Domed whetstones?

[Barry's Knives]

In sword sharpening it seems common practice to use convex or domed whetstones. Does anyone have any experience using these? And can anyone explain why its beneficial?

 

[Helmore]

Aren't they just using their stones that way because they don't bother to flatten their stone?

 

[Tristan]

The only reason they wouldn’t bother to flatten when sharpening is their livelihood is because it either doesn‘t hurt the outcome or it helps improve the outcome

 

[Barry's Knives]

Aren't they just using their stones that way because they don't bother to flatten their stone?

As far as I can tell they are shaped that way purposely. Whetstones tend to dish when used, not become convex

 

[spaceconvoy]

Have you ever gotten errant scratches from the raised edge of a dished stone? If they didn't convex the stone, they'd have to flatten it every few minutes. The only real advantage to a flat stone is to simplify the geometry for amateurs who struggle with finding the proper angle. If this is your life and you're being paid > $10K to achieve a perfect mirror finish over the length of a full sword, a flat stone causes more problems than it solves.

 

[ian]

Have you ever gotten errant scratches from the raised edge of a dished stone? If they didn't convex the stone, they'd have to flatten it every few minutes. The only real advantage to a flat stone is to simplify the geometry for amateurs who struggle with finding the proper angle. If this is your life and you're being paid > $10K to achieve a perfect mirror finish over the length of a full sword, a flat stone causes more problems than it solves.

Interesting. It does seem to me that there would be significant downsides to having a domed surface, since then the knife would only be contacting the stone in a small area at any given time. If you have a flat bevel on a knife and a flat stone, you can have a larger contact area and hence cut your polishing time by a big factor. But I suppose that if all their bevels are convex, the flatness wouldn’t be an advantage. And I guess you’re right that they’re always using the full width of the stone, so the only parts that wouldn’t be hit, and so would become problematic with a flat stone, are the top and bottom, which are the lowest parts of the dome.

I don’t know anything about sword polishing. Interesting question.

 

[Barry's Knives]

As far as I know, they polish thousands of tiny sections and blend them with finger stones. So my best guess at the moment is that the curved stone contacts the convex edge at very precise points.

 

[childermass]

As far as I know, they polish thousands of tiny sections and blend them with finger stones. So my best guess at the moment is that the curved stone contacts the convex edge at very precise points.

And from my experience and talks with sword polishers that’s exactly what this is about
There are lots of techniques in sword polishing that make it very different from knife sharpening.
First and most important thing is that they don’t try to get the blade sharp but this will just happen along the way if done right.

 

[spaceconvoy]

... But I suppose that if all their bevels are convex, the flatness wouldn’t be an advantage ...

Exactly! Compared to a kitchen knife, a katana's cross section is highly convex. If you imagine the blade road of a standard deba approximated as a section of a cone - a surface with single curvature* - then the blade road of a katana would be approximated as a section of a sphere. Because we're not robots, freehand sharpening a katana on a flat stone would give a very slight advantage, while increasing the risk of scuffing on the stone's edge. That trade-off makes total sense for a relatively short kitchen knife that's sharpened for daily use, but not a katana with double curvature that's given a mirror polish once a decade entirely for aesthetics.

*Not really though. The blade road on a single bevel knife is very slightly convex, but an order of magnitude less than a katana. For the sake of argument we can approximate it as a surface with single curvature, because when freehand sharpening on a flattened stone the entire blade road from edge to shinogi line contacts the stone on each stroke - again, cause we're not robots.

Here's some cool stuff:

 

[dafox]

Seems to me that a convexed stone helps to prevent faceting and helps to blend all of the many sharpening angles used together, a muddy stone probably also helps.
What is the material that is being painted on in the last video?

 

[Barry's Knives]

Seems to me that a convexed stone helps to prevent faceting and helps to blend all of the many sharpening angles used together, a muddy stone probably also helps.
What is the material that is being painted on in the last video?

I'm guessing its nugui, typically a mix of compounds containing aluminium oxide

 

[Barry's Knives]

Exactly! Compared to a kitchen knife, a katana's cross section is highly convex. If you imagine the blade road of a standard deba approximated as a section of a cone - a surface with single curvature* - then the blade road of a katana would be approximated as a section of a sphere. Because we're not robots, freehand sharpening a katana on a flat stone would give a very slight advantage, while increasing the risk of scuffing on the stone's edge. That trade-off makes total sense for a relatively short kitchen knife that's sharpened for daily use, but not a katana with double curvature that's given a mirror polish once a decade entirely for aesthetics.

*Not really though. The blade road on a single bevel knife is very slightly convex, but an order of magnitude less than a katana. For the sake of argument we can approximate it as a surface with single curvature, because when freehand sharpening on a flattened stone the entire blade road from edge to shinogi line contacts the stone on each stroke - again, cause we're not robots.

Here's some cool stuff:

Trying to polish a shig deba, I find the super convex edge quite tricky to finish well without fingerstones. So perhaps I'll keep my eye out for a cheapish curved stone.

 

[spaceconvoy]

What is the material that is being painted on in the last video?

at 7:55 the video mentions 'a compound of powdered deer horn.' I got curious and went down a sword polishing rabbit hole and found this interesting tidbit:

"Before actually performing hadōri, Mr. Abe does something rare in the world of sword polishing. He uses a mixture of powdered deer antler to paint the hadōri-line onto the blade. This is a special technique he learned as part of the Ono School of sword polishing, a trademark of Ono Kokei, the Living National Treasure sword polisher. In essence, this step allows the polisher to foresee what the hadōri will look like when completed and make any last minute changes to his stone selection, line creation, and so on. The goal of creating the hadōri is to create a gracefully flowing line that highlights the work of the smith while maintaining visual balance."

From Google Books: Japanese Swords

 

[nutmeg]

Sword polishing is more like sculpting metal so they need to adapt stones to some blade shapes very different from kitchen knives.
For example, with my Tamahagane blade from Yasha Yukawa I had to give a slightly convex shape to my stones in order to get the job done.

For kitchen knives I don't see many advantages to work on unflattened stones.

 

[Barry's Knives]

Sword polishing is more like sculpting metal so they need to adapt stones to some blade shapes very different from kitchen knives.
For example, with my Tamahagane blade from Yasha Yukawa I had to give a slightly convex shape to my stones in order to get the job done.

For kitchen knives I don't see many advantages to work on unflattened stones.

Is this to exert more pressure on precise parts of the blade to essentially 'sculpt' it more accurately? Is it more common on lower grit stones?