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 stone is a natural stone, which is often called an Arkansas stone, from one of the regions where it is found. The actual mineral is called Novaculite, and is a microcrystalline configuration of silica. The stone I review was sold by a German sharpening supply stone and has the distinct and characteristic translucent grey colour. It’s a very interesting stone to me, as I have been using Arkansas stones for several years to deburr a fixture or vise at my professional day-job! They are very common in the German manufacturing world.

Let’s take a look under the microscope!

Optical micrographs of the Arkansas stone. Instrument: Leica Emspira 3.

It’s a mostly homogenous stone, with some lighter coloured particles that conglomerate a bit together. A few black or dark particles are visible.

SEM micrographs of the Arkansas stone. Instrument: Zeiss Gemini 560.

The first thing that jumps out under the scanning electron microscope is how regular this stone is! We’ve had a couple of natural stones on this blog before, and most were a wild and inhomogeneous mix between flakey matrix and small abrasive grains. Meanwhile here, the stone consists out of an uncountable amount of small, blocky grains. Because of this structure, the mineral novaculite is considered a microcrystalline one, which also explains the artificial “grit size” many vendors assign to this type.

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

The EDS analysis shows what we expect – the mineral novaculite is quartz after all, so an abundance of Si and O can be found in this analysis. Quite a bit of carbon and a trace amount of iron can be found. I would guess that this stems from the manufacturing process and probably the transport / storage as well.

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 stone has a high surface ratio and is quite smooth. This correlates to the feel of it during sharpening – especially with a bit of oil, it is gliding with very little resistance back and forth. Because of a couple of high spots, there is some microvibrating feedback expected.

ISO 25178 parameters.

Let’s take a look at how this stone sharpens a blade!

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 Arkansas stone. Instrument: Thermo Fischer PhenomXL SEM.

The blade has a nice edge to it. Only at very high magnifications, micro burring becomes prominent. A couple of deeper scratches are created, as well as some prow formation on the side of these. As with most natural stones, the large size distribution of the abrasive particles can sometimes produce exceptional results, but also give you periodic deep scratches. Nevertheless, the Arkansas stone created a very nice, homogenous surface finish that approaches a glossy mirror finish. The blade itself was just barely sharp enough to shave and tested in at around 140 BESS. Compared to the yellow Belgian coticule (which I loved!) from the last blog entry, I would say that this stone is much harder, does more of a burnishing action with less material removed and leaves a rougher finish. I will continue to use these for deburring fixtures for the CNC machine, but I don’t think this will become my favourite for sharpening!

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.