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 has turned into a lesson for me. I read about this stone on the internet, saw a couple of youtube reviews where it gets hyped. According to the manufacturers page, it’s a mixed abrasive stone with a high diamond concentration, some SiC particles in a dense, ceramic matrix. I ordered two different ones – the #400 and the #1000 grit stone. Unfortunately, this stone packs a “surprise” I have been expecting for a long time, but did not expect to meet in this one.

Let’s start with the optical micrographs:

Optical micrographs of the KINTIF #400 mixed abrasive stone. Instrument: Leica Emspira

The stone shows a mix of black and green particles. In the randomly choosen FOV, a large yellow spot is visible. Overall, it looks like mixing could be improved here, but also that a low of abrasive is in this stone.

The #1000 grit stone is lighter in colour, as the smaller particles typically refract the light differently. It is more inhomogeneous as well, with large white spots (probably the ceramic binder?) visible.

Optical micrographs of the KINTIF #1000 mixed abrasive stone. Instrument: Leica Emspira

Let’s start on the SEM pictures! Because the stones are very large, very heavy and porous, but also not very expensive, I chipped off a large piece from the corner – this gives us a nice view inside the composition, and reduced vacuum pumping time. Furthermore, there’s less danger of contaminating the inside of the SEM!

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

We can see two different species of abrasives here – bulky diamonds with a pretty regular size distribution, as well as clear SiC grains that are more irregular, both in shape and size. While zooming in to 1000x magnification, I stumbled across something I didn’t ever want to stumble across. Here it is in very large magnification for you:

The sharpening stone contains nests of thin, sub micrometre fibres with a large aspect ratio. If you look on the 500x or 1000x magnification, more of these can be spotted.

I quickly checked on the #1000 stone: here, we can also find these nests of fibres, with the rest of the morphology similar, just at a smaller scale.

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

Now, why do alarm bells go off in my head when I find fibres in a non-western world sharpening stone? For this, I have to expand a bit. Most abrasives you see on this blog aren’t specifically designed for knife sharpening. They were made to be used in CNC machines, where they rotate at enormous speeds. Some are adapted to better fit the task of hand guided sharpening, but overall: the R&D effort to produce a sharpening stone tailored to hand sharpening is immense, and the market is not.

Now, on rotating abrasives, there are immense centrifugal forces. To give you a sense at what surface speed steel is ground: The general suggested rotational speed is in the magnitude of 35-100 m/s – that’s 360 km/h or 220 mph. Spinning that fast puts enormous forces, especially on large and heavy wheels. Ceramics are not well known to be very strong in the tensile regime. Commercially, large wheels are therefore reinforced, for example via glass fibre or cotton matts. Now, finding chopped, sub micron fibre, let’s a very specific alarm bell go off in my head. You see, I like to collect old books. Specifically, old books about resin technlogy, grinding and abrasives. One of my favourites is the “handbook of plastics – Vol X – Duroplaste”, which came out in 1968, and only exists in German language.

The reason alarm bells go off in my head is, a fantastic fibre reinforcement, that was very popular before we knew what it did is… asbestos. Asbestos really ticks all the boxes in what we want as reinforcement for a grinding wheel: It’s lightweight, good tensile strength, good coating behaviour, heat resistant, cheap, easy to mix and chop to the desired length. Oh, it’s also without doubt super bad for your health as it’s highly carcinogenic.

I’m not a forensic investigator. I know about asbestos, and some basics facts. It’s a fibre, sometimes spikey, sometimes it looks like cooked, limp spaghetti. It’s sub micron, and has large aspect ratios (much longer than the diameter). Let me pull up the picture of that fibre nest again:

Sooooo. Maybe you want to be scared, with me? We can further dig into this, by doing elemental analysis. Asbestos consists out of Mg, Si, O, sometimes with Fe, Ca or Ka. Depending on the mix of these elements, it has different fancy geological names. Let’s take a look at the chemical composition of this fibre nest:

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

Now, is this conclusive? Not really. Because the interaction volume of the electron beam is quite far reaching, and the fibres are thin, and behind it is a ceramic stone that also contains Mg, and O. But if you look at the individual channels, you can make out: no Carbon in the fibres, but the Mg and O channel are very clearly visible in the shape of fibres. This means it’s definitely no organic fibre (like cotton). At this size, any inorganic fibre will be hazardous, quite possibly carcinogenic if it enters your lung. And sharpening creates ultra fine particle dust due to the abrasion.

I am not a forensic expert and have thus far not encountered real asbestos fibre in my SEM life. But this makes me cautious enough, that I won’t be using nor testing this stone, and will dispose of it as special waste. I seldom give hard recommendations in this blog, but with this one, I stand quite firm:

If I was you, I wouldn’t use this stone. And because of it’s origin, I wouldn’t trust any explanation by the manufacturer. Because if this was harmless fibre reinforcement, it should have been part of the marketing pitch – and while I can’t conclusively declare that this is asbestos, I can conclude it’s inorganic fibre and will be bad for your health.

Better be safe and buy a slightly more expensive sharpening stones. There are good alternatives out there.

And on a sidenote: when we zoom out, one can see that there is very little diamond in this stone and mostly SiC – so why not get a nice AO stone like the shapton glass? It’s superb, safe, and a lovely, similar prized alternative.

Second sidenote: I always thought I’d encounter a scary stone one day. I so far thought it would be a historic, soviet era stone or a pre-WW2 german resin stone. Alas – here we are.