Altamira shale, lower member, Palos Verdes Hills, Los Angeles Co. CA
Middle Miocene shale, oldest of 3 sub-units of the Monterey Shale, contains volcanic ash (and possibly formed with silica in solution). Very pretty stone, it shows a slightly crystalline composition and looks somewhat similar to some (too)hard Montana shale that I have tried.
Starting from where we left off with the welded volcanic tuff, this picture shows the results of 1 minutes work....nothing. No black particulate, no slurry, edge may be burnishing slightly, but not enough to note. Very hard, moderately sticky, feedback not too good.
This is after a rinse and using a DMT Xfine P2400 diamond plate to raise a very thin slurry, then working the edge for 1 minute. A little black, no additional slurry generation, very slight polishing. This is looking like another garden stone, I guess. Not too much action, feedback was improved by the diamond slurry. Only slightly sticky now.
Rinse, ODC nagura, 1 more minute. Creamy slurry formed and darkened immediately, edge is polished, steel is bright mirror, iron is dark mirror, even haze.
Iron is reflective too, maybe around JIS 8000?
Check out the trees!
ODC made the difference. So now I am thinking that the ODC is responsible for ALL of the different behaviors shown by the various stones. It turned this one from a sticky, pretty, dud to a respectable performer. Not stellar, but fair/good. Feedback was very nice AFTER the ODC nagura. I am thinking that this stone is too hard and new particles aren't being released fast enough to really work the metal in a timely fashion.....come to think of it, it feels like an oilstone! Like a hard Arkansas/Novaculite.
I'm still trying to figure out this stone. I will revisit this one soon.
RIVETING, ISN'T IT?