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).