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 natural stone. Belgian coticules are a very well renown grinding and sharpening stone. They are commonly found in the Ardennes region of Belgium in central Europe. It’s a sedimentary rock with a high content of very fine garnet, a hard silicate mineral. Hence their suitability for sharpening! Because yellow Belgian coticule is quite a mouthful, I’ll appreviate it as “YB” for the rest of the article. I’ve previously analysed blue Belgian coticule – find it in the archive .

Optical micrographs of the “YB”. The magnification is visible in the lower right corner. Instrument: Leica Emspira.

Surprisignly, the garnets in this stone are of a different colour than in the blue stone! Let’s take a look under the SEM:

SEM micrographs of the “YB”. Instrument: Zeiss GeminiSEM560.

This stone overall is much finer. The flakey particles are thinner, and the garnet grains are definitely in the single digit µm range. A high, dense surface is created.

Let’s take it for an EDS analysis! If you are interested, I’ve written a brief introduction into the different analytical techniques of the SEM here.

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

I’ve written a bit about the possible chemical composition of garnets on the previous post, and there is fantastic research out there. Maybe more interesting here is the direct comparison of yellow and blue coticule from the same manufacturer:

EDS composition of BB (left/first picture) and YB (right/second picture)

The YB shows a much lower content of iron (1.2 vs 4.3%), with slightly higher levels of Mg and Al.

Let us now take a look at the surface topography!

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

The surface is smooth, even and shows an exceptional amount of material ratio at the top!

ISO 25178 parameters.

The ISO 25178 parameters confirm this – it is smoother, finer and more even than the BB stone reviewed in the last blog entry.

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 “YB”. Instrument: Thermo Fischer PhenomXL SEM.

The blade has a nice gloss to it, and very small, fine scratches all over it. The actual cutting edge is sharp and well defined – albeit with some serrating. These serations are typically right at the end of a scratch – I suspect larger, jagged particles, that tear out a bit from the cutting edge.

Overall, the stone gave a nice feedback and a very clean finish. This is a very fine, very cool stone. I like it and I can now understand the Internets fascination with natural stones! That something dug out of the earth leaves such a fine finish and wonderful edge with this preparation is quite remarkable.

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.