This is the first post presenting each stone in my range of japanese sharpening stones. First rock out is also the first japanese natural sharpening stone I bought, and I got it from Takeshi Aoki at Aframestokyo.
According to Mr. Aoki it's a very old pre-owned suita stone from the Oohira mine. It has a mustard or Kiita colour on the surface, but from the side it looks like it is an egg shell white, showing some renge or lotus towards the bottom, so the yellow colour is probably just oxidation and it a most likely a Shiro Suita. That would explain the exeptional hardness and very fine grit of this beautiful stone.
As you will see, I have mounted all my stones ,exept from a couple of two sided ones, to wooden bases (made by quartersawn Sibirean Lodgepole Pine) of exactly the same length. This eases up the sharpening process as I don't have to adjust my sink bridge for every new stone. I adjust the sink bridge for the first stone and the standardized bases allows me to just slip in new stones in a second. This base mounting also brings the shorter coppa stones towards me as I mount all stones equally distanced from one end of the base, giving me a lot better control and consistency in the contact between stone and knife.
The stone is very aggressive and pulls black iron filings from the first stroke, even without using a nagura. In the first picture below you can see the metal filings suspended in still clear water after just 10 passes over the stone. After about 25 more passes a black slurry is produced. Quite impressive for a stone this fine.
As you can see the stone has the usual su holes making it a suita. However, the stone is very clean without inclusions or renegade particles and with very few lines which are not at all affecting sharpening. It feels like it virtually sucks especially white steel into the stone, making a perfect connection between steel and stone. The stone also performs exeptionally well on blue steel and AS.
The stone finishes a kasumi to a very nice mist and haze making beautiful contrast between Jigane and Hagane and giving the edge a high grade of polish. Grit size is according to Japanblade definitions probably in the #40-42000 grit range. In the 330mate world it would be HGAL=5++/3++++/3++++/3++++. You can see some light scratches, but that is probably residual from a coarser stone.
As you can see the Oohira finishes the edge to a very nice and even finish. Just the right amount of light mist. This semi polished look is actually due to a very fine and complex scratch pattern from the natural stone. Looking at the close up picture above, the surface is actually matte or hazy to the look. This highly complex scratch pattern I believe is one of the main factors to get the edge really sharp and more wear resistant than an edge polished to a full mirror finish by fine synthetic abrasives. You can find some very good reading about this theory here http://thejapanblade.com/sharpvsshiny.htm. The edge off this stone gets the characteristic "stick" to it making it just slide effortlessly through sashimi or tomatoes rater than cutting them. It is absolutely amazing.
When using a diamond nagura pulling up a slurry and just letting it dry up, the colour of the powder is white, almost like potato flour. Another sign of a Shiro suita.
It goes without saying that this is one of my most treasured stones. It has become the reference I compare other finishing stones to.