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 “large brother” of one we had on the blog before – namely the Poltava CBN 2.5 µm. That one didn’t really work at all, for a number of reasons – too little CBN, too hard bronze binder, and some contamination. An avid reader of mine suggested I get a coarser one – and this goes hand in hand with what I concluded in the review: I could see this being a good stone at a larger grit size! Spoiler: it is!

Let’s take a look under the optical microscope:

Optical micrographs of the stone. Instrument: Leica Emspira

As usual, the bronze binder hides most of what we are looking for. But not to worry – this is the reason every stone get’s looked at under the SEM!

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

We can see that a number of dark, abrasive particles is visible. These are well within the stated size of the manufacturer. I wouldn’t call this a high concentration, but there definitely is some grit in this stone!

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.

Curiously enough, besides some SiC contamination, this stone also has some large titanium rich regions in it:

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

I am unsure why it is in here. It will make the binder much harder locally. If you have any suggestion or idea why it is in here, and not a sign of bad abrasive hygiene, I would love to hear it!

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.

We can see that this stone leaves a very serrated cutting edge. This will give you an edge on a blade that immediately grabs onto whatever you are slicing, and will feel much sharper. I think for an outdoor or everyday knife, this would be a cool, if sadly matte looking edge. The unfortunate downside is quite the wide apex – this is even visible from the side. This would not be considered a keen cutting edge.

The stone itself felt decently sharp and quick – I am a bit spoiled by my own design resin stones, and in the factory condition tested here, this stone felt a bit slower. I would guess that by etching this one, the grains would have a larger overhang, and would remove more material, easier.

Optical micrographs of the sharpened blade. The serrations but also matte surface is nicely visible. Instrument: Leica Emspira

I think this is a decent stone. I personally do not want to dabble in etching stones with chemicals, but unlike the 2.5 µm CBN stone, I would call this a working stone with a good feedback. The result is a serrated edge. If this is what you are after, this would be a very good stone to buy!

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.

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 very, very special. I humbly and very proudly present to you the result of spending a decade in the pursuit of REMOVING MATERIAL. The mad Dr. Marv bring’s you the fabulous:

Dr. Marv’s Scientific Sharpening Stones

This is my own design. I’ve written a bit more about this on the 20 µm stone review. Check it out here. I’ve also written about the 10 µm stone, check it out here.

This time we are looking at the 10 µm stone:

Optical micrographs of the stone. Instrument: Leica Emspira

The stone is a slightly greenish colour, which stems from the diamond type used. We can see regular stripes going from left to right, with a low roughness. All particles are either sparkling diamond, or if oriented in a way that no refraction happens, pure greenish diamonds. The bond itself is colourless and off white.

The stripes that can be seen are the result of the dressing employed. If you’ve read my article on dressing, or any other review, you’ve realised by now that flattening/dressing is a source of contamination. In order to avoid this, I use single point diamond chiseling – a very advanced manufacturing technique, where a very sharp, lapped diamond tool is moved in a linear motion across the surface. This has the advantage, that no foreign particles can be pushed into the surface, and exceptionally smooth and flat surfaces can be created. Here, the roughness is specifically adjusted to allow for a maximum of lubrication.

SEM micrographs of a broken through stone. Instrument: Thermo Fischer PhenomXL.

In order to better show the diamond density and distribution, I took a stone and broke it in half. What you are looking at above is the “cleaved” surface through the stone. We can see that a) this is only and purely diamond! no foreign particles! and b) there is a MASSIVE amount of diamond in this stone. Compare this to any other diamond stone you have seen before on this blog.

The standard for grinding abrasives states the concentration in percent – a C100 concentration is supposed to be 25% by volume. The actual measurement (volume, weight times density, volume before sintering, volume after sintering…) is to my knowledge not defined. I have decided to just state the percentage by weight – which is 50%. 50% of this stone is diamond, 50% is resin.

Moreover, I’ve taken special care in the QC and production of these stones. The resin, aluminium and work is all made in Germany. The diamond is sourced from abroad, but heavily checked under the scanning electron microscope:

SEM micrograph of the used diamond powder. Instrument: Thermo Fischer Phenom XL

This first batch has the following particle metrics:

Particle metrics for the 5 µm nominal sized stone.

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. For this blade, the edge was first set with a coarse F150 electroplated diamond stone, and then prepared with a F400 and F600 EP diamond stone from TSPROF. Afterwards, it is ground with Dr. Marv’s 20 µm stone.

SEM micrographs of the edge after preparation with Dr. Marv’s 20 µm stone. Instrument: Thermo Fischer PhenomXL SEM.

Then, the edge is prepared with Dr. Marv’s 10 µm stone. The stone is lubricated with a high performance honing oil. Afterwards, the edge is cleaned. No deburring/stropping is undertaken. We can see that a much smoother, more homogeneous edge is created. The burr is removed and a finer apex created.

SEM micrographs of the edge after preparation with Dr. Marv’s 10 µm stone. Instrument: Thermo Fischer PhenomXL SEM.

Finally, we take Dr. Marv’s 5 µm stone:

SEM micrographs of the edge after preparation with Dr. Marv’s 5 µm stone. Instrument: Thermo Fischer PhenomXL SEM.

We can see that the already very fine surface was further refined. It has become so smooth, that our small desktop SEM is approaching it’s resolution limit in terms of surface information – the noise form the electron source is now larger than the differences in height, a remarkable achievement for surface finish! We can also really see the beautiful carbide distribution. Because the surface is so much smoother, this information becomes more noticeable. The apex is refined and razor thin. This is a blade that easily slices – a BESS recording below 90 was taken on this!

The difference is also easily visible under the optical microscope (note the identical magnification, chamfers are just differently sized as I used separate pieces of steel).

Edge Morphology under the optical microscope. First/Left picture: finished with F600 EP. Middle/Second picture: Finished with Dr. Marv’s 20 µm stone. Right/Third Picture: Finished with Dr. Marv’s 10 µm stone. Fourth / last picture: Finished with Dr. Marv’s 5 µm stone. Instrument: Leica Emspira.

In order to further highlight the progression, here’s SEM pictures of the cutting edge at every preparation step:

SEM micrographs of the edge at different preparation steps. From left/ first picture onwards: EP F600, Dr. Marv’s 20 µm, Dr. Marv’s 10 µm, Dr. Marv’s 5 µm. Instrument: Thermo Fischer PhenomXL

Moreover, the gloss of the chamfer really starts to pop at this grit and scratches are barely visible:

Photo of the surface of the cutting chamfer. Note the very shiny reflection!

With some light stropping, this would make for an exceptional cutting edge!

That’s it folks. I’m very proud to announce that these are now available in a very limited handmade batch. Link is here or find it in the shop the top of the blog!

The medium grit set of Dr. Marv’s Scientific Sharpening Stones in their storage box. Proudly handmade by Dr. Marv in the German Alps.