Disclaimer: This post is probably going to trigger a lot of strong opinions. I don’t mind that. If you disagree with something I state, I’d love to have you show me where I went wrong! Contact details are in the impressum. If you also just want to shout at me because you disagree, that is fine as well.

If you haven’t read part 1, you should do that. Check it out here!

In part 1 of this, my good friend Roman Kasé and I started to look into the effect of stropping on a finely sharpened blade. Let me pull up two important pictures from that to start up our discussion for part 2!

Very high magnification SEM images of the apex (top view) in a Vanadis 8 (66 HRC) blade, sharpened without (First/left pictures) and with stropping. Instrument: Zeiss GeminiSEM560

I’d like to make a point about what we are seeing here. This is a very high magnification. The entire width of the image is 1.1 µm. Compare that to the theoretical limit of an optical microscope: The Abbe diffraction limit is defined as d = λ / 2 NA – where NA is the numerical aperture, a value that can be used to describe the resoluting capabilities of a lens. At best, this value can become 1. So the theoretical resolution of any optical system is half the wavelength. The wavelength of green light is around 530 nm, so a fancy, 3 digit k$ laser scanning microscope could at best have a resolution of 0.25 µm – just shy of 5 pixels would fill the whole image here! The burr you see on the unstropped blade is about 30 nm. Compare this to the lattice constant of martensitic steel (the distance between two atoms) which is 0.286 nm. So this burr is somewhere in the region of 100 atomic layers – this is unbelievably tiny, and would appear to any easily accessible test a “no burr” result. Surprisingly enough, with stropping, the burr got actually larger and more inhomogeneous.

For the 2nd part of this series, we are looking at a different steel, Boehler M398 at 65 HRC. This is also the steel used in the vast majority of the stone reviews in my blog. It’s a popular “super steel” for knives.

As a quick reminder, for the stropping, we used fresh leather strops loaded with different emulsions with the following approaches:

1.) No Stropping – pure ground edge! this is our base truth to what we compare the stropping

2.) “best practice”: 5 strokes with 1, 0.5 and 0.25 µm diamond emulsion on leather

3.) “overstropping” – 50 strokes with the 1, 0.5 and 0.25 µm diamond emulsion.

4.) “coarse grit” stropping – 5 strokes with 6 µm diamond emulsion.

Please check out part 1 for the approach to metrology, but also to read more about the sharpening progression and setup.

Results from approaches #1-#4 in M398

First, we had to establish the new “baseline”, aka: Images of the apex after just sharpening, no stropping:

SEM images of the unstropped M398 blade. (Approach #1). Instrument: Zeiss GeminiSEM560

One thing of note is – M398 doesn’t sharpen as nicely as the Vanadis 8 does! Nevertheless, the “no strop” approach gave us a very nicely defined apex in the range of 100 nm, with very low burr formation. The burrs are in the 50 nm range.

Next, we look at the “best practice” of stropping, so a few passes with increasingly smaller diamond emulsions (1, 0.5 and 0.25 µm diamond emulsion).

SEM images of the “best practice” stropped M398 blade (Approach #2). Instrument: Zeiss GeminiSEM560

Once again, we get a more pronounced burr, which is sticking out further in direction towards the cutting action. The burr has increased in width as well.

Next, we are taking a look at an overstropped result. For this, poor Roman had to strop 50 passes each with the 1 – 0.5 – 0.25 micrometre progression!

SEM images of the “overstropped” M398 blade (Approach #3). Instrument: Zeiss GeminiSEM560

I find the images here very interesting. The blade bevels are much more polished – details are harder to pick out, with a smoother surface. Moreover, the apex as gotten a lot finer – but also formed large, very fine foil type burrs. While we didn’t record higher magnification images on this, the pixel resolution allows us to measure the apex width in decent accuracy, giving us an apex width of about 50 nm. Weirdly enough, this blade was the slowest to grip and split a hair, even though the apex is defined the best of all the M398 blades analysed here. I once again have to compliment Roman on his stropping technique – there is no apparent round of the apex visible! But one deduction I can take from this is: with overstropping, a relevant amount of material is removed. If you don’t keep the pressure and angle under very tight control, you will actually be able to remove enough material to round of the apex.

Last, we’re going to look at the “coarse grit” stropped cutting edge.

SEM images of the 6 µm stropped M398 blade (Approach #4). Instrument: Zeiss GeminiSEM560

One can see that the surface of the bevel becomes more scratched, and the apex rougher. I think the often cited theory, that coarse grit stropping gives an edge more “bite” in terms of micro serrations is true! Nevertheless, it’s the widest apex of the 4 blades, with the worst BESS score.

Let’s compare the high magnification pictures of the first 3 blades:

Comparison of (from left/first to right/last) the unstropped, “best practice stropped” and “overstropped” blade. Instrument: Zeiss GeminiSEM560.

It is clearly visible, that the unstropped (aka: just ground) blade shows a very defined apex, with a very low amount of burr formation. The “best practice” burr formation gives a larger, forward facing burr. With overstropping, in this steel, the flanks become much nicer and more polished – but also refines the apex to a smaller width.

What key takeaways are to be deduced from this?

One point that should be made clear: The analysis of the apex is shown here for briefness sake at one location, but was homogeneous and comparable at several locations along the edge. The magnification used to show the apex here are very high, and the blade sharpness is unlike anything I’ve seen before in terms of SEM images on the internet.

I think we see a bit of a repeat performance from the previous part in Vanadis 8. The ground edge shows a clearly defined edge, homogenoues and relatively fine, with an apex just over 100 nm.

With stropping in what I would call the widely accepted best practice, a larger burr of slightly higher (120 nm) width is raised, facing outward of the blade. The perceived sharpness of this blade is very high – we were able to record an easy hair whittling video under the microscope on this blade:

Overstropping actually removed “a lot” of material, and refined both the apex as well as the surface finish. Nevertheless, this is a double edged sword: unless you are an OG at sharpening like Roman is, you will likely round over your apex with this approach.

So, is stropping finally dead, this time?

I don’t think so. This second data point adds a lot to the theory I am starting to form. Meaning, stropping raises a forward facing burr, and that burr performs better during the usual “proofs of sharpness”, such as whittling a hair or performing a BESS test. We’ve seen in part 1, that the stropped edge didn’t withstand the very limited cutting test as well. So while I still hesitate to call stropping dead, I am becoming cautiously convinced that maybe it’s not the best approach for a functional edge. More research is needed and I am willing to go down this rabbit hole.

A future part on this series, in a couple of weeks, will look at the effect of stropping vs no stropping in a cheaper, more simple steel.

Disclaimer: This post is probably going to trigger a lot of strong opinions. I don’t mind that. If you disagree with something I state, I’d love to have you show me where I went wrong! Contact details are in the impressum. If you also just want to shout at me because you disagree, that is fine as well.

Sharpening is a wonderfully complex and interesting process. The technique, steel and abrasive all influence the result to a major extent. Degrees of freedom and influencing factors are nearly uncountable. Moreover, the results we achieve are nearly impossible to visualise – the apex width on a sharp cutting edge is impossible to analyse with easily accessible means such as an optical microscope. For this reason, the standard approach towards quantifying sharpness, but also the methods used to sharpen a cutting edge are based on proofs of sharpness – for example, slicing through paper, splitting a hair or recording the cutting pressure on standardised tests such as a BESS test, where a test media (a nylon fibre) is cut through.

The common approach to sharpening is to use a coarse stone to set the bevel geometry, and then use progressively finer sharpening stones to refine that apex, until as a last step, a process called “stropping” is undertaken, where the blade is dragged across a (often pliable) media, sometimes coated in loose abrasive. Now, this process can be adapted infinitely, but generalised, this is what a sharpening process entails.

I have long been wondering, what happens during stropping. Why does it increase sharpness so dramatically, even if one strops just on the palm of ones hand or on a blank piece of leather? What is happening, at a microscopic level, to the apex?

My good friend Roman Kasé visited me, and together we set out to discover the secrets of sharpening and stropping. If you do not know Roman, you should check out his homepage – he is a masterful artisan making wonderful knives, but also an absolute beast and OG at sharpening.

What We Did (Experimental Setup)

Roman and I looked at two different steels – Vanadis 8 (66 HRC) and M398 (65 HRC), both wonderful high tech powder metallurgical steels. Every steel blank was prepared in an identical way:

Rough grinding was undertaken with an ATOMA F400 EP stone. The bevel was ground to an angle of 17 DPS, and the initial grinding was undertaken until the whole bevel was homogenous, with a clearly formed burr and no distinguishable carbide breakouts at 50x optical magnification.

Afterwards, a progression of resin bond stones is used to refine the apex and achieve a high degree of sharpness. For this, we used my own design stones – Dr. Marv’s Scientific Sharpening stones, going through a grit progression of 80, 60, 40, 20, 10, 5, 2.5 and 1 micrometre. Each stone was employed for 2 minutes, with the exception of the 1 micrometre stone which was used for a couple of passes only.

For the stropping, we used fresh leather strops loaded with different emulsions (Manufacturer: Stroppy Stuff) with the following approaches:

1.) No Stropping – pure ground edge! this is our base truth to what we compare the stropping

2.) “best practice”: 5 strokes with 1, 0.5 and 0.25 µm diamond emulsion on leather

3.) “overstropping” – 50 strokes with the 1, 0.5 and 0.25 µm diamond emulsion.

4.) “coarse grit” stropping – 5 strokes with 6 µm diamond emulsion.

Approaches 1 & 2 were applied to both the Vanadis 8 and M398, whereas 3 & 4 were applied only to the M398. For M398, check it out in part 2.

The Vanadis 8 was then used to cut 5x through a 16 mm manila rope, and wear was compared between the stropped and not-stropped blade.

How we analysed the results:

After sharpening, every blade was cleaned via a non contact process. For this, a steam cleaner was employed. There, heated water steam is ejected at around 3.5 bar towards the cutting edge. This can be considered a very gentle cleaning, as the force from the gas jet is very low. Afterwards, the blades are rinsed with ultra pure, analytical grade ethanol and blow dried with compressed (3.2 bar), ultra pure nitrogen gas. They are then inserted into a ultra high resolution SEM – a Zeiss GeminiSEM560 and plasma cleaned (2 min / 30W forward power). The SEM has a sub-nanometre resolution at all accelerating voltages. Images are recorded via the SE2 detector (sub 25kx magnification) and the InLens ultra high resolution SE1 detector above that magnification.

Let’s look at the results:

Let’s start of with the Vanadis 8 blade. This was first sharpened via my stones, and then analysed. Afterwards, we took it out, stropped it with approach #2 (see above), and analysed it again.

Sharpened cutting edge in Vanadis8, without stropping. Note the very nicely formed apex with low burr. Instrument: Zeiss GeminiSEM560

This is a wonderful look at the apex. A couple of things can be noted about the blade: We do have a couple of scratches, which are in the width of < 1 micrometre. The carbides are easily detectable as some very slight “bumps” across the flanks of the picture. The apex is homogeneous and very straight. Please also note the magnification of the pictures you are looking at – to get a sense of the scale, a got parameter in the databar is “width”, which shows you the width of the picture you are seeing. The maximum magnification shot you can see here features a magnification of 100kx (polaroid standard, look further down for those shots), so the picture shows you a 1.14 µm wide excerpt. This is out of this world in terms of magnification.

Next, let’s take a look how stropping changes the edge.

Sharpened cutting edge in vanadis 8 after stropping. Note the forward facing raised, nanometric burr. Instrument: Zeiss GeminiSEM560

Immediately visible, even at low magnifications is the improved surface finish of the cutting bevel. The diamond definitely abraded some material!

If we compare the two results with some measurements:

Comparison between the two apex: Left side/first picture: no stropping. Right side/second picture: stropped apex

It can be noted, that after stropping, a prowl shaped burr exists on the blade, that is facing straight outward at the cutting edge. This burr is significantly wider and more inhomogeneous than on the sharpened blade. The direction of the burr faces straight outward – which makes for a secondary, nanometric apex that is of a much sharper angle than the apex behind it.

Sharpness wise, both edges easily shaved and sliced through a piece of paper. The stropped edge showed a reduced BESS reading compared to the unstropped edge.

I’d like to also note that Romans stropping technique appears to be flawless – there is not discernible rounding of the apex here! He uses very light pressure and also lowers the angle by 0.5 DPS to compensate for the deflection of the leather.

Let’s take a look at the blade after 5 cuts through 16 mm manila rope. For this, we first tested the stropped blade, and then resharpened with identical stone progression, checked via SEM and then cut again. Pictures of the apex were taken at the location where the cut happened::

Vanadis 8 blade, unstropped, after 5 cuts through the 16 mm manila rope.

Even a low amount of 5 cuts already dulls the apex noticeably. Instead of a nanometric width, there is now a roughly 1 micrometre wide apex. At some positions, some brittle breakouts of carbides are visible.

Comparing this with the stropped edge:

Vanadis 8 blade, stropped after taking 5 cuts through a manila rope.

The apex is in a similar condition. It looks a bit more irregular, and folded/deformed compared to the unstropped blade.

Comparing the two apex side by side:

Comparison between the two used apex: Left/first picture: no stropping, right/second picture: stropped edge

The stropped edge actually became noticeably duller/wider than the unstropped edge.

What key takeaways are to be deduced from this?

One point that should be made clear: The analysis of the apex is shown here for briefness sake at one location, but was homogeneous and comparable at several locations along the edge. The magnification used to show the apex here is absurdly high – very few images such as these have been openly shown on the internet, if at all.

On this steel, and with our approach, stropping actually raised a nanometric, forward facing burr. It is undetectable by optical or (human) tactile means, as the size (both of the apex and the burr) is in the double digit nanometre range.

The burr improved the sharpness of the blade in the usual proofs of sharpness, such as whittling a hair or slicing paper.

After slicing just 5 cuts through manila rope, the apex has rounded over significantly. This would still be nearly undetectable with optical means, and it still felt like a very sharp knife. The stropped knife has a higher amount of apex wear than the unstropped one.

Does this mean you should stop stropping?

I don’t think so. This is a single data point in a single steel. Especially on steels where deburring is more difficult, stropping might behave differently. Moreover, the blade was sharpened with very specialised stones that are designed around a superior cutting action, thus further reducing burr formation.

I think this raises more questions – especially ones such as: where does the material for this burr come from? Is this burr responsible for the perceived increase in sharpness on soft and thin materials such as paper, hair or a bess test?

For me personally, I have stopped stropping when I started using my own stones. They produce a devilish sharp edge, with a very clearly defined apex. The increase in sharpness which can be undertaken from stropping doesn’t seem to translate into a longer lasting sharp edge. More data points for this are needed – which we will be giving in PART 2 (TBC).