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 benchstone – yes, dear readers, we’ve fast arrived at my “benchstone era”. It’s a Shapton glass in the #1000 grit. It’s a japanese aluminium oxide stone with a remarkable reputation.

Optical micrographs of the stone. Instrument: Leica Emspira

The stone itself is an off-white colour, very homogeneous. Touching it feels like touching a slightly gritty ceramic tile – after all, that’s pretty much what it is! Let’s take a look under the scanning electron microscope:

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

I always find it fascinating how the SEM manages to resolve what looks like a very homogeneous, flat stone under the optical microscope into distinct shapes. The stone here shows a flakey, crumbling matrix with lots of slightly flat, cubic abrasive grits in it. This probably is the defining characteristic of the grit rating in ceramic sharpening stones!

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 stone shows a nice, dense distribution of Al2O3 particles in a MgO matrix. Moreover, some larger SiO2 particles can be made out. I would guess that these wear out first, allowing for some porosity / dimples in the surface where swarf and lubricant can fit in.

In order to evaluate the sharpening performance and material removal mode of this stone, a blade was sharpened with it. As this is a benchstone, I’m using a Katocut Nowi Pro to sharpen the blade and an exact angle and remove the human error. Two blades are sharpened – one is a custom heat treated M398 (65 HRC), one is a commercially available Nitro-V Blade (60 HRC), which shows the stones behaviour in two wonderful steels near the opposite ends of the spectrum of knife steels. The stone was used wet and regularly splashed with water.

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: Thermo Fischer PhenomXL SEM.

The stone struggled quite a bit with the M398 edge. Instead of a clean cutting action, one could feel that it is slightly glitching over the surface of the stone, and the edge actually got a bit duller during use, compared to the edge preparation beforehand. In the SEM, we can see that the apex was pushed over – a clear sign that not enough cutting action, but instead a lot of plastic deformation is happening.

Now, in the NitroV steel, this looks completely different:

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

The edge is smooth, regular, no plastic deformation. A nice, toothy appearance is visible along the apex. The blade turned out quite sharp, with the surface much more homogeneous than in the M398.

The stone itself is wonderful- the feedback is constant, it is quick cutting in NitroV, and exceptional fun to use, especially as it is splash and go. A well made, affordable gem for sharpening, if your preferred knife steels are not super hard and high carbide content. The results speak for themselves!

I’m really surprised – but have to state that I absolutely love this stone and will use it regularly on my simpler kitchen knifes.