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’ve kind of been avoiding today’s sharpening stone – they are after all quite pricey! But, I am very glad I finally got around to it when they were on sale in Germany. Gentle readers, today we are taking a look at the very well reputed Naniwa Diamond Pro in it’s 600 Grit version!

Let’s take a look under the optical microscope!

Optical micrographs of the Naniwa Diamond Pro 600 stone. Instrument: Marvscope

This is going to be an interesting stone. We can see a mixture of a lot of different colours, but also different particle sizes.

Let’s take a closer look in the SEM:

SEM micrographs of the Naniwa Diamond Pro 600 stone. Instrument: Zeiss GeminiSEM 560.

The stone shows a standard, phenolic, grumbly and quite fine resin bond. The grains do not seem to be embedded very firmly – on the top layer, one can make out gaps between the grains and the resin bond. Moreover, there is a large difference in size of the particles: some small, sub 5 micron particles, but also larger particles around 30 micrometre.

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 Naniwa Diamond Pro 600 resin stone. Instrument: Oxford Ultim Max  ∞ 40mm² EDS sensor. Note that our EDS sensor doesn’t show elements lighter than boron.

As I suspected in the optical micrograph, this stone consists out of several different abrasives. We have the diamond, which shows a medium concentration, but also a slight tendency to clump together. My understanding is that Naniwa uses concrete style mixers, and I have to give them kudos here – I expected much worse. We can see several secondary abrasives (I learned that term from the TSPROF marketing guy to justify their new series parasitic grains, and I am now going to use it to the end of eternity…), mostly SiC which is used as a filler particle (that is the correct term) to increase hardness of the bond, but also some Al-Na-O-F compounds, which is typically the mineral cryolite (sodium hexafluoroaluminate). It helps to reduce loading, but also for the bond to break down and renew the surface. This is one hightech abrasive here, I expected nothing else from Naniwa.

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 differ from the usual process by using a Katocut Nowi Pro to keep the angle constant. 2 blades are sharpened, one in 65 HRC M398, one in 59-60 HRC Nitro V.

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

Let’s start with the harder steel – the M398 blade:

SEM micrographs of the M398 edge finished with the Naniwa Diamond Pro 600 resin stone. Instrument: Zeiss GeminiSEM 560

The stone left a slightly marred surface, but for 600 grit this is quite fine. The apex is very straight, albeit not super sharp. I would say for this grit size, this is totally fine and a good result. The bevel itself is slightly marred – one could feel the bond slightly breaking down, and the slush on the surface likely contains a large amount of rolling grains. This is reflected in the optical micrograph, which shows a very matte and diffusely scratched appearance:

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 as follows:

Sa	0.3037	µm
Sq	0.4004	µm
Ssk	-0.7666	–
Sku	4.773	–

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 rolling particles seem to have created a more diffuse and locally pitted surface in the softer steel, whereas at the same time the scratches appear less deep to me.

Optical micrograph of the NitroV bevel. Instrument: Marvscope

Very slight raising of the apex is visible in the WLI images – I would guess that the softer steel is slightly deformed here.

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.

Surface roughness is quite good for such a coarse stone:

Sa	0.2510	µm
Sq	0.3384	µm
Ssk	-1.031	–
Sku	5.741	–

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

Overall, the stone has a pleasant feedback. It is regular, homogeneous and for a resin stone quite hard. The reputation for freehand sharpening is well earned.

The results are totally fine for a 600 grit stone, with good, high-tech composition, decent mixing and abrasive grain density.

I think the only issue one can have with this stone is the (in Germany) quite high price tag for a very thin layer. I heard that they are prone to wearing a bit quicker than other stones, which makes this an expensive stone with a high performance. When you compare it’s performance to a shapton glass for example. I do not think it is justified. Nevertheless, I like it and am absolutely looking forward to trying it’s finer brother soon!