Abraham Anapolsky, one of the metallurgical collaborators that vetted the steel chapter in my book volunteered to shoot some scanning electron microscope (SEM) images of blades that Konrad Sauer sharpened with his 15,000-grit Shapton waterstone and his ultra-fine natural Japanese waterstone. If you haven’t read the first part of this story go here to get up to speed or what follows won’t make much sense.
Abraham writes: “I finished the data collection, and I have a few ideas I am working on especially about what makes a hard sharp edge (based on some analysis of the edge composition), but I am not finished thinking about everything. However, I do have an answer as to why the two blades look different. If you notice, in the images of the blade honed with the natural waterstone, the surface looks like crumpled paper, the higher mag picture shows this is due to random short scratches. The Shapton images on the other hand have large areas that are smooth and the scratches are all of a specific orientation. Both samples show a range of large scratches (~0.25um wide) that are continuous and very visible; I imagine that these are the remnants of the original grinding, or you used the same coarse stone on both of them. The random short scratches are consistent with a surface moving over an abrasive that is free to move (i.e. a slurry) whereas the specific direction scratches are consistent with a surface moving over a fixed abrasive. Note also that the edge on the natural-stone images has regular fine rounded serrations, whereas the Shapton edge shows no regular serration, but kind of random straight sections. Also, the Shapton edge is much thinner than the natural waterstone edge.”
Double-click on the images to see them larger.
The earlier entry suggested that the softer abrasive of the waterstone was selectively removing the softer steel around the carbides more readily than the carbides themselves, creating the matte surface. This new input suggests that the loose nature of the waterstone’s slurry is (also) contributing to the random, matte surface quality. The Shapton’s grit grains are better anchored in the stone surface which allows them to shear a straight groove in the steel, carbides and all, thus creating the flatter surface.
Mystery solved? I think so. Let me know what you think.
By the way, in Konrad’s defense I need to repeat something from my book (page 213):
I recently mentioned to Chris Schwarz, editor of Popular Woodworking Magazine, something about using a simple, USB, toy microscope to look at edges before and after sharpening. His reply was, “I got to play with one of those gizmos and it was very… humbling.” I know what he means. Magnifying a sharpened edge can spotlight a dismaying array of results. Even an edge that you are certain is as sharp as any living being could make it, one that passes all the sharpness tests and performs beautifully, will show scratches, nicks and abject irregularities that you’d swear couldn’t possibly be there. It’s not for the insecure. If Chris’s experience with the toy microscope was… humbling, mine with the SEM was undoubtedly… more so.
So, don’t worry Konrad. Those edges you sharpened may look nasty at 3000x but I, for one, know how sharp they really are.
My thanks to Konrad for asking the interesting question and providing the samples for the SEM, and to Harrelson, Gary and Abraham for offering interesting answers.