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

I’m always hesitant to review Chinese sharpening stones. I feel like any review is only a snapshot of a moment, a fleeting statement: this was the one I bought, no idea what the next batch will look like, as they are constantly changing, evolving, backtracking in their recipes. The company who made today’s sharpening stone even reached out, offering me a collaboration. I politely refused, and instead ordered this stone with my own money. Let’s dig into Forever Superabrasives Resin bonded diamond Stone – 400/1000 grit. I’ll split the important picture galleries in two parts, some might combine the sides. Please refer to the caption to see which is which.

Let’s take a look under the optical microscope!

Optical micrographs of the (left/first/green) 400 side and the (right/second/red) 1000 grit side of the Forever Superabrasives Resin bonded diamond stone. Instrument: Marvscope

The stone is probably coloured, as the difference between the two sides is quite stark. The coarser, #400 grit side shows an overall green appearance, with some copper coloured agglomerates. The finer, #1000 grit side is more homogeneous coloured, but has large, white-grey particles or agglomerates.

Let’s take a closer look at the #400 side in the SEM:

SEM micrographs of the #400 stone. Instrument: Zeiss GeminiSEM 560.

We can see a plethora of different particles here. The bond itself looks like a phenolic resin bond, based on it’s grumbly appearance. A large amount of small particles is interspersed. We can also make out some voids that look like the contour of bubbles. Grain adhesion doesn’t seem to be super high, as most of the larger grains are already showing some separation from the bond.

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 EDS analysis shows a medium diamond concentration. The large, copper coloured particles seen in the optical micrograph are actually Copper and Zinc – with quite a bit more Zinc than copper. There is also quite a bit of chromium oxide, correlating to the fine particles, but also the green colour of this side. Lastly, the usual SiC that creates the haptic feedback can be found. Overall, mixing looks like it could be improved – while the diamond is distributed quite nicely, as is expected of this particle size – the other components of this bond seem to agglomerate, and break up the regular diamond distribution. Moreover, I do not see why a manual bond would require any metal components – those are typically added for heat conductivity, and there’s not a lot of heat generated at manual speeds.

Let’s take a closer look at the #1000 side in the SEM:

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

It seems to me like this is a very similar bond- but struggling more from larger agglomerates of fine filler particles. This side also looks to me like it got baked warmer – the phenolic is less crumbly and more solid.

Let’s check out the #1000 grit side’s EDS:

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 visual impression from the SEM pictures is confirmed in the SEM. Most of the components from the #400 grit side can be found, but also large amounts of Iron as well as a much higher concentration of sodium. I would guess that the sodium (Na) is maybe some pressing agent, that did not fully debind during initial baking? Very curious. Mixing is a bit worse, which was to be expected – smaller particles are harder to distribute.

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.  As this is a benchstone, I use a Katocut Nowi Pro to keep the angle constant and get comparable results without much of a human error.

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

First, let’s dig through the 400 grit side – and we’ll start with the harder steel – the M398 blade:

SEM micrographs of the M398 edge finished with the 400 stone. Instrument: Zeiss GeminiSEM 560

Two things stand out immediately to me: Quite a lot of burr, bend over, but also a remarkably smooth bevel at this grit size. The scratch pattern is pretty reglular, with some deeper scratches. The apex is still quite wide – this definitely is the stone to remove material, and not finish a blade. It doesn’t look like the stone is really freely cutting, instead there’s a lot of burnishing, which of course helps with the surface roughness.

Optical micrograph of the M398 bevel. Instrument: Marvscope

The WLI measurement confirms the large burr, and regular scratch pattern:

3D surface height map of the M398 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.

With the surface roughness parameters as follows:

Sa	0.1211	µm
Sq	0.1552	µm
Ssk	-0.4174	–
Sku	3.965	–

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

Let’s take a look at the NitroV edge:

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

The softer steel is less polished, and the apex quite a bit finer. The stone cuts more easily into it.

Optical micrograph of the NitroV bevel. Instrument: Marvscope

This is reflected in a more irregular surface in the WLI measurements:

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.

Which result in a higher surface roughness:

Sa	0.2022	µm
Sq	0.2674	µm
Ssk	-0.4928	–
Sku	4.637	–

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

Now, let’s see how the #1000 grit side performs!

SEM micrographs of the M398 edge finished with the #1000 stone. Instrument: Zeiss GeminiSEM 560

The large burr formation, even folding over the burr completely, in combination with the surface morphology that shows a lot off miniature burr and prow formation shows that the stone struggles a lot with cutting into the hard M398. It is of course removing material, but also damaging the apex and matrix of the steel here due to the plastic deformation.

Optical micrograph of the M398 bevel. Instrument: Marvscope

Which is further visible in the white light interferometer measurements of the bevel: a diffuse, marred surface:

3D surface height map of the M398 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.

With the surface roughness parameters improved quite a bit compared to the #400 grit side:

Sa	0.0946	µm
Sq	0.1212	µm
Ssk	-0.2354	–
Sku	3.518	–

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

Let’s take a look at the NitroV edge:

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

The #1000 grit seems to be a quite a bit cleaner in this softer steel. The finish on the bevel is matte, with some irregular streaks caused by rolling grains.

Optical micrograph of the NitroV bevel. Instrument: Marvscope

Overall, the stone has typical , high filler particle resin bond feedback. There’s a certain vibration, firmness that makes it easy to keep a good angle when freehanding. It’s speed is average, the composition and manufacturing could be improved.

The results are okay. We recently had the #1000 grit Edgeworks DMT Resin stone in this blog – all objective parameters point to that one being ever so slightly better, but not decisive.

Price wise, it’s hard to beat this stone – you get diamonds, a decent feedback, in a very affordable (around 120€/$ at the time of this review) package that consists of actually two stones. This stone does get a knife sharp. It does refine the apex and bevel.

The problem is: performance is okay. Just okay, not superb, and objective measurement shows it doesn’t live up to the hype on the internet, pushed forward by cooperations and affiliate links. This leaves a slightly bitter feeling, and I think you can feel in these last paragraphs how disappointed I am.

If you don’t have powder steels, I’d instead get a Shapton Glass. I find the feedback on that one unbeaten – and the results are exceptional homogeneous. It is a little bit cheaper individually, and made by a wonderful company in Japan – with a proven record of quality & consistency.