Thread: JNat Hardness: remind me please

Remind me please why high hardness jnat stones are preferred (or seem to be) for razor honing?

The sharpest edges are usually put on with straight water, and a softer stone will usually self-slurry at least a little bit. This gives a tiny bit of a rounding off of the edge at a microscopic level and also can make the stone out of flat. That said, some prefer the edge from a very light slurry as the edges on straight water can sometimes be a little harsh. I believe the slurry on a harder stone is also usually made up of smaller particles as when a slurry is made, smaller bits come off than with a softer stone.

Thanks! I couldn't remember.

Also hardness generally equates to fineness. Not always true but is 95%+. I kind of feel that a little softer stone, like a kiita versus an asagi, can give a smoother edge.

Cheers, Steve

Yup, that's what I was referring to in my post. On one of the forums somewhere I saw a reference to a study of JNat slurry from a lot of different stones that determined that all Jnats have the same abrasive particle size - in the neighborhood of 2 - 3 microns. I think it was done in Japan and mentioned by Iwasaki in his sharpening treatise maybe?

Anyway, so it would seem that the slurry is in particle form resembling a (possibly layered) corn flake type shape on a microscopic level. When slurry is made, the particles that come off determine the coarseness or fineness of the resulting finish. So in short, it seems that harder stones give up smaller particles and give a finer finish, softer stones give up larger particles and give a coarser finish.

Finishing on a hard stone with light slurry rather than straight water gives a slightly smoother finish in the same way that finishing on straight water with a softer stone will - because the softer stone will give up some slurry/particles by itself even on straight water.

Originally Posted by eKretz

Yup, that's what I was referring to in my post. On one of the forums somewhere I saw a reference to a study of JNat slurry from a lot of different stones that determined that all Jnats have the same abrasive particle size - in the neighborhood of 2 - 3 microns. I think it was done in Japan and mentioned by Iwasaki in his sharpening treatise maybe?

Anyway, so it would seem that the slurry is in particle form resembling a (possibly layered) corn flake type shape on a microscopic level. When slurry is made, the particles that come off determine the coarseness or fineness of the resulting finish. So in short, it seems that harder stones give up smaller particles and give a finer finish, softer stones give up larger particles and give a coarser finish.

Finishing on a hard stone with light slurry rather than straight water gives a slightly smoother finish in the same way that finishing on straight water with a softer stone will - because the softer stone will give up some slurry/particles by itself even on straight water.

My understanding on this is, the softer stones readily release those bigger particles. For harder stones, it's the stronger binding, probably made from the same material, that do not readily allow the whole sized 2-3 micron, as said above, particles to be released easily.
Using clear water, the shape of the embedded particles brake down into smaller ones because of that and in the same time, the "coarse" surface of the stone is burnished by this action, making it smoother (the surface of the stone, not the edges), and with lighter cutting strength.
From a very interesting post or article from Alex that I can't seem to find, he explains the shape and mineral of those cutting particles, and they are more 3D shaped, not like flakes, as far as I remember. The Thuringian particles look more flake shaped.
I have to agree that the use of slurry offers a smoother, mellower edge, while making the edges slightly less sharp,and I personally prefer this tradeoff.
In any case, a good Kyoto Jnat will offer an excellent shaveable edge, whether it's a kiita or Asagi (the two most famous types), very hard or softer, stone from the western or eastern mines. They have their differences, but most of the vintage ones, not collected from the disregarded pieces around the mines, are good stones.