreciating your blog for some time. Yes, these are SEM images. They are different than other images you have seen in that I use low electron energy and I clean the steel prior to imaging. In fact, we briefly discussed some of them on the Wicked Edge forum last year.

      In this post I have presented just one or two images that demonstrate each of the effects. I agree that burnishing is the most difficult to see, and really deserves a separate post and additional images.

      In the image series I present here, stropping on linen causes the edge (which is initially about 150nm wide) to be broken (or micro-chipped) away leaving and “squared-off” edge 300-400nm wide. This part of the process is abrasion, in the sense that micro-chipping is the removal of metal. However, continued stropping on the clean linen results in plastic flow (burnishing) of steel from those squared corners UP and over the existing edge, first forming a crown-like shape and then a new apex. Further stropping on clean leather will abrade the edge, thinning it and improving keenness.

      To be clear, this is not a successfully honed straight razor; however, the process of reforming micro-chips through stropping is the mechanism by which a blade is maintained in shave-ready shape with stropping between uses.

      Burnishing also occurs on the bevel face where scratches are smoothed out; this is easier to see at higher magnifications. Close examination of the “abrasion” series does demonstrate the burnishing effect.

      Todd.

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  2. Todd, This is a great blog! Thanks to Mark for posting it on the Wicked Edge forum 😀 Thanks for all the research and work you are doing!

    This article is interesting… Your work on plain leather/burnishing/re-aligning the edge seem to contradict Dr. Verhoeven’s findings in his paper, “Experiments on Knife Sharpening” in which he concludes:

    “The experiment with stropping clean leather confirms the previous experiments. The natural abrasives present on clean leather are not adequate to remove edge burs or surface abrasion grooves on stropping.” (bottom of page 36, http://www-archive.mse.iastate.edu/fileadmin/www.mse.iastate.edu/static/files/verhoeven/KnifeShExps.pdf).

    I would love to see more research on this topic. Thanks!

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    • Josh,

      My interpretation of the comments in John’s manuscript is that the stropping effect is not visible at the resolution and contrast provided by his SEM images.

        The effect of stropping with clean leather is subtle and occurs primarily over the last few microns from the apex and requires much better quality images to observe.

        Todd.

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  3. I apologize but I do not follow. Stropping on linen degrades an edge? Should I not use my linen strop. I have a pure flax linen and was unaware it was making my edges duller.

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    • After more than a year and thousands of visits to this page, you are first to comment on this surprising result…

      The answer will depend on your point of view. We could call this removed metal a “burr” or a “false edge” and be happy to have removed it. It is important to understand that the “broken-off” edge will continue to be refined by the linen strop, so this is one (of many) approaches to honing and stropping a razor. A razor which has been successfully finished with a pasted strop (micro-convex) will not be damaged by the linen, although it may not be improved either. A razor micro-convexed by honing with slurry may be unaffected or improved by the linen. A razor honed on very high grit synthetic hones (triangular apex) is usually degraded (initially) by linen, as shown here. You can easily test your own situation with the hanging hair test or by tree-topping arm hair.

      In my experience, metal that is removed by the linen strop would not survive the first shave. Undoubtedly, there are people who happily shave with burr-edges.

      In the longer term, linen will refine micro-chipped areas of the blade, and this is certainly a positive effect.

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      • I’ll have to try going right to leather and skipping the linen. I finish on a Black Arkansas and the edges I get are agreeable, smooth and sharp. Maybe they will improve without linen?

        I just found it odd after all the praise of using linen and all the other plethora of synthetic materials before going to your leather strop.

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    • I recommend that people experiment for themselves using the information here as a starting point and also to help understand their observations.

      That said, I personally only use the linen component for cleaning the blade after shaving with 4 or 5 very short, light strokes. I typically use linen or denim with abrasive as part of my honing routine, however.

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  4. Have you taken a look at the comparative effects of different “second component” materials? Opinions on some of them vary widely within the community of straight razor shavers, eg. some folks think nylon webbing is the bee’s knees while others think it accomplishes nothing at best. Strop tension and pressure are sort of elephants in the room in these discussions, usually, but it stands to reason that different materials would act differently ceteris paribus.

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    • I have looked extensively at the results from flax linen (Scrupleworks strops) and denim strops (Levis 501). The primary effects are basically indistinguishable between those two. I only looked briefly at the results from nylon weave (Walking Horse strop) and can’t make any definitive comment on that. There will be differences in speed. The challenge, for someone without an SEM, is how to compare results from different fabric components. Fabric is not (in my experience) suitable as a final strop, so observing properties related to keenness (like shave quality) is not particularly informative or even misleading.

      Strop tension and pressure primarily determine the rate that the effect is achieved. For example, heavy pressure for 10 laps on flax linen gives the same result as 100 laps with light pressure.

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  5. I do agree with all of the ideas you’ve introduced in your post. They are very convincing and can definitely work. Nonetheless, the posts are too short for novices. May you please prolong them a little from next time? Thanks for the post.|

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    • My goal with this blog has been to show and explain what actually happens during common sharpening/honing/stropping processes. I’ve never intended this to be a guide for beginners, but rather to help people understand what is happening when as they develop their own techniques and methods.

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    • I don’t have that particular hone, but I would expect it performs similarly to the Shapton 16k. Triangular, but typically not quite keen enough without stropping to slightly micro-convex the apex. Leather alone should be sufficient. Obviously you can just try for yourself.

      Stropping on linen will remove “weak” steel as part of the process of micro-convexing the apex. So “ripping off” the edge isn’t necessarily bad, provided you strop sufficiently to re-profile (micro-convex) that broken apex. This same process occurs with use (shaving) in any case, so you can’t “preserve” that weak steel by avoiding the linen.

      My personal recommendation is to use a pasted denim (linen) strop as part of the honing procedure to avoid this concern altogether.

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  6. When I look at your micrographs, I don’t see a sharper edge after stropping. This seems consistent with Verhoeven’s work, and with what Howard Schechter says about finishing with a Shapton 30K stone and touching up on a hone weekly. Have I misinterpreted? (BTW, just out of interest — have you tried a Shapton 30K?)

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    • Using my definition of “sharp” you are correct, stopping does not sharpen. What stropping does is improve keenness through micro-convexity.

      Unfortunately, Verhoeven’s SEM images (from the unpublished manuscript) are not of sufficient quality to reveal what actually happens to the apex during stropping.

      In my experience, a typical straight razor with a 16 degree bevel angle cannot maintain a triangular geometry for any length of time. With edge-leading strokes only very high grit stones like the Gokumyo 20k or Shapton 30k can produce a triangular bevel with sufficient keenness for shaving. Coarser stones (eg Shapton 16k or Naniwa 12k) can also achieve this with care (usually a couple of edge trailing strokes will do it).

      With use, or when stropped on linen, those triangular bevels will micro-chip and lose the initial keenness. With sufficient stropping (particularly on linen), keenness is restored through micro-convexity. With repeated use and stropping, all blades become micro-convex.

      The approach that Howard suggests is to maintain the triangular bevel by “sharpening” with the Shapton 30k after each use and only stropping on leather to apply lubricant (strop oil) to the blade. As I have mentioned before, you must avoid the fabric strop if you want to maintain the triangular bevel.

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  7. Great blog posting. I had only thought that stropping would remove burrs but you show convincing evidence that it can both reform metal and add lubricant. Most impressive work with the SEM. Opticians recommend using microfiber cleaning clothes instead of tissue paper because the cellulose products have microabrasive inclusions (maybe some sort of calcium silicate?). I’m guessing your linen strops are taking advantage of that fact.

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    • I don’t know where the “natural silica” myth comes from. I’ve analyzed a variety of strop leathers and not found even trace amounts of silicon in any of them.

      The reason for using soft cloth is that foreign particles (dust) falls into the weave and so there is not sufficient pressure to cause a scratch. This is why denim/linen strops are so effective and why we can achieve a mirror finish with metal polish containing 100 micron aluminum oxide particles.

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