I found a flat rock...

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Bert2368

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Actually, dozens of them. All sizes, from large gravel to too big for me to pick up.

It seems to be some kind of semi metamorphicized sedimentary stone, I'd guess it MIGHT be called a "slate".

Has layers that are very hard to discern when looking at them edge on, but become obvious when our farmer renter hits & splits them on the layer boundaries with a chisel plow or disc, or I whack one with the big rototiller.

I grabbed this one, scrubbed it, ran it through the dishwasher and took a stab at sharpening a white #2/iron nakiri, without even flattening it. Just tap water and perhaps 20 alternated strokes per side, then stropped on my forearm a couple of strokes (hey, it works, I have an abrasive personality...)

It worked surprisingly well. I sliced up a 4.5 lb. Chateaubriand after roasting on the BBQ grill with the edge it produced- I liked the performance, will certainly do a tomato well also.

The color after washing was a lot closer to yelliw/beige of an antique cotticule I have than the dark grey the stones have when I pull them out of the soybean field or my gardens- There are also tiny little sparkles all over the smooth, cleaned faces, glinting in the light.

First picture shows "unimproved" naturally cleaved face I sharpened the nakiri on.

Last picture is a close up, trying to show the "sparklies", will try for a better shot
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musicman980

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Do you have any pictures of the surface prepped and wet? I think it looks promising enough to test out some more.
 

Bert2368

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Below are a few pictures after I started flattening the rock.

It is a bit easier to flatten with silicon carbide paper than the other naturals I have tried to flatten/smooth (yellow cotticule, hard translucent Arkansas, jadeite).

The mud when flattening looked like caffe VERY o lait, milky with a touch of brown. When sharpening steel and during final bit of flattening with a diamond plate, it was barely tan.

First picture: mud from diamond plate flattening:

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Second: dry surface after flattening:

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Third: same surface, clean with only water on it, has not yet been used.

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Final flattening was with an old, very worn smooth diamond plate, surface of stone was almost shiny after.

What appear to be white "dots" scattered over surface of the smoothed down areas are the reflections from the "sparklies".

I sharpened the nakiri again, doing as is sometimes done with cotticules, diluting the mud more and more, pressing lighter and lighter as I went, then stropped a few times on newspaper. It shaved my arm cleanly and bites into a tomato very nicely.

After flattening, the stone has a feedback like a faint version of the sensation you get when a stone you are flattening on a diamond plate without venting grooves wants to STICK to the plate by vacuum?! Different from anything else I've used, almost like the rock is sucking the blade against itself. Weird, but not unpleasant.

Definitely sharpens Carbon steel effectively, but what doesn't?

Will try other steels after I have a larger flat area ground onto it.
 
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jwthaparc

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Sounds like what jnats do. They tend to have that "suction" effect too.
 

Bert2368

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Sounds like what jnats do. They tend to have that "suction" effect too.
Ah! I've put off buying J nats so far, generally expensive and I don't feel I know enough.

Got 5 Japanese man made water stones, they're among my favorites for results but don't give anything like the feel this one seems to have. I could get used to this chunk-O-rock. I'm going to drag a few more of its family back to the barn as I see them.

I've looked at how professionals lap smooth faces on stones using motorized Aluminum discs, different grits of SiC, running water. I've got a diamond blade, water cooled tile saw, could at least make a start on flat sides with that. Takes forever to lap this by hand!
 

aaamax

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Terrific post and hopefully you'll have some more to tell soon.

I have long since thought that local rock should be useful to some regards in sharpening. Like you have said, it's tough to get flat.

Cheers
 

cotedupy

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Excellent stuff! Sounds like a good stone :)

Some random thoughts from my (limited) experience...

‘Some kind of semi-metamorphosed sandstone’ would probably be what I’d call it too. Precise definitions of stones seem to exist on a slightly burry continuum afaics, but from the pictures and your description of the quartz in it - I’d say this looks to sit kind’ve in the middle of sandstone and shale. Perhaps tending slightly more toward being fine-grained quartzite than slate (?) Which might be quite a good thing I imagine. You have some mud there that looks slate-like, but hopefully get some of the cutting ability of sandstone/quartzite.

(^ The above is conjecture, I'm not a rock expert by any means).

In the same vein* - I've sharpened a fair bit on local slates and sandstones (and did a couple of threads earlier this year about them if you can be bothered), and have had similar relative success. Something I can say for certain is that the most important quality of any natural stone you find is going to be bind.

In your stone you have some larger twinkly bits of quartz you say... that's not necessarily a bad thing if the stone in general is hard enough. What you don't want is larger particles in softer stones, that get released and start getting randomly scratchy. If they stay in the stone itself they can just wear down over time. A fairly obvious point to make I know, but it's worth bearing in mind if looking for more stones. It may be slow and a bit tricky, but you can sharpen a knife reasonably well on pretty much any stone you find if it's hard enough - almost everything contains quartz. But with soft stones they really want to be fine and even.



* Ho ho. See what I did there? They're rolling in the aisles at geology conferences when that one comes up.
 
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