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 bit of an unusal one – in terms of: I look at an abrasive, but also the mechanism used there. It’s probably the most popular sharpening mechanism currently sold in my home country – a Horl 3 Rollschleifer. At the time of the review, it retails for roughly 170 Euro (200 dollar), which is quite pricey for a sharpening device that targets a non-hobbyist group.

It is a rolling barrel. The idea behind it is:

You have a wedge with some magnets, which holds the knife at a fixed angle. Then, you have the abrasive fixed in circular form to the barrel, and roll it along the edge.

According to Horl, the abrasive side consists of extra blocky diamonds, and there is a “deburring/polishing side” as well.

Let’s take a look at the abrasive under the optical microscope!

Optical micrographs of the stone. Instrument: Marvscope

We can see a densely packed electroplatd diamond side. The distribution of the grains looks relatively homogeneous, and they are of standard shape. I cannot support the marketing argument that it’s a very blocky grain – to me, it looks absolute standard micro grit powder.

The deburring side is clearly some ceramic compound, with some darker particles embedded that have a metallic sheen. Interesting!

Let’s take a closer look in the SEM:

SEM micrographs of the EP side of the Horl. Instrument: Zeiss GeminiSEM 560.

The grain size is not super tight – we can see a wide spread between 50 and 80 µm. The grains are embedded in a smooth metallic binder, with the “proper” covering – meaning most grains are embedded about half, which is what one usually aims for in quality electroplated abrasives.

Quite a bit more interesting is the second side, the oxide ceramic deburring stone:

SEM Micrographs of the deburring Side of the Horl. Instrument: Zeiss GeminiSEM 560.

From the morphology, I am near certain that this is created via thermal spraying – the molten, impact like look is quite distinct. Interesting! I guess it is a very workable approach to coat a round workpiece with a thin layer.

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 EP side. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

The EP side, just as the morphology suggested, doesn’t have any surprises: A standard, nickel based EP binder.

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

The oxide ceramic side is revealed to be a standard thermal spraying compound – mostly aluminium oxide, with a bit of titanium oxide and calcium oxides in it.

Sharpening Performance

This review differs a little bit from others – no pre working was done on the blade, it was just sharpened with the sharperner and the included two abrasives. I only used the NitroV blade (59-60 HRC) for this review – geometry constraints! I sharpened 5 cheap kitchen knives to get used but also break in the abrasive – just as the manufacturer suggests.

The edge is then analysed in the electron microscope for breakouts and morphological appearance.

SEM micrographs of the NitroV edge. Instrument: Zeiss GeminiSEM 560

The edge shows some clear rounding over. The scratch pattern is very inhomogeneous, due to the rotating nature of the device, and shows the typical, much deeper gouges created by grains that stick out. The apex itself shows some waviness and a number of deeper dips – I would guess that some larger grains ploughed through there and created the recesses.

Optical micrograph of the NitroV bevel. Instrument: Marvscope

This observation is mirrored in both the optical micrographs, but also the white light interferometer height map:

3D surface height map of the NitroV Bevel. Instrument: Zygo NewView 9000, Objective Lens: 20X. Metrological filter chain: LS-Plane to orient data, cutoff 0.1/99.9 percent to remove outliers.

The surface roughness of the bevel is correspondingly high:

Sa	0.4508	µm
Sq	0.5858	µm
Ssk	-0.4068	–
Sku	3.813	–

ISO 25178 surface roughness parameters. S-Filter: 2.5 µm (gaussian), L Filter: 0.25 mm (gaussian). No F operation besides LSQ leveling.

The aluminium oxide side smooths over the scratch marks, giving the bevel a slight gloss:

Optical micrograph of the edge after using the deburring / polishing side of the Horl. Instrument: Marvscope.

Overall, I’m very torn. The device is super easy to use. It’s relatively compact, which means it’s easily stored in a kitchen. The positives end round about there – in my opinion, the sharpening result is very bad. There’s clear rounding of the apex, the scratch pattern is very inhomogeneous due to the movement of the grain. The lifetime of the abrasive will be limited – especially if one sharpens a high tech steel. I think every reader of my blog knows how quickly electroplated stones wear out – which is typically fine, as they are very affordable. The EP side is available individually for the Horl, as a replacement. In my domestic market, it costs 69 euros – which is frankly atrocious for the amount of abrasive on gets.

The angle wedge has some soft rubber above the magnets, which makes the whole fixturing very pliable – the result is the mediocre apex shape. Moreover, due to the shape of the sharpener, it is impossible to reach into the ricasso on most knives.

Will it sharpen a knife? Yes. But not exceptionally, and I would go so far and say not even “good”. In my opinion there are many better choices out there – any cheap guided system should give you better edges and a system where you are not locked into the manufacturer, but can choose the abrasives freely.