The Pasted Strop – part 1

The primary goal of stropping is to reduce the edge width by increasing the bevel angle near the apex (micro-convexity).  Abrasive particles (paste, spray, etc) are applied to the strop to increase the rate of abrasion.  The edge geometry that is achieved will depend on the type and size of abrasive, the strop material and the pressure and number of stropping laps applied.  Although simple in principle, consistent results can be difficult to achieve due to the number of variables and their effects.

Abrasive stropping differs from conventional edge-leading honing in two ways; the trailing edge direction and the compressible nature of the substrate.  Sharpening on a hard hone or stone in the edge-trailing direction provides the simplest case of pasted stropping; where compressibility does not play a role.  As a starting point, I will discuss a comparison of edge-leading and edge-trailing honing.

When honing a blade in the edge-leading direction,  steel is removed in two ways; abrasion of the bevel surface and the ‘breaking’ or ‘micro-chipping’ away of the apex.  This micro-chipping process prevents the formation of a burr, but also limits the level of keenness that is achieved.  In the edge-trailing stroke, the occurrence of micro-chipping is reduced to the point where a burr or foil will almost inevitably be formed.  Two examples are shown below; honing on the Chosera 1k and on the Shapton Glass 16k. In both cases, the blade was honed normally with edge-leading strokes and then given 20 pairs of edge-trailing strokes approximately 10cm long.

Chosera 1k

Comparing edge-on view images, at the same magnification, shows the dramatic difference between edge leading and edge-trailing strokes.

edge-on view images after Chosera 1k

Edge view SEM image after Chosera 1k with edge leading honing strokes.  The edge width is irregular,  in the range of 0.5 to 1.0 micron.

CH1k_back_01

Edge view SEM image after Chosera 1k with edge trailing honing strokes.  The edge width is less than 0.1 micron, although obscured by the presence of a foil on the edge.

The cross-section images confirm the formation of a foil edge in the edge-trailing case.

CHO_1K_X_measured

Cross-section after Chosera 1k with edge leading honing strokes. The near-apex bevel angle is slightly larger than the 16.5 degree spine-defined angle.

CH1k_back_02

Cross-section after Chosera 1k with edge trailing strokes. The near-apex bevel angle is 4 degress less than the 16.5 degree spine-defined angle; a foil-edge burr by definition.

Shapton 16k

Shap16k_E

Edge view SEM image after Shapton 16k with edge leading honing strokes.

Sh16k_plus20back_01

Edge view SEM image after Shapton 16k with edge trailing honing strokes. An edge burr is visible.

Sh16k_plus20back_02

Edge view SEM image at 50,000x magnification after Shapton 16k with edge trailing honing strokes, showing a slight burr with sub-100nm edge width.

A comparison of the cross-sections confirms the formation of a burr in the edge-trailing strokes on the Shapton 16k also.

Shap16k_X

Cross-section after the Shapton 16k with edge-leading honing strokes. The measured near-apex bevel angle is equal, within uncertainty, to the 16.5 degree spine-defined angle.

Sh16k_plus20back_03

Cross-section after the Shapton 16k with edge-trailing honing strokes. The measured near-apex bevel angle is one degree less than the 16.5 degree spine-defined angle.  In addition, there is metal beyond the projected apex, indicative of a burr.

Sh16k_plus20back_05

Cross-section after the Shapton 16k with edge-trailing honing strokes. The 30kX magnification image shows that the bevel angle in the last micron of the edge, at 10 degrees, is significantly less than the 16.5 degree spine-defined angle, indicative of a foil edge.

In summary, edge-trailing strokes can produce a keener edge than edge-leading; however, they generally result in the formation of an undesirable burr or foil.

In part 2 of  The Pasted Strop the effect of a compressible substrate will be discussed.

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26 responses to “The Pasted Strop – part 1

  1. Great post! I am amazed how much difference results from a simple change in honing direction. I wonder what happens when you hone in a back and forth motion (ie. alternating trailing and leading strokes).

    Also, what about honing side-to-side?

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    • The back and forth motion (half-strokes) will lead to a burr, just as with any honing that does not alternate sides. This is not necessarily a negative, depending on what you are trying to achieve. Other than during the final stages of honing, this is a very efficient way to remove metal.
      As to strokes parallel to the edge, I have not looked at this, but I would expect the same result as edge-trailing.

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      • It would be very interesting to look at the effects of strokes parallel to the edge, as this is the effect of many of the simpler sharpening gadgets sold for sharpening kitchen or utility knives. From my own experience experimenting with these types of gadgets, sharpening parallel to the edge produces a less effective cutting edge than either edge leading or edge trailing strokes. I have never had a microscope powerful enough to be able to prove it, but I imagine that strokes parallel to the edge end up tearing and rounding the edge instead of creating a keen apex with the micro-serrations that are so effective in cutting through the fibres of meat and vegetables.

        If ever you have the time to extend the edge-leading / -trailing experiment to include parallel strokes, I would love to see the results.

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  2. How much of a factor is the type of steel? Do you think using a couple of edge trailing strokes after the final polishing strokes on the finishing stone would create a sturdy Mimi bevel?

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    • There will be a number of factors, including the type of steel that will influence the result. The examples shown here should not be generalized, but rather serve as a starting point for further experimentation.
      In the example I have shown, there is an increase in keenness with each edge-trailing stroke which, after 20 laps, leads to the production of a foil edge. At some point the apex was very keen, but a foil had not yet initiated. So yes, a few edge trailing strokes would be an effective finishing technique in this case.

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  3. Todd, I may have missed it, but could you specify approximately how much force was used and if you alternated sides on each pass or did multiple passes per side? thanks!

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    • Josh,
      These strokes were honed alternating sides. As a rule, I am using a force-equivalent on the order of 100 grams. Pressure is more relevant here, and that will decrease with increasing width of the bevel – this razor has a 0.3mm wide bevel.

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  4. 1st, thank you for your work. I keep coming back and thinking about what I see and your conclusions. Do you think micro chipping size is a function of grit size at the hone, speed of collision with the grit particle and the layout of interstitial impurities in the lattice of the razor steel? If mostly from grit size, maybe the bigger chips are when the grit stays with hone and the smaller when the grit fractures or comes loose.
    Or, if the effect is mostly from the razor, random lattice imperfections would dominate the chip geometry.

    Ive seen both positions reported on the razor forums; some state a certain hone is bad to microchip, others state a certain steel. Of course the chips they reporting are much bigger optically observable.

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    • In the (edge-leading) example shown here, the edge is abraded as micro-chips as we hone “through” the plane of the apex. It appears that for the coarser stone, the resulting edge is thicker and less linear implying the micro-chips become smaller with increasing grit. This is why the edge is observed to become keener as we progress through the hones.

      The grain structure does not seem to play a determining role in these carbon steel razors; the edge geometry can be finer than the grain size. You can see the grain structure in this image

      Certainly we should expect chipping would occur at weak points.
      The micro-chipping referred to above is not visible with magnification below 500x. I have a suspicion that the effect described as micro-chipping in the context of the effect described as “over-honing” is related to the formation of a foil-edge burr. A foil edge will appear to be “micro-chipped” at modest magnification. It is known that burr formation depends on the properties of the steel.

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  5. The bur that is created from honing with back and forth motions/circles up to 5000k,
    will it be removed by continued honing with only edge leading strokes with 5000k and 8000/10000 grit?

    Thanks for all good posts!

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    • Generally, a few edge-leading strokes alternating sides seems to remove any burr created by back and forth or edge trailing strokes. This does require contact of the apex to the stone, so this is not necessarily true for freehand honing or convex bevels.

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  6. Todd,

    Incredible data. Myth busting at the highest level. I am still confused by the issue of deeper scratches by finer grit stones. I think I understand the force per size concept. As an alternative to the wood sanding analogy, would sword polishing progressions with Jnats add to the discussion. The finer and finer stone produce an increasingly mirrored finish which I assume is the result of diminished scratch depth. Is it possible that with increased honing at finer and finer grit that the depth of scratches diminishes on the bevel. It seems you did only a few laps on each grit which resulted in deeper scratches as you progressed. In other words would 100 laps on 600 grit following 325 yield a different result.

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    • This effect is peculiar to diamond plates, in my experience. I believe that a small number of over-sized diamonds on the plates are responsible. There is an explanation in the “Diamond Plate Break-in part 2.”

      The format of my blog articles may give the impression that these are one-time experiments or progressions. In fact, I have produced and examined edges honed on each of these plates numerous times under a number of conditions – varying the starting point (16k, 1k, etc), the pressure, number of strokes, until I establish what is typical for each stone/hone/strop. The images I show are typical, representative images that display the characteristic features of blades sharped with those particular diamond plates. In answer to your question, no, the scratch depth will not change from what I have shown with additional laps or different hones preceding.

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  7. You have done an excellent job on this subject. I have enjoyed the images, and confirmed some believes as well as learned new information! Thank You

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  8. If I end with 5 light edge trailing strokes on my Suehiro 20k on a hard blade 62-63 hrc would this result in a foil burr? I’m trying to develop a very keen edge but not a fragile one based on your research. Thank you!

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    • When used edge-leading (alternating sides), the Suehiro 20k produces a shaving-keen, triangular apex without microconvexity or a foil burr. Edge-trailing strokes after this point will produce a foil burr fairly quickly. It is difficult to give a number, since it depends on whether the hone is freshly lapped and on the width of the blade bevel. I would suggest trying one or two short strokes if you just want to increase the keenness a bit.

      On a typical 16 degree inclusive angle straight razor, this geometry is too fragile for shaving a coarse beard. Also, stropping the blade on clean linen will break off the apex and then micro-convex this broken apex by burnishing – it could take hundreds of laps on clean linen and leather to bring the apex back to shaving keenness. If you want to experience shaving with the pure triangular geometry, do no strop on linen, other than 3-5 short light strokes to clean the blade after shaving. Stropping on leather will not damage this fragile apex as linen does. You may also find that stropping on leather part way through the shave has a noticeable effect.

      If you want to follow my abrasive-loaded hanging denim approach, you can go directly from the SG 20k with edge leading strokes to the denim.

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      • Ok thank you for the advise. I ended a honing of a hart steel with one layer of tape with 5 edge railing very light strokes. The result was a very keen edge that did not seem to deteriorate by the third pass and resulted in probably the best shave of my life. I am assuming this edge pretty much held up. I don’t know the exact geometry but it’s a 1/4 hollow honed with a layer of tape. The edge angle is probably at the high end. I will try an extra hollow Ralf Aust tonight with just two edge trailing strokes like you suggest. Also to note by the fifth edge trailing stroke the razor smoothed out on the hone and there was virtually no feed back on the Latigo strop I used. Not sure why the strop would have almost no pull unless I really straightened out and keened the edge but it shaved very well. Thank you again! No linen for me!

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  9. Wow I just found this site, amazing work.

    I have one question about the microchipping. Do you think it also happens (probably to a lesser extend) on hard fixed abrasive hone like arkansas stone or sintered ceramic ? I would love to see a magnified view of an edge from such hones.

    When sharpening (knife) with my chosera 1000 I always end with a couple of trailing passes but I have to be very careful of pressure and number of passes because I end up compromising edge retention. If I end with edge leading passes I can’t make the edge push cut phonebook paper.

    On a hard stone like a spyderco ceramic (kept clean to limit burnishing) I can acheive a very high keeness from edge leading passes with very light pressure.

    Anyway thanks !

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