Tuff - welded volcanic ash
Paraphrased from GSS...mined as scouring agent in toothpaste, paint filler and oil absorbing compound. Same composition as granite, but not crystalized, blasted from volcanic vents, hardness varies with temperature and degree of welding. Particularly clean, almost pure ash.
I was thinking back to the mid-80's and Mount St. Helens......BOOM! That ash went everywhere, got into everything and totally ate up what it touched. Volcanic ash is VERY abrasive. Now if only it could be found in a solid form...
Size is about 150-90-22 mm, some areas of darker particles, most is fairly smooth and moderately dense. Mottled appearance, very white.
This is the back face and side. Check out the stratification. The back looks more marbled and for some reason doesn't cut well (abrade metal). The sample as received had one smooth face but the other side was more textured. This is the textured side. I lapped it flat so that I could test it too.
The front side abrades well, though! This was 2 minutes (after the hard gray shale) and the slurry is nice and dark with steel particles. The scratches from the HGS are pretty well gone. Some scratches are visible, but I don't know if they are from this stone, the HGS or the DMT diamond. Nice polishing action. Feels a bit "sticky".
So for S&G'ls, I rinsed the stone off and used the ODC as a nagura stone to form a thin slurry, and worked the chisel for 1 minute. VERY different feel! The ODC feels gritty but slippery and very smooth. Abrasion was fast, thin slurry thickened and turned dark almost immediately. Finish is a slight haze and very even.
Nice hazy mirror, maybe 4000 grit (JIS). Some coarser scratches are visible but I still don't know where they are coming from.
The stone by itself was pretty easy to use, a bit sticky, and I got the feeling that if I was patient, it would completely polish the edge to a full mirror state. There is some scratching, however I am not sure of the source. More testing! For being a light, low density stone, it is surprisingly stable dimensionally and shows little tendency to cup. It feels like it would dish out super fast, but it doesn't. Wear is fairly even. The areas with the darker particles has a different feel, with the specks as small bumpy patches. Not particularly scratchy specks, though. Might be a good high polish stone. Maybe an uchigumori stone?
The difference between the front and back side's cutting power is strange. It doesn't seem likely that the difference would be sooo dramatic.....MORE TESTING!
Using the ODC nagura made this stone really come alive. Very easy, smooth and fast. Even finish, light touch, even wear. Still didn't make that much difference on the back side, however. That tells me that the abrasive action is NOT solely due to the ODC nagura, but that there is a different particulate material in that strata.
Thumbs up! I have a bigger, more homogeneous stone on the wish list.