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 Naniwa. I had the Chosera on this blog a bit earlier, this time it’s the “super” series. Overall, this stone feels a bit less premium, but this might also be because it’s the colour of cleaner! 🙂

Let’s take a look at this horrendously yellow stone:

Optical micrographs of the stone. Instrument: Leica Emspira

We can see that this is a very flat stone, with some larger, dark spots on it. It also contains some coloured particles. Overall, the grit is quite fine. Let’s take a closer look under the SEM!

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

The stone consists mostly out of very small grains. Some regions have agglomerations of even finer, dust like particles, whereas a couple larger pieces can be made out at lower magnification. Overall, for a synthetic stone, this is not a very good size 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.

The EDS analysis of this stone is wonderful in it’s colour composition! We can see that it is a wild mix of aluminium oxide and silicon oxide. Some SiC particles can be found as well!

3D surface height map of the stone. Instrument: Bruker Alicona µCMM, 50X objective lens, 3×3 FOV high resolution focus variation scan. Data is leveled and outliers removed (0.25%).

The 3D height map shows some very flat spots, but also some deeper spots. A large scratch is also visible in the lower portion of the frame. This is also visible in the ISO 25178 parameters, where a large roughness, and low material ratio Smr is identifiable.

ISO 25178 parameters.

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 cutting edge surface is very homogenous and matte with this stone. Moreover, the apex is very rounded over – the whole blade gives the appearance of having been smoothed over in an abrasive slurry. While sharpening, this is also what happened. The loose stone became very quickly a slurry of particles. Seeing as the majority of the abrasive in this stone is of the oxide type, this slurry is not very hard and struggles to cut instead of burnishing the M398 steel of this blade.

Overall, I liked the Chosera better – but that might also be because of it’s grit difference! Or because of the not-obnoxious colour! 🙂

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.