It's hot in Hiroshima during the Obon festival.
"Japan is practically subtropical now!"
"This cold beer is amazing!"
My optimistic younger brother, who shares my late father's love for beer, said this as we returned to our hometown for a class reunion and to visit the family graves. We talked about how our grandfather must have been a craftsman, discussing his knife-making and sharpening skills while admiring the bamboo crafts, carvings, and woodworking he had done.
For woodworking tools, a brilliant cut and a precise mortise joint rely on a finely honed edge. I've acquired some incredible finishing whetstones for this purpose, and I even gave one to my brother.
My childhood admiration for my grandfather's whetstone, which he never let me use, still lingers.
However, as a modern engineer, I'm prone to saying,
"In the end, sharpness is just a matter of Ra (Roughness Average), the alignment of two flat surfaces.
You just need to be able to polish a flat surface without damaging the material."
In reality, to achieve the ultimate sharpness required for razors and finishing planes, the steel itself must be of extremely high quality. This includes fine-grained carbon steels with minimal carbide content (e.g., Hitachi Metals, Shirogami #1) or low-carbon, high-chromium stainless steels with very fine carbides (e.g., Böhler Uddeholm, AEB-L).
By "ultimate sharpness," I mean an edge with a sharpness level equivalent to a microtome, which can slice cells without destroying them. This is an Ra ≤ 50 nm.
For the final, microtome-level polishing of carbon steel blades, commercial alumina suspensions like Baikalox® 0.05CR Polishing suspension work quite well. However, when it comes to the user experience of sharpening, it feels like there's room for improvement. The combination of an acrylic plate, the suspension, and the carbon steel doesn't feel perfectly matched for flat polishing.
After returning home, I was looking for a better way to disperse the 0.05 CR alumina powder when my eyes fell on a dried-out "Kieiro Pit" glue stick.
"Ah, this is PVP (PolyVinylPpyrrolidone)! And it has a pH indicator (thymolphthalein)!"PVP has polar, hydrophilic chains that act as a stable "basic/neutral steric dispersant." This makes it an excellent colloid stabilizer, preventing bridging. Alumina has an isoelectric point that is slightly alkaline (around pH 8–9), causing it to flocculate easily as the pH approaches this point. It is most stable in a neutral to slightly acidic range (pH 6–7).
I was excited. I spread the Kieiro Pit glue on an acrylic plate and kneaded it with about 50 parts of purified water until the color disappeared. As the atmospheric carbon dioxide neutralized the solution, I added a spatula-full of the Ra = 50 nm Baikalox 0.05 CR alumina powder, along with a small amount of isopropyl alcohol to act as a defoaming agent and viscosity modifier. I continued to knead the mixture into a smooth, thick slurry. The nano-polishing slurry was complete.
When I used this slurry to sharpen a yokote-kogatana (traditional Japanese single-bevel craft knife), it felt different from the commercial suspension. It glided smoothly without that grating sensation, yet it polished firmly. As I worked, the back of the blade transformed into a mirror-like surface. After cleaning it with a spray of isopropyl alcohol and wiping it with a Kimwipe, its edge took on a hazy, almost optical-camouflage-like appearance. It's difficult to photograph a perfectly clean mirror.
Meanwhile, the front side of the blade looked like a cross-sectional observation surface, revealing the layers of soft iron and the effects of forging.
I then tried slicing the end grain of a piece of Hinoki cypress. The cut was incredibly smooth and glossy.
Cross-section of the Hinoki-cypress (8×8 mm)
The PVP from the Kieiro Pit glue stick seemed to support the Baikalox like a healing spell from a white mage in a fantasy novel.
"Hey, how about we go for a motorcycle ride at the end of summer to relax in a hot spring with a Hinoki cypress bath?"
"Sounds great! After a hot ride, nothing beats a cold beer!"
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© 2000 Takayuki HOSODA.
