Konrad Sauer called me the other day with a sharpening question that I couldn’t answer. I hate that. Not the call, but the drawn blank with which I responded to his question. The issue is esoteric but that just makes it more interesting (to me, anyway). Konrad sharpens his blades using a Shapton 15,000-grit stone for the final polish. He says he notices an improvement in the wood surface quality and edge retention when he goes the extra mile (or fraction of a micron, if you will.) He recently tried a natural waterstone that he believes to be about 20,000-grit (natural abrasives can be rather vague in this regard). The surfaces left by the two stones look very different and with that, as Holmes said, the game’s afoot.
Here’s Konrad’s mystery: why is the surface left by the natural stone a uniform matte while the Shapton 15K leaves a polished near-mirror? I was stumped. I’ve noticed something similar when I used wet-or-dry sandpaper for honing but never thought much of it. Now I was on the spot. The sharpening “expert” stumped by one of his customers. I couldn’t come up with any decent reason for this at all so I emailed another expert, Harrelson Stanley of GetSharper.com, the Shapton importer in the USA.
Harrelson’s first point is that “the very best natural waterstones top out at about 10,000 to 12,000 grit” so Konrad’s final polish with his natural stone may not be contributing to his goal in this case. But I know that “near-mirror” starts at much coarser grits so this doesn’t answer the matte question. Harrelson next suggested: “the hardness of these particles is about 6 on the abrasive hardness scale; ceramic is 9 and diamonds are 10. The particles in the natural stone are very round … and do not leave sharply defined geometric walls to the grooves they cut. Therefore they do not refract light very well. The very fine ceramic stones leave extremely fine grooves … with very angular walls that refract light well.” Hence the matte surface. He also gave me a contact: Gary Runyon is a geologist with metallurgical training. I sent him my questions, then he and I had a great conversation that propelled the investigation forward.
Gary told me that the abrasive particles in natural waterstones, being softer and rounder than the grit grains in manufactured stones, have trouble abrading the various carbides in tool steel. The iron matrix is not quite as hard as the carbides it contains so the grains wear it away faster, leaving the carbides as slightly proud, rounded domes. The matte surface is essentially a relief map of the microscopic carbides exposed during honing. The harder and sharper aluminum oxide grains in the Shapton stone cut through the carbides as well as the surrounding matrix, leveling the surface to an optical mirror.
So, where to go with all this? Or… so what? Not sure, but I’ve always found interesting the way things work as they approach their limits. The notion of exposing and making visible the very grain structure of steel is intriguing to me and, I hope, to you, too. And I am intrigued when an action as seemingly simple as scratching steel with a (very small) rock yields unexpected results.
I’m sending Konrad a couple of small blades to hone on the two stones and then send back. I’ll have them photographed with an electron microscope and post the results so we can see what’s going on close up. Stay tuned.