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.