Dr. Marvin Groeb – Abrasive Solutions

Category: Allgemein

Sharpening Mechanisms – Part 1 – Katocut Nowi Pro

This is the start of a new blog series, where I try out sharpening mechanisms. I’m not yet fully sure about the format – I will write about things that matter to me, such as build quality, capability to sharpen, but also very subjective things like how it feels to sharpen with this. If I was you, I would expect this to be a rare blog segment. The reviews will be independent from the manufacturer, without control over what I test or write. I am not paid nor do I receive anything from the manufacturer if you decide to buy one.

If you make a sharpening device and you want me to test it, feel free to reach out.

KATOCUT Nowi Pro

This sharpening mechanism is made by the Austrian engineer Alexander Hackl. He operates an engineering company under the name KATOCUT. The exact model for this post is the “Nowi Pro Black Edition”, which at the time of this review retails for around 1600 Euro, including VAT.

The sharpening mechanism is based on what is typically called a “Bogdan” principle. This means the knife is held at the desired angle by fixing that degree of freedom, while leaving most other directions free to move. I’ll dig deeper into this later on, but basically this means the angle is not dependent on the knife geometry or the height of the sharpening stone – which is also a principle, much in contrast to guided rod sharpeners, where every point along the curve of the knife later is sharpened to the same, precise angle.

Basic configuration to put the Katocut together with the clamping mechanism. And a very pretty custom Shard from Roman Kasé in Rex 121.

The Katocut comes in a very high quality, solid case with a precise foam cutout. The baseplate is heavy, 8 mm thick steel that looks powder coated. The vertical rod is a 25 mm steel rod with a brushed finish. The arm is black anodised aluminium, with a (vertical) thickness of 20 (first link) and 14 mm (second link). The kit includes different mounting options, such as a clamp as well as a large and small magnet. The vertical guide rod features the angle-adjustable mechanism and there is a spring to adjust preload / weight compensation of the apparatus.

A digital angle meter as well as all needed tools plus some food safe oil to lubricate the bearings and mechanism is included. No stone holder is included, but I would guess that you already own one, so that’s not a large issue.

Build Quality

The build quality of the black edition is very high. All parts are nicely anodised or coated, with no rack marks visible. The parts are made well – good straightness, no machining marks and homogeneous chamfers really create a high quality appearance. The 8 mm steel baseplate in combination with the 25 mm steel vertical rod give the whole mechanism quite some weight, further increasing the qualitative appearance. The threads are well machined, and everything fits together nicely without major play or struggles. The high price of the device is mirrored in the build quality, much more so than on other sharpening mechanisms. The machinist in me can only enjoy this!

A couple of shots from the Nowi Pro.

Working Principle

The angle adjustment mechanism. This constrains the angle of the blade, relative to a horizontal surface. Note the secondary swivel bearing below the mechanism.

The idea behind any of these “Bogdan-Style” sharpeners is that a mechanism locks the angle of the blade, relative to the stone. On this system, this is achieved via a series of swivel joints and a threaded part of the rod. By setting a reference, for example via a digital level box, the angle can be adjusted very finely. Afterwards, the two screws are locked around the angular joint via supplied wrenches.

Bevel box sitting on the parallel ground spline of a production Shard – if you have a fully ground blade, you need to compensate for the blade bevel angle.

The reason this principle works so well can be put down to the principle of “constraints”. Every mechanism that moves has some degrees of freedom. Imagine a linear rail: you can move it back and forth, but not sideways or up and down. It is therefore constrained in 1 dimension.

What happens here on the Nowi pro is similar – in that our system is also constrained – by the linkage joint shown above. This angular constraint makes sure that our knife only ever touches the stone at the angle we want:

This angle will stay constant, even if we rotate the vertical rod – it will just shift the contact point somewhere else along the stone! This is also true for moving the blade back and forth, or up and down – the angle won’t change, as it is constrained.

By arranging a second bearing, and this one after the constrained angle, it is possible to tip the blade from side to side – which will allow you to sharpen every point of the blade, even the tip, at this specific angle.

What this means is that the system is pretty robust in terms of knife geometry (length or width of the blade does not matter!), but also height of the sharpening stones. Where on other systems you have to fiddle and adjust if your sharpening stone progression has different heights, this system by design and constraints always has the same angle.

Sharpening Process

The use of this device is pretty straightforward: Clamp your knife, either in the supplied centering clamp, or attach it to one of the magnetic bases. The adjust the angle of the joint mechanism via the digital level box, make sure the screw nuts are tightened, and start sharpening!

Adjusting the angle of the blade – first by setting the reference (“zero”) on the stone, and then adjusting the joint mechanism. This Kasé shard was then sharpened to 17 DPS. Note that this blade features a parallel ground section on the spline where the measurement is easy – if you have a fully ground blade with no parallel section, you need to account for the blade angle as well.

The sharpening action itself is – and I have to say this is of course very subjective – wonderful. This is what I think sharpening should feel like: relative movement between the blade and the sharpening stone, with no fiddling, no adjustments. Just you and the movement!

When I started sharpening over a decade ago, I probably bought the same entry level stones everyone has lying around – some whetstones that you had to soak. The sharpening action on those was mostly defined by my skill – if I held the knife at the wrong angle, or wobbled, the result was a dull knife. But the freehand experience seems to scratch some primal itch, it is, for lack of better words how a knife should be sharpened.

This mechanism fixes this issue in a big way: skill is not really needed here. I find this to be super true, as the first knife I ever sharpened on this mechanism probably was my sharpest knife to date… and it has only gotten more impressive with a little bit of practice. Check the action out below:

Sharpening on the Nowi Pro – note the swivel mechanism of the clamp.

Conclusion

There is a lot of fantastic things about this mechanism. I absolutely love the way the sharpening movement happens here – if you ask me, this is how sharpening should be. It’s the freehand experience for those of us who lack the skills to sharpen freehand.

The build quality and packaging of the device is stelar, as one would expect at this price point. Pair this device with a set of high quality benchstones (for example, Dr. Marv’s Scienitfic Benchstones), and you will have fantastic edges:

A Kasé Shard (NitroX) sharpened on the Katocut Nowi Pro. Benchstones used: Dr. Marv’s Scientific Benchstones in 180 – 40 – 10 – 2.5 µm progression.

I’ve had the Nowi since November of last year. It has replaced all other mechanisms and devices I own to sharpen my own knives. I don’t think I can give any higher praise.

Moreover, this device will enable me to expand the blog reviews on sharpening stones towards benchstone sized ones – look out for some cool new reviews coming!

26. January 2026
A brief study on sharpening stones – Part 44 – PDTools Diamond Vitrified 100 grit (Vitrified, Diamond)

This is part of a series of blog posts – looking into the appearance and composition of commercially available sharpening stones. If you are interested in the previous episodes, check out the archive for them.

If you have some suggestion on what I should look at next, or want to share your super secret DIY stones, I could be persuaded to open the bag of analytical devices… hit me up on Instagram under @marvgro for that.

Disclaimer: I’m not for sale. Every review you see on this blog is bought with my own money. I have no affiliation to any manufacturer.

Review

Today’s sharpening stone is another PDT. After I was super disappointed in the much hyped CBN vitrified, and equally in the even more hyped silver, I thought: why not spend more of my money on another PDT stone. At least this time, it contains my favourite abrasive, diamond. Diamond is only metastable, meaning at around 680°C, it becomes graphite. Making a vitrified stone, where ceramic components need to at least achieve a glassy phase, below that temperature, is quite tricky. Let’s take a closer look!

Optical micrographs of the PDT Vitrified diamond stone. Instrument: Leica Emspira

These are some chunky diamonds! But also, lots of other particles we can peek here. This will be an interesting stone under the SEM!

SEM micrographs of the stone. Instrument: Zeiss GeminiSEM 560.

We can see lots of coarse, abrasive particles inside a brittle matrix. This looks a lot like their previous vitrified stone – but in this case, with diamond as the abrasive. One question here would be: did they manage to stop the diamond from becoming graphite? This is quite hard to detect, a first hint can be given by switching sensors back and forth. For this, I used the InLens detectors of our fantastic Zeiss SEM – switching the detected electron type, and also energy filtering. Especially the EsB sensor is very sensitive, graphite shows up in a different brightness than diamond.

SEM micrographs of a diamond, comparing it’s appearance between the SE1 (InLens) and EsB (Backscatter InLens) detector. Looks very homogeneous! Instruments: Zeiss GeminiSEM 560.

I did not find any difference here, even at lots of filtered energies – a good sign!

Let’s look at the chemical composition! For this we are going to use an advanced SEM technique called EDS. If you want to know more about this, I’ve written extensively about SEM microanalysis here on this blog.

EDS analysis of the stone. Instrument: Oxford Ultim Max ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

We can immediately identify the diamond, and also in a decent concentration and distribution! The matrix is a standard vitrified bond, consisting of different oxides. I find the addition of titanium quite curious – this probably gives the bond some tensile strength when used in CNC applications? I’m now very excited to try this out!

In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. I am using a standardised testing procedure, read about it here. Nevertheless, it’s 65 HRC M398, and sharpened to 17 DPS with resin bond diamond stones down to 10 µm. Afterwards, the tested stone is used, first in a back and forth movement until the surface becomes homogenous, and then alternating strokes (5-5-3-2) on each side, for a total of 20 strokes towards the apex per side. No pressure is applied but the weight of the apparatus.

The edge is then analysed in the electron microscope for breakouts and morphological appearance.

SEM micrographs of the edge finished with the stone. Instrument: Thermo Fischer PhenomXL SEM.

The stone itself feels ultra coarse. There’s a massive amount of vibration/feedback, and one can really see how it removes material. It also stinks like no tomorrow. This is something I find with a lot of eastern stones – I’m unsure what they do to these, and when I use them they have been thoroughly cleaned and survived high vacuum inside the SEM. Nevertheless, the stone quickly removed a lot of material. The hype about this stone is in some part understandable – after just a few stroks, one can feel something akin to a burr! Taking a closer look under the SEM, this “burr” is revealed not really as a burr, but as a massive, deformed apex. I generally sharpen without any pressure but the weight of the aparatus, and the whole bevel is bend at nearly 45°, forming a super wide apex of > 10 µm. Brittle spots where the matrix cracked and individual carbides are visible can be identified. I’d say that without a high resolution optical microscope, it can be easily mistaken for ultra quick burr formation. But what we typically look for in a burr – a properly formed apex, isn’t visible here. Instead, massive plastic deformation prevails, which will probably make it harder to achieve superior sharpness later in the process.

I’m really disappointed – I was hoping for a long lasting, bevel setting coarse stone. My guess is that the very hard vitrified bond pushes against the steel matrix and thus mostly deforms and pushes the material.

Moreover, the bevel shows very deep, coarse scratches. I think this could be an option if you are a knifemaker and really need to remove a lot of material before forming the first apex, but for every knife that already was sharpened before, I find this stone to be more of a “wreck your steel” than “prepare that bevel” solution.

Optical micrographs of the bevel sharpened with the PDT Vitrified stone. Note the large breakouts and folded over bevel. Instrument: Leica EMSPIRA 3.

Sharpening disclaimer: I use a standardised approach to sharpening, which basically follows how most manufacturer of guided systems tell you to use this system. I am very aware, that every stone could perform much better than this, in terms of sharpness, but I want a comparable approach. The sharpening segment mostly shows the material removal mechanism – is it burnishing? is it cutting? is the cutting pressure too high so that carbides crack? Is there massive burr or prow formation? The BESS value definitely doesn’t highlight the ultimate sharpening performance of the stone, but was an often requested information. Over time, this blog will show BESS values for different edge morphologies, but by the holy endmill – don’t read it as a „this is the max value this stone can achieve“. I would also suggest to familiarise yourself with the works of Immanuel Kant, it’s absurd I need to write such a disclaimer here.

25. January 2026
A brief study on sharpening stones – Part 43 – Shapton Pro Ha-Nu-Korumaku 1000 Grit (Aluminiumoxide)

This is part of a series of blog posts – looking into the appearance and composition of commercially available sharpening stones. If you are interested in the previous episodes, check out the archive for them.

If you have some suggestion on what I should look at next, or want to share your super secret DIY stones, I could be persuaded to open the bag of analytical devices… hit me up on Instagram under @marvgro for that.

Disclaimer: I’m not for sale. Every review you see on this blog is bought with my own money. I have no affiliation to any manufacturer.

Review

Today’s sharpening stone is a japanese whetstone – the Shapton Pro Ha-Nu-Korumaku at 1000 grit. It’s a coloured alumina oxide stone. Shapton has a stellar reputation in the sharpening world. The 1×6″ version I bought was purchased through a German online shop.

Let’s take a look under the optical microscope:

Optical micrographs of the stone. Instrument: Leica Emspira

We can see some larger, very white particles in an orange matrix. The white particles are most likely the aluminium oxide – the purer, the whiter it becomes!

SEM micrographs of the Shapton Pro stone. Instrument: Zeiss GeminiSEM 560.

I’m not sure about you, but I am not surprised at this point! It’s exactly what one would expect: AO particles of the correct size in a hard, brittle matrix. This is not surprising, as it’s a japanese stone and they have a reputation for honesty and hihg quality products. I’m curious though, what the chemical composition is like!

Let’s look at the chemical composition! For this we are going to use an advanced SEM technique called EDS. If you want to know more about this, I’ve written extensively about SEM microanalysis here on this blog.

EDS analysis of the stone. Instrument: Oxford Ultim Max ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

The EDS analysis shows it’s a mix between different oxides – mostly Al2O3, MgO and to some lesser extend CaCO3 and SiO2. It’s quite tricky to make any oxide ceramic ultra pure, and probably also not needed for this stone. I think it’s therefore save to declare that it’s Al2O3 particles as the main abrasive, in a matrix of other, softer oxides that make up the bond design.

In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. I am using a standardised testing procedure, read about it here. Nevertheless, it’s 65 HRC M398, and sharpened to 17 DPS with resin bond diamond stones down to 10 µm. Afterwards, the tested stone is used, first in a back and forth movement until the surface becomes homogenous, and then alternating strokes (5-5-3-2) on each side, for a total of 20 strokes towards the apex per side. No pressure is applied but the weight of the apparatus. As this is a water stone, I’ve used water instead of the typical oil.

The edge is then analysed in the electron microscope for breakouts and morphological appearance.

SEM micrographs of the edge finished with the stone. Instrument: Thermo Fischer PhenomXL SEM.

The stone struggled quite a bit with the M398 steel I use for the tests. The apex was rounded over a little bit, and overall during use, it felt like the stone looses more than the steel edge I’m trying to sharpen. Moreover, some larger particles introduced pronounced scratches. Overall, I think the blade got noticeable duller due to the preparation with this stone. But from it’s composition, general well made quality, I would guess that this is an exceptional (and very affordable!) stone for less high-tech steels.

24. January 2026
Musings about material removal – Part 5 – Dressing sharpening stones and inevitable contamination (continued…)
TL;DR:

Lemma: When flattening / dressing a sharpening stone, what’s the best approach to avoid contamination? Continuation from a previous post, expanding with other methods.

Methodology:
- Made some contamination free 10 µm resin diamond sharpening stones (identical to Dr. Marv’s Scientific sharpening stone)
- Flattened 1 sample on a glass plate with toothpaste
- Flattened 1 sample on an ATOMA F400 electroplated stone under running water
- Flattened 1 sample on a SiC flattening Stone
- Flattened 1 sample on a coarse resin stone (TSPROF Alpha)
- Looked under the SEM for contamination – using a backscatter detector which shows elemental contrast and EDS, which identifies elements and thus nails contamination down.
Results:
- Flattening a resin stone on the ATOMA F400 works fantastically. Very fast, and because the japanese totally nailed their bond, it doesn’t seem to release any diamonds!
- All other methods showed varying success. The SiC flattening stone proved completely unsuitable, leaving large tracks of SiC on the stone surface. The toothpaste left very little contamination, but was super slow in renewing the surface of the stone. The coarse resin stone left some particles, but worked.
Actual Science and long version:

Sharpening stones experience uneven wear. This is because they are inherently anisotropic in their composition, but also because we as humans use them inequally. Often, the end parts of the stone do not get used, as you do not want to fall “off the edge”. Moreover, different movements, pressures and just general wear sometimes require you to flatten, dress or renew a stone. For simplicity’s sake, I will from this point onwards call the process “dressing”, as it is the technical term. What you apply this to (renewing the surface, flattening the stone or actual dressing, e.g. creating a surface morphology suited to the application) is irrelevant, as the general mechanic is a 3 body abrasion on a flat surface. In the last instalment on this, we looked at popular choices. Since then, I’ve sharpened a lot of knives and used a lot of different sharpening stones. I’ve experimented with dressing and I think I found a wonderful solution, which I want to share here!

Let’s take a look at the methods:

I.) Toothpaste on a glass plate

I’ve tried several “home methods” that could be considered abrasives. Something I had high hopes for, but didn’t work at all was salt – I thought some coarse salt, very lightly wetted, would probably abrade the soft matrix. Sadly, after a lot of rubbing it around, I didn’t see any change on the stone. Same with coffee grounds! The last thing I took a look at was toothpaste. I used a charcoal containing “whitening” one, as those have a high content of polishing particles in them. (Exact brand, without wanting to make advertisements for them: “Colgate Sensation White: Aktivkohle”)

The toothpaste worked … kind off. Removal is very slow, and it takes a long time to remove even a bit of material. Nevertheless, it smells fresh and minty. That’s nice.

Under the SEM, the surface looks like this:

Because we are using the “BSD” sensor of the SEM, heavier elements appear with a lighter colour. We can make out a lot of particles here on this surface -but all “medium grey” ones are actually the diamond. A couple of small, lighter coloured ones are visible. Via the SEM’s EDS sensor, we can identify these:

These particles can be identified as “SiO2”, which actually is the abrasive of many toothpastes. I’m surprised that it’s pretty much the same size as the diamonds – but I guess 10 micrometre SiO2 means that it’s not yet ultrafine particulates, and still is small enough to not feel like a mouth of sand.

SiO2 is hard enough to scratch martensite, so at finer stones, I wouldn’t advise to use toothpaste to prefer the surface. Nevertheless, it’s a gentle and smooth way to renew a resin stone and bring new diamonds to the front.

II. ATOMA F400 under running water

Yes – another EP stone! Last time I used a relatively cheap and low technology EP stone. There, we found larger diamonds, corresponding to the EP stone grain size. In the time since then, I’ve reviewed the ATOMA F400 as a sharpening stone – and was not only thrilled by it’s performance to set a bevel (still my favourite stone for this!), but also by the high-tech, super strong bond ATOMA employs. Because this bond is so strong, I tried dressing the resin probes with this one, to see whether it would hold the grain better. For this, I first took a piece of relatively soft stainless steel (cheap kitchen knife) and vigorously rubbed it over the stone’s surface for some minutes, under running water. This got rid of any “looser” particles. After this, I rubbed the sample specimen in a figure 8 movement directly under the stream of water.

Let’s take a look at the SEM:

We can see a smooth, regular surface. No foreign particles are visible in the BSD – and also no “dulled” or shattered diamond particles. Frankly, I don’t see any resin bond being strong enough that two diamonds can crush each other – and for me, this is the “premium” method to renew a resin stone, which is also what I’m using on my personal sharpening stones. It’s relatively affordable, wonderfully quick and leaves the sharpening stone at a smooth, low roughness. Because the ATOMA plates are very well made, the sharpening stones become ultra flat, too! At some point, it will feel like the stone gets “sucked” onto the ATOMA.

The EDS analysis shows no foreign particles:

III. SiC dressing block

Very readily available, and dirt cheap are so called SiC dressing/flattening stones:

First, let us take a look at these under the SEM,because I’m curious what they actually are:

SEM micrograph of a “SiC flattening stone”. Instrument: Thermo Fischer PhenomXL

This is pretty cool! Looks like super large, sintered and slightly fused SiC particles, with lots of porosity in between. EDS analysis only showed Si and C – so this really is what the label says.

A larger excerpt (stitched image of about 5×4 mm):

Dressing on this one is QUICK. Like, this thing is a file. I used it under running, warm water. Unfortunately, in the SEM, but also to the naked eye, brown streaks are visible after a short amount of dressing:

SEM micrograph of the SiC flattening stone “dressed” surface. Do note the angled, bright coloured streaks which are SiC.

In the SEM, these are immediately visible as light coloured, heavier element particles. EDS analysis shows that this is SiC which rubbed of onto the diamond stone surface:

Detail of the streaks:

EDS showing the Si channel, visible all over the sample. Instrument: Thermo Fischer PhenomXL.

IV. Coarse resin stone

A good sharpening contact and fantastic customer told me he uses an old resin stone to dress his stones. I was curious whether this would loose some particles, and tried it out myself. For this, I used a TSPROF alpha 120 µm stone – because it is very coarse, resin based and I’m not a particular fan.

This showed larger, bright particles in the SEM. EDS analysis:

We can see some Sodium and aluminium in these particles – and if we compare this to the composition of the TSPROF stone:

So – while it worked surprisingly well, and quickly created a relatively smooth surface, we also abraded the softer fillers of this stone into the diamond resin stone sample. I would say this is a decent method, if you have a diamond resin stone which contains some filler – the particles seem to be in a similar size to the stones grit size, although this could also be just happenstance. Overall, I think this is an “okay” way to dress a stone, but falls short of the ATOMA performance.

Final conclusion:

I originally planned (and had some designs!) for a contamination free dressing device to offer to similar enthusiasts as I am. But for my personal sharpening stones, I think the ATOMA F400 is a fantastic choice in dressing them. It’s relatively affordable, super quick, smooth surface and lasts quite a long time. I was able to dress about 30 sharpening stones before it became a little bit slower. In the SEM, I wasn’t able to find any foreign particles, or larger loose diamonds. Customers have reported great success in dressing even fine, ultra pure stones with this. As I personally also use an ATOMA to set bevels, I’ve just created a rotation – first use them to sharpen, and when they become slightly dull, they get relegated to “dressing duties”.

If you have any input on methods for a 3rd instalment of “dressing methods, please reach out to me!
13. January 2026
A brief study on sharpening stones – Part 42 – PDTools Silver 160 Grit (125/100 µm, CBN, Metal Bond)

This is part of a series of blog posts – looking into the appearance and composition of commercially available sharpening stones. If you are interested in the previous episodes, check out the archive for them.

If you have some suggestion on what I should look at next, or want to share your super secret DIY stones, I could be persuaded to open the bag of analytical devices… hit me up on Instagram under @marvgro for that.

Disclaimer: I’m not for sale. Every review you see on this blog is bought with my own money. I have no affiliation to any manufacturer.

Review

Today’s sharpening stone is the coarser brother of the PDTools 650 grit silver we’ve analysed before. According to the manufacturer, the bond is a “hybrid vitrified” one – we’ve proven on the 650 grit that it’s mostly a markting buzz word bingo, as it’s just a very hard metal bond. Let’s dive into the coarser one:

Optical micrographs of the PDT Silver 160 grit stone. Instrument: Leica Emspira

The lovely thing about LARGE abrasive media is – they are easy to identify under an optical microscope! The 160 grit stone has very visible CBN particles. They are pretty evenly distributed and blocky in their shape. In between the CBN grains, a silver bond is visible.

Let’s focus on this stone under the SEM:

SEM micrographs of the PDT Silver 160 grit stone. Instrument: Zeiss GeminiSEM 560.

The view from the optical microscope is confirmed – we have large, blocky CBN grains inside a metal bond. The metal bond looks pretty regular and homogeneous. There are smaller abrasive particles interspersed between the gigantic (well, at least in the SEM…) CBN grains.

Let’s look at the chemical composition! For this we are going to use an advanced SEM technique called EDS. If you want to know more about this, I’ve written extensively about SEM microanalysis here on this blog.

EDS analysis of the PDT Silver 160 grit stone. Instrument: Oxford Ultim Max ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

Once again, we are shown a wonderful colourscape of different elements! The CBN grains are easily identified as such. The matrix consists once again out of copper, tin and iron. Moreover, a large number of decently fine SiO2 and SiC abrasive particles can be seen. SiC is often added as a filler to grinding tools to make them harder. This is in tune with the statement that these will last nearly forever! One can also make out some Al2O3 particles as well as trace elements. Overall, this is a very complex bond with lots of filler particles. I’m not a huge fan of fillers – I personally prefer to get more superabrasive!

In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. I am using a standardised testing procedure, read about it here. Nevertheless, it’s 65 HRC M398, and sharpened to 17 DPS with resin bond diamond stones down to 10 µm. Afterwards, the tested stone is used, first in a back and forth movement until the surface becomes homogenous, and then alternating strokes (5-5-3-2) on each side, for a total of 20 strokes towards the apex per side. No pressure is applied but the weight of the apparatus.

The edge is then analysed in the electron microscope for breakouts and morphological appearance.

SEM micrographs of the edge finished with the PDT Silver 160 grit stone. Instrument: Thermo Fischer PhenomXL SEM.

This sharpening stone is for sure a very coarse one! We are left with a ragged, wavy and “teethy” edge. The teeth are spaced apart about as far as the CBN grains are wide. This goes hand in hand with the feeling that this stone works like a file – quick material removal, with a coarse finish. Unfortunately, there is also a lot of cracking happening near the apex. The very hard bond most likely creates a lot of pressure that introduces these damages.

Optical micrographs of the sharpened edge. Instrument: Leica Emspira

Overall, this is probably the coarsest and roughest finish on a bevel I’ve had so far on this blog. Even the very coarse TSPROF alpha is not at this level. This stone is a decent choice if you need to remove a lot of material, fast. Nevertheless, it’s still slower than my favourite EP stone – the ATOMA, and leaves you with a higher degree of damages near the apex, that subsequent stones need to remedy.

I don’t detest this stone – but I think there are better choices out there if you need a really coarse rework stone, EP stones being at the very front of that list!

Sharpening disclaimer: I use a standardised approach to sharpening, which basically follows how most manufacturer of guided systems tell you to use this system. I am very aware, that every stone could perform much better than this, in terms of sharpness, but I want a comparable approach. The sharpening segment mostly shows the material removal mechanism – is it burnishing? is it cutting? is the cutting pressure too high so that carbides crack? Is there massive burr or prow formation? The BESS value definitely doesn’t highlight the ultimate sharpening performance of the stone, but was an often requested information. Over time, this blog will show BESS values for different edge morphologies, but by the holy endmill – don’t read it as a „this is the max value this stone can achieve“. I would also suggest to familiarise yourself with the works of Immanuel Kant, it’s absurd I need to write such a disclaimer here.

5. January 2026
A brief study on sharpening stones – Part 41 – Nanohone (10 µm, Diamond)

This is part of a series of blog posts – looking into the appearance and composition of commercially available sharpening stones. If you are interested in the previous episodes, check out the archive for them.

If you have some suggestion on what I should look at next, or want to share your super secret DIY stones, I could be persuaded to open the bag of analytical devices… hit me up on Instagram under @marvgro for that.

Disclaimer: I’m not for sale. Every review you see on this blog is bought with my own money. I have no affiliation to any manufacturer.

Review

Today’s sharpening stone is something quite special – a structured stone! Structuring grinding media is a relatively “novel” thing, in terms of many recent research endeavours. In my day job, this would be called “engineered grinding wheels”, EGW – and typically has massive improvements in terms of cutting pressure, cooling behaviour and swarf transport. Nanohone do 3D printed (yes! and apparently FDM printed?) sharpening stones, where the sharpening media is embedded into the filament. Thus, they are able to structure the stone surface. Let’s take a look under the microscope:

Optical micrographs of the Nanohone 10 µm stone. Instrument: Leica Emspira

If you excuse my choice of words – WILD! I did not expect this. As a side note – the anodised blank is absolutely top notch quality. Freeform fillets and rounded, very nicely anodised – this is probably the fanciest aluminium blank I have come across yet.

Let’s take a closer look under the SEM. Because the filament is very non conductive, we need to image in modes that allow for lower charge up:

SEM micrographs of the stone. Instrument: Zeiss GeminiSEM 560.

Quite wild! We can see a lot of macro-structures here – not sure where they stem from! If one zooms in a lot, the diamonds become apparent in a decent distribution.

Let’s look at the chemical composition! For this we are going to use an advanced SEM technique called EDS. If you want to know more about this, I’ve written extensively about SEM microanalysis here on this blog.

EDS analysis of the stone. Instrument: Oxford Ultim Max ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

Pretty much what one would expect from a simple thermoplast – this is filament and diamond, and not much else. The diamond is very nicely distributed, but not in very high concentration. I would guess that a higher concentration would wreck absolute havoc on their 3D printer nozzles…

In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. I am using a standardised testing procedure, read about it here. Nevertheless, it’s 65 HRC M398, and sharpened to 17 DPS with resin bond diamond stones down to 5 µm. Afterwards, the tested stone is used, first in a back and forth movement until the surface becomes homogenous, and then alternating strokes (5-5-3-2) on each side, for a total of 20 strokes towards the apex per side. No pressure is applied but the weight of the apparatus.

The edge is then analysed in the electron microscope for breakouts and morphological appearance.

SEM micrographs of the edge finished with the stone. Instrument: Thermo Fischer PhenomXL SEM.

Trying out this stone was…a bit disappointing. It felt super slow, like rubbing a piece of 3D printed plastic across the bevel. Probably, because that’s what it’s doing. After an extensive amount of rubbing, some swarf was visible – and some micro scratches started appearing on the blade. I would guess that both the lower surface area, but also the low concentration do not really help with the pursuit of a fantastic cutting edge. The final result is frankly like a slightly ragged and worn down edge from my pre-preparation. It got noticeably duller and less keen than my 5 µm finish I did in preparation. Under the optical microscope, larger breakouts and dimples were visible at the apex (compare below).

I’m not sure what to make of this. Their approach is novel, probably very difficult to achieve (mixing their own filament with a lot of abrasives?), it looks cool and is produced in a superb craftmanship. It’s just…after the first 20 strokes of this stone, I was wondering whether it works at all, and it didn’t get any better. With a much higher concentration, and no structuring, this could be a good sharpening stone, but then their unique selling point is kind of gone. Overall, I don’t think I’ll pick up this stone again.

Optical micrographs of the blade finished with the nanohone stone. Instrument: Leica Emspira

Sharpening disclaimer: I use a standardised approach to sharpening, which basically follows how most manufacturer of guided systems tell you to use this system. I am very aware, that every stone could perform much better than this, in terms of sharpness, but I want a comparable approach. The sharpening segment mostly shows the material removal mechanism – is it burnishing? is it cutting? is the cutting pressure too high so that carbides crack? Is there massive burr or prow formation? The BESS value definitely doesn’t highlight the ultimate sharpening performance of the stone, but was an often requested information. Over time, this blog will show BESS values for different edge morphologies, but by the holy endmill – don’t read it as a „this is the max value this stone can achieve“. I would also suggest to familiarise yourself with the works of Immanuel Kant, it’s absurd I need to write such a disclaimer here.

30. December 2025
A brief study on sharpening stones – Part 40 – PDT Expert Pro 7/5 (diamond, resin)

This is part of a series of blog posts – looking into the appearance and composition of commercially available sharpening stones. If you are interested in the previous episodes, check out the archive for them.

If you have some suggestion on what I should look at next, or want to share your super secret DIY stones, I could be persuaded to open the bag of analytical devices… hit me up on Instagram under @marvgro for that.

Disclaimer: I’m not for sale. Every review you see on this blog is bought with my own money. I have no affiliation to any manufacturer.

Review

Today’s sharpening stone is another PDTools sharpening stone. It seems like they have unlimited R&D and are publishing a new best stone every week, so I’m barely able to keep up with the reviews on these. Today’s stone is their high end resin stone, which according to the manufacturer has a “resin-metal bond, ideal to produce a perfect cutting edge”. Let’s take a look under the microscope:

Optical micrographs of the stone. Instrument: Leica Emspira

The sharpening stone has some earth, copper like colour to it. At higher magnifications, larger particles and some inhomogeneities are visible. This will be one interesting stone under the SEM!

SEM micrographs of the stone. Instrument: Zeiss GeminiSEM 560.

The inhomogeneous look under the optical microscope is further confirmed in the SEM. We can detect a clear resin bond – to me, it looks to be mostly phenolic resin based (starting from a powder which is then heated to create the matrix), with lots of metal powders, but also much larger, hard abrasive particles in it. Exceptionally large, hard grains can be made out that are multiple times larger than the stated abrasive size. This typically points towards either poor abrasive hygiene in manufacturing or the “fortification” of a bond by adding filler particles – and SiC typically has a fantastic bonding behaviour with phenolic resins, making these much harder and tougher.

Let’s look at the chemical composition! For this we are going to use an advanced SEM technique called EDS. If you want to know more about this, I’ve written extensively about SEM microanalysis here on this blog.

EDS analysis of the stone. Instrument: Oxford Ultim Max ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

This is one colourful EDS analysis! Wonderful. Let’s dig deeper: First, we can easily identify the diamond. It’s shown in red colour (Carbon, C) and is distributed in small nests of agglomeration. Moreover, the large, massive particles are most likely SiC (large Si peaks), and we can see lots of metal particles (mostly copper ), which is typically added to CNC tools as heat-conducting filler particles. I’m a bit stumped by the Bismuth we can find here in decent quantities. Bismuth is not used a lot in industry. It has poor heat conductivity and is very brittle, so I don’t really see the appeal to add it to a grinding bond. Sometimes, it is a byproduct of copper production, but it is also very heavy (density similar to lead). Maybe it was added to give the stones more weight and create a more premium haptic feel? I am unsure. If you know more than me, I’d love to hear your thoughts!

In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. I am using a standardised testing procedure, read about it here. Nevertheless, it’s 65 HRC M398, and sharpened to 17 DPS with resin bond diamond stones down to 10 µm. Afterwards, the tested stone is used, first in a back and forth movement until the surface becomes homogenous, and then alternating strokes (5-5-3-2) on each side, for a total of 20 strokes from the spinde towards the apex (edge trailing) per side. No pressure is applied but the weight of the apparatus.

The edge is then analysed in the electron microscope for breakouts and morphological appearance.

SEM micrographs of the edge finished with the stone. Instrument: Thermo Fischer PhenomXL SEM.

The stone has quite a bit of haptic feedback. This probably stems from the much larger SiC particles in it. The cutting edge is okay. Some waviness to the apex is detectable, as well as some rounding of the edge. Some smearing and burnished pro formation is visible closer to the apex. There is very little detectable burr. This is a standard resin finish and expected from a resin stone that contains lots of fillers.

Overall, I found the stone rather slow in it’s work. A perfect or even near perfect mirror was hard to achieve, because it constantly creates scratches and imperfections, likely from the large, hard particles in it. I’m a bit spooked by the composition of the stone, did not expect Bismuth in it. Once again, if you have any insight into why this is – please reach out!

29. December 2025
A brief study on sharpening stones – Part 38 – Dr. Marv’s Experimental Series CBN (30-15-5 µm)

This is part of a series of blog posts – looking into the appearance and composition of commercially available sharpening stones. If you are interested in the previous episodes, check out the archive for them.

If you have some suggestion on what I should look at next, or want to share your super secret DIY stones, I could be persuaded to open the bag of analytical devices… hit me up on Instagram under @marvgro for that.

Disclaimer: I’m not for sale. Every review you see on this blog is bought with my own money. I have no affiliation to any manufacturer. Note: this review is for my own product and in certain countries can be considered as advertisement. Therefore: beware, WERBUNG!

Review

Today’s sharpening stone is something pretty new to me. You might have gotten the very correct impression that I am a huge fan of diamonds. I firmly believe, and I think it is starting to come through when looking over the reviews in this blog, that diamond seems to result in a superior cutting action, which is in part because it’s a superabrasive. Super, as in super hard.

There is one other abrasive that can be considered such – and that is CBN. CBN has a similar crystallographic structure such as diamond, is much softer (about half the hardness), BUT: is chemically inert to the steels we are sharpening. I wrote a bit more about CBN here when I had the first CBN stone on the blog, check it out here. There’s a lot of myths going around CBN, and you can find many CBN stones on the market. I personally love CBN when high speed grinding in my dayjob – but have never found a decent, pure CBN stone for purchase that would allow me to explore their interaction with a cutting edge in detail. All commercial stones I’ve had on the blog so far had massive amounts of either SiC or Al2O3 as a filler in them. Hence I set out to make one myself, where the pure effect of CBN in handsharpening can be observed!

Today’s triplet of stones is a new product line which I call “experimental series”. Experimental as in: I do not think that these stones will outperform my Scientific Sharpening Stones. I actually would be surprised if they have a higher performance. But I am unable to test them in every condition, every steel and also – sharpening is a very subjective thing. Maybe some people will love the edge produced by this. I think some of my avid readers might be interested in trying this out – and become the scientist themselves through their experiment! 🙂

These stones are produced with the same principles as my diamond stones are: very pure, no filler, homogeneous grain distribution both in size and location (aka: no agglomeration). You could say, these are identical to my diamond stones, but feature CBN. How much CBN? Well, so much that they are fully black, without any colouring in them:

A set of Dr. Marv’s experimental series CBN stones – grain size: 30 µm, 15 µm, 5 µm.

This gives the stones a wonderful, cool look, black but sparkly:

The 30 µm Dr. Marv’s Experimental Series CBN stone, right after dressing, before the first use.

Let’s take a look at the composition and appearance of these stones:

Optical micrographs of the stones: First two pictures: 30 µm, Second two pictures: 15 µm, last two pictures: 5 µm. Instrument: Leica Emspira

Let’s take a look under the SEM – stone by stone. For this, I’ve taken both images of the powder used, but also broke a stone in half to enable us to look at the cross section:

SEM micrographs of the 30 µm CBN stone as well as the used CBN powder. Instrument: Zeiss GeminiSEM 560.

SEM micrographs of the 15 µm CBN stone as well as the used CBN powder. Instrument: Zeiss GeminiSEM 560.

SEM micrographs of the 5 µm CBN stone as well as the used CBN powder. Instrument: Zeiss GeminiSEM 560.

I think this experiment can be considered a success at this point! The stones show a uniform distribution of CBN grains, with no agglomeration and a very decent concentration!

In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. I am using a standardised testing procedure, read about it here. Nevertheless, it’s 65 HRC M398, and sharpened to 17 DPS with resin bond diamond stones down to 10 µm. Afterwards, the tested stone is used, first in a back and forth movement until the surface becomes homogenous, and then alternating strokes (5-5-3-2) on each side, for a total of 20 strokes towards the apex per side. No pressure is applied but the weight of the apparatus. For these CBN stones, they are also applied in their natural progession: so the blade that is sharpened with the 15 µm CBN stone is sharpened beforehand with the 30 µm CBN stone. As I consider these a “set”, it is only natural to use them to prepare the whole bevel.

The edge is then analysed in the electron microscope for breakouts and morphological appearance.

Let’s start of with the 30 µm stone again:

SEM micrographs of the edge finished with the 30 µm CBN stone. Instrument: Thermo Fischer PhenomXL SEM.

The surface shows a more matte, scratched appearance. There are a couple of deeper scratches, but they are evenly distributed. Zooming in, one can see a folded over (towards the observer) wide burr. Between the deeper scratches, heavy prow and burr formation in the apex plane can be identified – typically a sign of burnishing and not cutting. Near the apex, a couple of cracks are visible, albeit small.

Followed by the 15 µm stone:

SEM micrographs of the edge finished with the 15 µm CBN stone. Instrument: Thermo Fischer PhenomXL SEM.

The 15 µm stone shows a more refined apex, but the edge is a bit toothy now. I find this very interesting! I could image that this edge, when either stropped or further optimized via a fine diamond stone could give you a fantastic working edge.

And finally the 5 µm stone:

SEM micrographs of the edge finished with the 5 µm CBN stone. Instrument: Thermo Fischer PhenomXL SEM.

There are some weird, spidery structures near the apex on the 5 µm stone. I took this one out 3 times, cleaned it and cleaned it again. I’d say these structures, which look like heavy plastic deformation really are there. Most curious!

Let’s compare the edge in their progression: 30 – 15 – 5 µm:

A look at the morphological appearance under the optical microscope:

Optical micrographs of the edge. First two pictures: 30 µm, second two pictures: 15 µm, last two pictures: 5 µm CBN stone.

I have to say – I’m quite surprised. The surface created by the 5 µm stone is superb – a nice, glossy reflection! I have a lot of very expensive swiss CBN grinding tools at my dayjob, and I can’t produce such a surface with those. Seems like purity and good particle distribution really are key to fancy finishes!

Reflection on the 5 µm finished surface!

But if you compare the 5 µm CBN with my 5 µm diamond stone:

Comparison between the 5 µm CBN (first/left picture) and the 5 µm diamond (second/right) picture stone.

I feel like the diamond stone just… left clearer edges. More refined apex. Less burr and burnished prow formation.

I’m intrigued. My suspicion that CBN cuts less clean than diamond seems to have a first data point. I’ll revisit this in a bit with some other steel, and also some deeper look into how CBN behaves while cutting metal.

If you want to experiment with these stones – they will be available beginning of december in a very limited, individually numbered run. Just…don’t expect the same level of performance I promise from my diamond stones!

30. November 2025
Musings about Material Removal – Stropping is dead – or is it? (Part 2)

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.

20. November 2025
Musings about Material Removal – Stropping is dead… or is it? (Part 1)

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).

19. November 2025

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