TL;DR:

Lemma: When flattening / dressing a sharpening stone, what’s the best approach to avoid contamination? Continuation from a previous post, expanding with other methods.

Methodology:

• Made some contamination free 10 µm resin diamond sharpening stones (identical to Dr. Marv’s Scientific sharpening stone)
• Flattened 1 sample on a glass plate with toothpaste
• Flattened 1 sample on an ATOMA F400 electroplated stone under running water
• Flattened 1 sample on a SiC flattening Stone
• Flattened 1 sample on a coarse resin stone (TSPROF Alpha)
• Looked under the SEM for contamination – using a backscatter detector which shows elemental contrast and EDS, which identifies elements and thus nails contamination down.

Results:

• Flattening a resin stone on the ATOMA F400 works fantastically. Very fast, and because the japanese totally nailed their bond, it doesn’t seem to release any diamonds!
• All other methods showed varying success. The SiC flattening stone proved completely unsuitable, leaving large tracks of SiC on the stone surface. The toothpaste left very little contamination, but was super slow in renewing the surface of the stone. The coarse resin stone left some particles, but worked.

Actual Science and long version:

Sharpening stones experience uneven wear. This is because they are inherently anisotropic in their composition, but also because we as humans use them inequally. Often, the end parts of the stone do not get used, as you do not want to fall “off the edge”. Moreover, different movements, pressures and just general wear sometimes require you to flatten, dress or renew a stone. For simplicity’s sake, I will from this point onwards call the process “dressing”, as it is the technical term. What you apply this to (renewing the surface, flattening the stone or actual dressing, e.g. creating a surface morphology suited to the application) is irrelevant, as the general mechanic is a 3 body abrasion on a flat surface. In the last instalment on this, we looked at popular choices. Since then, I’ve sharpened a lot of knives and used a lot of different sharpening stones. I’ve experimented with dressing and I think I found a wonderful solution, which I want to share here!

Let’s take a look at the methods:

I.) Toothpaste on a glass plate

I’ve tried several “home methods” that could be considered abrasives. Something I had high hopes for, but didn’t work at all was salt – I thought some coarse salt, very lightly wetted, would probably abrade the soft matrix. Sadly, after a lot of rubbing it around, I didn’t see any change on the stone. Same with coffee grounds! The last thing I took a look at was toothpaste. I used a charcoal containing “whitening” one, as those have a high content of polishing particles in them. (Exact brand, without wanting to make advertisements for them: “Colgate Sensation White: Aktivkohle”)

The toothpaste worked … kind off. Removal is very slow, and it takes a long time to remove even a bit of material. Nevertheless, it smells fresh and minty. That’s nice.

Under the SEM, the surface looks like this:

Because we are using the “BSD” sensor of the SEM, heavier elements appear with a lighter colour. We can make out a lot of particles here on this surface -but all “medium grey” ones are actually the diamond. A couple of small, lighter coloured ones are visible. Via the SEM’s EDS sensor, we can identify these:

These particles can be identified as “SiO2”, which actually is the abrasive of many toothpastes. I’m surprised that it’s pretty much the same size as the diamonds – but I guess 10 micrometre SiO2 means that it’s not yet ultrafine particulates, and still is small enough to not feel like a mouth of sand.

SiO2 is hard enough to scratch martensite, so at finer stones, I wouldn’t advise to use toothpaste to prefer the surface. Nevertheless, it’s a gentle and smooth way to renew a resin stone and bring new diamonds to the front.

II. ATOMA F400 under running water

Yes – another EP stone! Last time I used a relatively cheap and low technology EP stone. There, we found larger diamonds, corresponding to the EP stone grain size. In the time since then, I’ve reviewed the ATOMA F400 as a sharpening stone – and was not only thrilled by it’s performance to set a bevel (still my favourite stone for this!), but also by the high-tech, super strong bond ATOMA employs. Because this bond is so strong, I tried dressing the resin probes with this one, to see whether it would hold the grain better. For this, I first took a piece of relatively soft stainless steel (cheap kitchen knife) and vigorously rubbed it over the stone’s surface for some minutes, under running water. This got rid of any “looser” particles. After this, I rubbed the sample specimen in a figure 8 movement directly under the stream of water.

Let’s take a look at the SEM:

We can see a smooth, regular surface. No foreign particles are visible in the BSD – and also no “dulled” or shattered diamond particles. Frankly, I don’t see any resin bond being strong enough that two diamonds can crush each other – and for me, this is the “premium” method to renew a resin stone, which is also what I’m using on my personal sharpening stones. It’s relatively affordable, wonderfully quick and leaves the sharpening stone at a smooth, low roughness. Because the ATOMA plates are very well made, the sharpening stones become ultra flat, too! At some point, it will feel like the stone gets “sucked” onto the ATOMA.

The EDS analysis shows no foreign particles:

III. SiC dressing block

Very readily available, and dirt cheap are so called SiC dressing/flattening stones:

First, let us take a look at these under the SEM,because I’m curious what they actually are:

SEM micrograph of a “SiC flattening stone”. Instrument: Thermo Fischer PhenomXL

This is pretty cool! Looks like super large, sintered and slightly fused SiC particles, with lots of porosity in between. EDS analysis only showed Si and C – so this really is what the label says.

A larger excerpt (stitched image of about 5×4 mm):

Dressing on this one is QUICK. Like, this thing is a file. I used it under running, warm water. Unfortunately, in the SEM, but also to the naked eye, brown streaks are visible after a short amount of dressing:

SEM micrograph of the SiC flattening stone “dressed” surface. Do note the angled, bright coloured streaks which are SiC.

In the SEM, these are immediately visible as light coloured, heavier element particles. EDS analysis shows that this is SiC which rubbed of onto the diamond stone surface:

Detail of the streaks:

EDS showing the Si channel, visible all over the sample. Instrument: Thermo Fischer PhenomXL.

IV. Coarse resin stone

A good sharpening contact and fantastic customer told me he uses an old resin stone to dress his stones. I was curious whether this would loose some particles, and tried it out myself. For this, I used a TSPROF alpha 120 µm stone – because it is very coarse, resin based and I’m not a particular fan.

This showed larger, bright particles in the SEM. EDS analysis:

We can see some Sodium and aluminium in these particles – and if we compare this to the composition of the TSPROF stone:

So – while it worked surprisingly well, and quickly created a relatively smooth surface, we also abraded the softer fillers of this stone into the diamond resin stone sample. I would say this is a decent method, if you have a diamond resin stone which contains some filler – the particles seem to be in a similar size to the stones grit size, although this could also be just happenstance. Overall, I think this is an “okay” way to dress a stone, but falls short of the ATOMA performance.

Final conclusion:

I originally planned (and had some designs!) for a contamination free dressing device to offer to similar enthusiasts as I am. But for my personal sharpening stones, I think the ATOMA F400 is a fantastic choice in dressing them. It’s relatively affordable, super quick, smooth surface and lasts quite a long time. I was able to dress about 30 sharpening stones before it became a little bit slower. In the SEM, I wasn’t able to find any foreign particles, or larger loose diamonds. Customers have reported great success in dressing even fine, ultra pure stones with this. As I personally also use an ATOMA to set bevels, I’ve just created a rotation – first use them to sharpen, and when they become slightly dull, they get relegated to “dressing duties”.

If you have any input on methods for a 3rd instalment of “dressing methods, please reach out to me!