How to Flatten a Stone (Lapping an Ark)

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captaincaed

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Inspired by threads about SiC powder flattening, I thought I'd post up how I flatten and dress a stone. Wrote this up for the razor forum, figured I'd cross post here as long as it's written up. Useful for all kinds of stones, not just Arks.

Today I'm dressing a hard black Ark with unknown branding, a $20 used tool store find that was finished very rough, maybe 36 grit on all surfaces when purchased. I was hoping I might luck into something like Keith Johnson's favorite, opaque black ark. This one is totally opaque, with some lighter figuring/veins much like the one he's fond of. I doubt this one is equal, but the search is always the fun part.

Anyway, here goes, and you can see how things change slowly along the way.

I've been working with a couple baggies of SiC powder I got from a kind knife forum guy. Recently invested in a full set, and it's definitely made things easier. You only need 1/4 - 1/2 tsp. of each grit for a stone this size.

I like to use a dead-nutz flat marble counter offcut, with sacrificial high quality sandpaper to save the marble surface from dishing (Cubitron or Precision Pro Grade for coarse grits, Rhynowet for finer grits). These papers are incredibly durable, and I can lap many stones on a single sheet. I wet the back and front of the paper with Windex or some cleaner to make it adhere to the stone, prevent slipping, and lubricate the SiC powder.

What does the cutting speed depend on?
  1. Sharp abrasive
  2. Pressure (the fraction of force divided by area, in a literal sense)
  3. Distance travelled over the abrasive
How does this affect the process?
  1. Refresh your abrasive every so often, and clean off the old. You don't need much, but you want good contact between the stone and the loose grit. If you let a bunch of worn grit accumulate on the paper, fresh abrasive dulls itself on old abrasive. If you only use fresh, it'll cut faster. I don't have a recipe for how often - be observant.
  2. More pressure cuts faster, but too much can be destructive to the paper and the stone. Just be aware of how you angle the stone. Even 800 grit powder will round a sharp corner really quickly because of the small surface area. Work on keeping the stone flat, and not rocking it, since you can create a slight convex surface if you rock the stone, or have really thick SiC powder that doesn't allow flat lapping. Food for thought.
  3. Long, sweeping arcs/figure-8s will get things done quickly. Remember to keep things lubricated enough for easy SiC powder movement, since the grit needs to move freely over the surface to cut efficiently.
Here are some pics through the process. This was flattened with 60 grit (not pictured, sorry), then dressed with 120, 240, 320, 400, 600 and finally 800 (probably overkill, but going back down a grit is the easy part).

Note how much powder I'm using, how the surface changes at each grit, and how much more the finer grits will clump up, almost like clay.

120
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220

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I should have cleaned off the old powder first, oops.
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Despite many uses, this Cubitron is still going strong
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captaincaed

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Process continued:

320
Wetting the paper back to prevent slipping.
I'm using paper of finer or equal grit to the loose SiC powder at each step. In this case, a single piece of 800 grit paper will serve to take me from 320 grit to 800 grit powder.

Between 320-400 grit, you can really see the surface start to flatten out, and become less pebbly. I think you'd see the best surface if you had the patience of a saint, and spent a lot of time (1-2 hours) at 220 or 320 grit, to really flatten the peaks down to meet the valleys. Sadly, I am no saint.

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I try to used pressure on all 4 corners, or a wide, flat palm when my fingers get tired (since I'm holding the camera, imagine my other hand is also on the stone). This distributes pressure equally over the stone surface. If I rock the stone, I'll wear the surface unevenly, and it won't get flat, or it won't all be dressed to the same surface texture.
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400

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600

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captaincaed

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800
After lapping, it seems like this is not a totally homogenous stone - some of the lighter inclusions seem to be softer, and were more easily worked away, leaving some pitting on the surface. Not a big deal by any means. The razor will ride on the remaining flat surface, and hone nicely, I believe. I lapped it once previously, and it produced a nice initial shave. I was hoping to restart at 120 grit, flatten down past the pits, and get a flatter surface, and had a little luck by spending more time at the mid grits between 120-400 (to really even out the the lumps left by the coarse 60 grit flattening process). But it's not perfect. C'est la vie, that's just the nature of this particular rock! All in all, $20 well spent, and after some elbow grease, I'd say it's worth at least $10 now! You can see the original rough state on all the other sides in the video. This lapping process was about 2 hours of work on a stone this hard.

If anyone has lapping tips, tricks or insights, I'd love to hear them. I'm always learning too. Hope this helps beginners hoping to get started. It's much faster to use powders than other methods I've tried, much more economical, and also leaves a much more homogenous surface with no striations, as opposed to sandpaper alone, or diamond plates.


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Thanks for the detailed write up. I’ve had a heck of a time flattening Arks. First time around I learned the hard way that Arks love to eat diamond plates. They also ate through sandpaper in short order. I also didn’t know that high quality sand paper could hold up so that’s new info to me. If that’s true, then sand paper on a flat surface plus SiC is brilliant! I will have to give this a try.
 

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This is awesome. Thank you for taking the time to put such a detailed write up together.

For something as hard as an ark, which I’m making the assumption they don’t release much mud or grit, this kind of multi step lapping seems ideal. Would one condition jnats in the same way? I find most of them lose the conditioning quickly as you work, negating any prep, but it’s entirely possible my process is less than ideal.
 

captaincaed

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Thanks for the detailed write up. I’ve had a heck of a time flattening Arks. First time around I learned the hard way that Arks love to eat diamond plates. They also ate through sandpaper in short order. I also didn’t know that high quality sand paper could hold up so that’s new info to me. If that’s true, then sand paper on a flat surface plus SiC is brilliant! I will have to give this a try.
The abrasive on the paper wears super fast, but the rubberized backing is designed for hard duty, and holds up very well to having SiC rubbed on it for hours. I’m astonished. A budget paper gets wet, tears, is useless in a few minutes.
 
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captaincaed

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This is awesome. Thank you for taking the time to put such a detailed write up together.

For something as hard as an ark, which I’m making the assumption they don’t release much mud or grit, this kind of multi step lapping seems ideal. Would one condition jnats in the same way? I find most of them lose the conditioning quickly as you work, negating any prep, but it’s entirely possible my process is less than ideal.
The arks do release slurry during the process, you’re really grinding it away. I flattened my first translucent Ark with a diamond plate. It just killed the plate, but it also left a clean novaculite slurry. It cuts steel very fasts. I used a translucent ark with its own slurry like a 2k synthetic, it was a monster. It’s not an economic or sustainable option, but a cool experiment.

If you wanted to rough/lap/condition the surface of a Jnat, it would work just as well. SiC is a 9 on the Mohs scale, very hard, should cut the sandstone that Jnats are made from quite well. Just be careful to get ALL the SiC scrubbed off before polishing. Remaining grains could really mess up a polishing job. Although if you’re only doing a shallow surface condition, any ceramic sandpaper will probably do quite well, and be less messy (Cubitron is ceramic, as are some Norton papers, but Cubitron is cheaper). Cubitron cuts forever as well, 4x as long as any other paper (at least in woodworking).
 
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natto

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The abrasive on the paper wears super fast, but the rubberized backing is designed for hard duty, and holds up very well to having SiC rubbed on it for hours. I’m astonished. A budget paper gets wet, tears, is useless in a few minutes.
Good Point!

Is #800 your final step with sic on hard arks?

Btw, with the rims of sanding paper coming loose, I managed to convex a stone. A first try with spray glue on glass was not bad. This allows exchanging dry sheets with one layer of glue. Flattening should be fine with glue on the paper, to avoid cleaning the glass.
 
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Thanks @captaincaed. I have been doing this all wrong for years! Well maybe not all wrong, but definitely different in a lot of ways that are probably less efficient and effective than what you've outlined here. I always love a good excuse to buy some more sandpaper and dime bags of silica grit.
 

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I like your idea of putting sandpaper down to supplement the loose grit.

Do you find any difference in stone performance between 120 vs 800 grit lapping? When I used to use SIC for lapping my synthetic stones, I didn't go past 120/220. I personally found that the stone smooths itself out during use and the "coarse finish" didnt have any negative impacts on the resulting finish on my blades. Curious about your experience.
 

captaincaed

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Man I had no idea people wanted to see pictures of me rubbing rocks on tiny artificial rocks.
Is #800 your final step with sic on hard arks?
This is intended for razors, and I've heard the sweet spot is 600 grit. I took it higher since I could, and it's easy enough to go back down.
I always love a good excuse to buy some more sandpaper and dime bags of silica grit.
Every project is an excuse for a new tool.
Do you find any difference in stone performance between 120 vs 800 grit lapping?
Yes. For a stone this hard, I honestly don't see much benefit to coarse lapping. The surface is bumpy, uneven, but doesn't really cut very fast. Fast cutting is usually due to particle shape and friability, neither of which are the Arkansas's talents. I found that 400 grit + gives good friction and feedback from the stone surface. I just expect these stones to be fine finishers and burnishers, and lap them to a grit that matches that talent. It's weird to think that lapping at 600 grit produces a stone surface that finishes in the neighborhood of 4000 grit, but there you go.

If you want fast-cutting novaculite, think about Washitas (and take a peek at the Washita thread here).

Last, this video covers a lot of ground on the complex topic of particles size, shape and perceived grit for all types of natural stones. A long watch, but one of the best comprehensive overviews I've seen. This guy's videos are usually a bit long for my taste, but this one delivers. Unfortunately the title says nothing of what's inside. It's titled as an "Arkansas" video, but it's really a general discussion .

 

inferno

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i just do the powder on glass. the powder breaks down in about 1 minute and its then gets finer, you can hear this. so there is really no need for all the various grits imo. i use the coarsest one until its fine, add more, add more, i think 60 grit is my coarsest. and then when everything is flat i use the finest one, 400 grit, then its off to the dmt 325 to smooth things out. then i usually finish the surface with a coticule nagura. its also my cleaning stone.
 
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