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. As this is a review of my own product, you could consider this an advertisement. I have applied the same scientific principles and morale vigor as I do to every review – nevertheless, I feel like I should mention: this is WERBUNG.
Review
I know – I know. We’ve had this before:
The thing is – I am a big fan of Kaizen (改善). You see, one reason why I started making abrasives was my frustration with the quality of abrasives on the market. And that desire to improve, to innovate is a constant in my actions. When I launched my sharpening stones a year ago, I was quite happy with the performance – it felt, and still feels like a very, very high performance abrasive. But time hasn’t stood still – I have constantly improved. Compared to a year ago, the stones are cleaner. They are nicer made. Distribution of particles has been fine tuned, improved and the concentration varied until it hit a sweet spot.
Moreover, in the reviews, I’ve acquired and started to incorporate better metrology – and last but not least, I still owe the reviews for the 1 and 0.5 µm stones.
Therefore, I felt like it was time to update the review.
Let’s start with the outer part: 4 mm blank, fully black anodised. Cleaner interface between stone and resin.
Moreover, after having a small batch (produced at the end of 2025) that had a tendency to come apart when stored improperly, I have improved on the bonding technique. The aluminium blanks now feature mechanical interlocking, laser structuring and are plasma cleaned. It has increased bond strength between the abrasive and the blank so much that I actually can’t take them apart, no matter what I tried. I am very convinced that any future purchases will be very strongly bonded and not come apart.
There’s a new case design, that’s a bit smaller and sleeker. The engraving now features a very proud “Made in Germany” on it, and has also improved in quality and resolution by switching from a blue diode to a MOPA laser.
Let’s take a look under the optical microscope!
Optical micrographs of the stone. Instrument: Marvscope
The stone still features what I believe is important – diamond, the binder and nothing else. This is already very apparent at the high resolution optical microscopy shot above.
Let’s take a closer look in the SEM:
SEM micrographs of the 2.5 micrometre stone. Instrument: Zeiss GeminiSEM 560.
The picture above is taken from the dressed, factory new surface of the stone. We can see a very fine distribution of particles, with no apparent clumping. The funny thing about such small diamond particles is – because the molecular weight between binder and diamond is so identical, contrast is very low, and we can’t really see them visually if they are hidden below the binder. If you compare this with the optical image above, it seems like the concentration is much lower. Fortunately, one can “make” the SEM look “deeper”, by increasing the accelerating voltage. Then, the electrons start to tunnel, and go further into the material, increasing the interaction depth.
This is exactly what I did for the EDS measurement – we now have interaction not just on the very surface of the specimen, but also much deeper – a couple of microns deep! This immediately shows the high concentration of the stone:
EDS analysis of the stone. Instrument: Oxford Ultim Max ∞ 40mm2 EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.
Something I want to highlight here – the distribution is frankly outstanding for such a small stone. First, there is no parasitic particle, no foreign abrasive or anything but the carbon that forms the diamond visible. Moreover, if you look at the spacing and clumping of the 2.5 micron particles – I am personally speechless. There is no agglomerate whatsoever and a very fine distribution.
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, edge trailing 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 M398 edge finished with the stone. Instrument: Zeiss GeminiSEM 560
We can see a very smooth surface. There are some nanometric scratches remaining, only visible at the higher magnifications. The whole bevel has a polished appearance to it – the carbides of M398 are wonderfully visible. A fine, sub micrometre apex is also visible, with no apparent burr at the magnifications shown.
Optical micrograph of the M398 bevel. Please note the increased magnification compared to other reviews – this is at the limit of optical microscopy. Instrument: Marvscope
The optical high resolution shot shows a similar image – very visible carbides, and sub micron scratches. This is a wonderful, very regular result for a mirror-like apex. The blade itself is blazingly sharp – just like it should be!
A sidenote: I have been constructing my own microscope for quite some time, to have better optical images. The above image is as far as I remember the first high resolution shot on the blog – taken with a fantastic NA 0.8 lens. I’ll do a post on the build and microscope in a few months, but can proudly state: I have achieved diffraction limited resolution with that build.
Let’s take a look at the WLI height map:
3D surface height map of the M398 Bevel. Instrument: Zygo NewView 9000, Objective Lens: 50X. Metrological filter chain: LS-Plane to orient data, cutoff 0.25/99.75 percent to remove outliers.
The white light interferometric measurement shows a near constant shape over the observed region – this is a very flat (sub 0.1 µm) bevel! Moreover, very little stray scratches are detectable,
With the surface roughness parameters as follows:
| Sa | 0.0042 | µm |
| Sq | 0.0053 | µm |
| Ssk | 0.2987 | – |
| Sku | 2.913 | – |
ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.025 mm (gaussian). No F operation besides LSQ leveling.
A single digit nanometric surface roughness is approaching optical specifications – and is a stunning statement to the cleanliness of this stone.
The stone itself still is “quick”, for it’s micron rating. There’s a bit more feedback than on previous generations.
Let me be very clear here though: If you already are a customer and have a fine set, there is very little sense in upgrading. Don’t have regrets because you bought an earlier set – they are fantastic, high performing and stelar abrasives. The performance gain on this on, just like every time you approach peak performance, is of course of the diminishing kind. It’s not 100% better than the previous ones, but a constant improvement over time – in the spirit of Kaizen!
If you don’t have one already – this stone probably is the reason why I make abrasives. It is spectacular, so clean – increasing sharpness, gloss and finish of a bevel by a wide margin. I love it and it gets used on every knife I sharpen for myself.
Leave a Reply