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 blue Belgian coticule is quite a mouthful, I’ll appreviate it as “BB” for the rest of the article.

Let’s take a look under the microscope!

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

It quite definitely is a natural stone, and the reddish garnets are easily visible, even at low magnification! Let’s take a closer look under the SEM:

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

The morphology of the stone is a mix between very fine, flakey particles and rhombic particles. The majority of distinguishable particles is in the size range of very low double digit µm. The stone is quite dense, with very few voids.

To dig a bit deeper into the chemical composition, we will be using an EDS sensor to identify elements. 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.

The chemical analysis shows a typical natural stone – it’s bright, colourful and there’s a lot of elements into it. Garnet is a mix of Mg, Fe, Mn, Al, Si and O, which are all elements we are able to find above. In addition, there’s Ti and K, Na.

Whenever I look at natural stone, I’m reminded at how complex mineral geology is. These other elements are all easily replaced inside the minerals, as their chemical behaviour is similar. One can see that the actual garnet on this stone is not super dense – looking to the signal from Fe, Ti and Mn should point out the garnet distribution in the upper interaction volume of the stone.

There has been a massive amount of research, mineralogy and history of whetstones undertaken in regards to the belgian coticule. Besides modern high resolution SEM analysis, I don’t think I have the expertise to add something to it. There’s a fantastic, very long document initiated by Henk Bos: “Grinding and Honing Part 4: Belgian Whetstones INFO 20M”.

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 measurement shows a fine, dense surface. A high material ratio points towards a highly active surface area, reducing the force per grain. I believe that the typical application of these stones is to create a slurry of abrasive garnets on top, which aid in lubrication but also material removal.

ISO 25178 parameters.

The ISO 25178 parameters confirm this – a relatively smooth stone, with a low spread for parameters such as Sdc and Smc.

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 “BB”. Instrument: Thermo Fischer PhenomXL SEM.

The blade has a multitude of small scratches and prows along those scratches. The actual cutting edge is sligthly blunted, and there is some amount of carbide cracking near the edge. Compared to the preparation with DrMarv stones beforehand, the edge lost some gloss and mirror finish. I think this stone would work fantastically as a roughing stone for less high-tech steels, where the carbide content is lower.

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.