Making a whetstone from wild slate

[Luftmensch]

A thread @cotedupy posted about 6 months ago must have stuck in my head: Some questions about slate...

Recently I was on a walk by the ocean and passed some slate. The thread popped into my mind and I decided to stop and take two whetstone-sized pieces home. I thought cutting them into a whetstone might be fun. So here is a mini, visual story.

I decided to cut the largest rectangle possible out of the stone:

The rectangle measures about 250 x 100mm. I cut the stone to size using an angle-grinder (@inferno ) with a diamond masonry blade:

I anticipated lots of dust so I wore a respirator and ran a vacuum during the cuts. I wasnt wrong... it was very messy. Ideally, if you had the right tools, you would use water to suppress the dust! The blade has a thin kerf (1-2mm) and the stone is soft... so the whole thing was pretty quick and easy:

I wanted to flatten the honing surface and clean up the sides. The stone is soft... really soft. I figured my cheap and gutless 914 x 100mm belt sander would be up to the task. I used a worn out 40-grit belt:

I did not put a huge amount of effort into making perfect geometry and crisp edges but it came out okay:

Flattening the surface brought out the pretty sedimentary pattern in the stone:

Lots of small detail going on:

up close:

Given how soft the stone is, no surprise, it is very muddy:

I would say it is equivalent to 800~1000 grit:

I did not have high expectations for the stone. I did this for fun! I never thought it would be anything more than merely functional. I would have preferred to find a stone that was as uniform as possible. The geology in that area was not conducive to that goal. The different sediments do appear to wear away at different rates. After using the stone for a while, the surface develops gentle humps and dips. I also hoped the stone would function as a mid grit. One area where it has been well behaved is how gentle it is. I thought there was a good chance the stone would release large scratchy particles occasionally. So far it has not - although, it is already abrading on the lowish end.

Stone aside, if I were to do it again, I would be more modest with the size. It doesnt need to be 100mm wide. The standard 70mm width would be easier to flatten with a diamond plate. My stone holder cant accommodate the 250mm length either! So it is a bit cumbersome to use. I am still deciding whether I should bother resizing it or abandon it.

All in all it was a forgiving and gentle introduction to cutting a whetstone. I think the angle grinder and diamond wheel would hold up to a hard stone (albeit with more effort). I have doubts whether the belt sander would be a viable method for flattening a hard stone. A hard stone might just savage the belt and only permit for a few minutes of productive cutting/grinding before the abrasives are shot.

 

[cotedupy]

Excellent stuff!

I've had a fair few stones like that. When I started I thought they would be the better ones, as you could tell on the belt sander that they were softer. And in my mind: softer = muddier = better and easier to use. However my experience of them when used is similar to yours; softer stones, or softer parts of stones, often seem to be coarser and more heterogenous in grit. An example is stone 6 in my more recent thread, or the one on the right of the 2nd picture in the post you link to above. And I've just noticed I said exactly the same back then: 'The one on the right here raises the most mud, but is fairly heterogenous, and parts of it seem quite coarse'

I've come learn that really I want to err on the side of trying to find harder stones, as the grit is more consistent. Though even then not every one I take home to flatten and try will work. I've had many stones that I've abandoned because no matter how I flatten them the surface always seems to have one or two larger, scratchy particles in it. And there's no real way to tell that until you try them.

Yours is certainly pretty though, as you said!

You're spot on I think about other things as well: harder stones certainly take a toll on sanding belts. And yes - you may well find the occasional rogue particle getting released, especially on softer stones. Fun though

 

[Luftmensch]

softer = muddier = better and easier to use

It was obvious when fossicking that the stones were going to be soft and muddy. Some stones out there were harder than others but the average was soft. I thought that might be alright for kasumi finishing? I suppose that is why I was hoping the stones would finish higher than ~1000

I've come learn that really I want to err on the side of trying to find harder stones, as the grit is more consistent.

Or at least uniformly soft? I can see very niche use in a high-grit, uniformly soft stone for kasumi finishing. Maybe you could achieve this with a lime stone or natural chalk?? It would be hopeless on core steel, but it may haze up cladding nicely? If it was very fine and cut slowly, it might do a very, very gentle kasumi?

Otherwise, yeah. Hard!! I dont see much utility in a soft low grit natural with a slow cutting speed.

I've had many stones that I've abandoned because no matter how I flatten them the surface always seems to have one or two larger, scratchy particles in it.

That is a bummer. It is a bit of a lucky dip. I suppose the frustration is part of the fun... at least it is a cheaper alternative to trying endless expensive J-nats

Fun though

I had a good time

I am going to keep my eyes open the next time I am on a bush walk. I am hoping one of my relatively local walks will have some interesting geology that I didn't notice (just sandstone everywhere as far as I can tell!).

 

[cotedupy]

It was obvious when fossicking that the stones were going to be soft and muddy. Some stones out there were harder than others but the average was soft. I thought that might be alright for kasumi finishing? I suppose that is why I was hoping the stones would finish higher than ~1000




Or at least uniformly soft? I can see very niche use in a high-grit, uniformly soft stone for kasumi finishing. Maybe you could achieve this with a lime stone or natural chalk?? It would be hopeless on core steel, but it may haze up cladding nicely? If it was very fine and cut slowly, it might do a very, very gentle kasumi?

Otherwise, yeah. Hard!! I dont see much utility in a soft low grit natural with a slow cutting speed.




That is a bummer. It is a bit of a lucky dip. I suppose the frustration is part of the fun... at least it is a cheaper alternative to trying endless expensive J-nats





I had a good time

I am going to keep my eyes open the next time I am on a bush walk. I am hoping one of my relatively local walks will have some interesting geology that I didn't notice (just sandstone everywhere as far as I can tell!).

Yes, exactly! Uniform, soft-ish, fine, but with cutting ability is the ideal. It's kinda why I called my stone 8 the 'Goldilocks' stone - it has all of the good aspects of others I have, but in one piece. Though a few that I've tried in the last month or so are very good too.

Some of them are genuinely excellent stones. My wife, who is quite used to our knives being sharp, commented (for the first time ever) the other day on how sharp one was, when she was cutting some potatoes... it was the petty that I had made really very blunt, and then sharpened on stone 8 the evening before.

Interesting idea about limestone/chalk. And it'd be quite easy to try out I think - the chalk for a pool/snooker cue would be appropriately sized for a fingerstone I'd have thought...

 

[cotedupy]

None of the ones I've kept btw would be as soft as yours, I don't think. They're quite difficult to slurry - you'd want a diamond plate.

 

[inferno]

angle grinders are the best!