honegrown- Geological Specimen Supply (GSS) -

October 26

Geological Specimen Supply

Hard gray shale - Inyo range, CA

This was a sample that Rudy had kicking around that he thought I might like. It is a siliceous shale. Silica is good and sharp. I DO like it! He was right.

Surface #1 composition is uneven, with the darker 1/2 being more dense and slower wearing. Size is about 150-70-15 mm. 

Looking at the side, you can see that the layers of sediment are not perfectly parallel with the working surface. If I can get a thicker sample, I will lap the surface planar.

Side #2 is uniform in overall composition, but has inclusions of soft, rusty material. It doesn't scratch, but does wear unevenly.

This sample was so easy to lap, I thought that it might be too soft to work well. A while back, I found some nice muddy shale that abrades metal well but is so soft that it practically disappears just by looking at it. This stuff gets softer as it saturates, so even though it is a thirsty stone, I am not going to let it soak. After a few spritzes, it settles down. Actually, I need to try this one dry! It doesn't seem the type to load up or glaze. Hmmm......

Starting from P1200 grit diamond, lamination barely visible.

30 seconds and diamond scratches look to be gone. Lamination line is sharply defined.

Grit numbers are so funny. They seem very relative, although they are a measure of size and should be absolute. A P1200 grit sandpaper is pretty fine, on a stone it seems coarse. An abrasive particle can make a sharp, deep scratch or a rounded shallow scratch. I would guess this to be in the neighborhood of P1000 grit, JIS 1100, but these scratches are remarkably even and gentle and the stone is FAST! The well worn DMT leaves scratches that are deep AND sharp. As I was testing I thought that this stone was maybe as fast as my Tanba? which is my fastest natural stone. Of course I had to test it....not even close. This one is way faster! Slightly more coarse and a teeeny bit scratchy, but then again, it IS more coarse.

I am trying to get one of these shales that is bigger and of a more consistent composition. It's on my wish list. For a first shot and a guess, this is a really incredible stone. It is soft, so it wears pretty fast, but not any faster than my Tanba?, really. I want a really big one so that I don't have to worry about it wearing out. And to be clear, these were not intended to be sharpening stones, they are teaching aids. That they work at all is great, that they work REALLY well is amazing! I may sell Tanba?!

I use a DMT duo-sharp fine/x-fine because it is somewhat of a familiar standard for hobbyists and because it is flat-ish and stays that way, but as I do these comparison tests, I end up polishing blades way more than I otherwise would. THAT'S when you see how deeply diamonds scratch, even well worn ones. I plan to experiment with using a kanaban, but if that doesn't seem like a fit for me, I'll be getting an Atoma diamond plate, not another DMT.