essexalan":64smys5y said:
Are you saying that Japanese natural stones contain Aluminium in the form of AlOx as used in synthetics or in some other compound often found in sedimentary mud? The prime abrasive in all natural stones is Silica AFAIK or sand deposited and bound together with clay, Novaculite is a bit different but still Silica. I always thought that the big advantage with water stones is their ability to shed grit as in they are not tightly bonded so how do these hard JNats work? Is it like an Ark stone where the abrasive gets blunted with use so giving an effectively finer stone and you are not depending on renewing the abrasive or what? I did read the the lowest grit size in natural stones is around 1 micron so pretty near to a 12K synthetic. I still get the best finish on my Japanese chisels from an old King 8K which either means it is a stone I am used to using or I am just not getting the maximum out of my Sigma 13K, I am talking hand honing here. Experiments using CWs and LIs seem to indicate that an even finer finish is possible and these are very hard stones. Such a fine finish is probably irrelevant for woodworking tools except possibly for carving. My kitchen knives are all Japanese and get finished at either 3K or 8K .
Yes, aluminum oxide. IIRC, Kyoto (mt. atago, I believe is the name of the mountain with all of the mines on it) is sediment from volcanic eruptions. Here is a page that has analysis of four separate stones:
http://rk-trading.ocnk.net/page/28
(look down the page to "awase-do analysis table") - about 16-17% alumina on average, mid 60s silica. I was surprised to see the consistency, because some of the stones are strong cutters and some are weak. The composition maybe doesn't explain it that well, who knows? If you can slurry hard stones, even the ones with a reputation as weak cutters, they do cut pretty well.
Now, people selling japanese stones could say anything, so I tracked down tables of volcanic ash (knowing that I know nothing about volcanic ash, and that it might be different from one volcano to the next, I figured I'd at least find something corroborating the analysis table above). See the following:
http://www.scielo.br/scielo.php?script= ... 6000300012 (1/3rd of the way down the page, table III shows something strikingly similar - about 55% silica and 14-15% Al2O3)
Anyway, the bit about shedding particles is sort of a generalized statement - and perhaps attached to the early generation of synthetic stones (the king types). Natural japanese stones come pretty much across the board. Releasing particles easily, releasing them only with pressure, releasing them only with pressure and laminated tools (laminated tools also keep the surface of oilstones a little bit fresher, too), to not releasing particles at all. In the case of the final type, they're a lot more like arkansas stones in use, but not quite as tough - and potentially finer in some situations. The fast shedding particle comments made on behalf of waterstones often resonate with people with low sharpening skill (but certainly preference for them doesn't mean that's definitely the reason), especially if the early generation were combined with comments about how they are finer than an oilstone (they are not, properly hardened steel can be driven to higher sharpness with trans or black ark than with a king 8k, but it takes a little bit more skill and a more specific technique. As you say, LI can do the same, as can charn, but charn are very variable from stone to stone. I've only had two LIs - still have them, actually - they remind me a lot of a trans or black ark).
Your statement about not needing anything beyond the king 8k in woodworking is 100% correct. There is a point with the ultrafine stones where the chance of success in getting everything out of the stone and actually getting a finished edge is low enough that someone is better off just using a finisher that's a little less fine and that's a little faster. Sharpening something for 5 minutes and then taking a paring cut is really a poor test of the applicability of a sharpening setup. A better one is half an hour into work, what's an edge look like when you refresh it and your mind is still stuck on finishing a surface. People chase the finest and hardest japanese natural stones, too, and pay a bunch of money for them, but the more practical stone is one that's just a touch softer. Unfortunately, the market is starting to catch up to that fact, so good versions of those aren't cheap.
Particle-wise, I've heard alex gilmore say that the lower limit for japanese naturals is about 3 microns. However, in looking for the volcanic ash info above, one of the sources said that volcanic ash can have particles as fine as a micron, so though I was about to say that 3 is the correct number, who knows what it is? I think for stones specifically, at least the cutting particles, probably 3 is closer, because it takes technique to get the most out of them, whereas applying a microbevel in a guide with a 0.73 micron sigma power 13k pretty much produces the same result no matter how you do it.
(just a side comment - I have touted the washita in a couple of videos on youtube. I don't get that many viewers - that's not my objective. I want enthusiasts in the feedback loop, not everyone, which is what happens if you start making nice videos that are well edited and entertaining. At any rate, about once every 3 months, i get a comment from someone who had given up on sharpening but went and tracked down a washita. The follow up comment is that they've never had tools as sharp as they have when just using the washita stone. That means, to me, that fine and slow cutting media wasn't for them - because you can certainly get sharper than a washita, but if you can't do it consistently as a user, then the washita is a better stone to use).
