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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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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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.
 
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.
 
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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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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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.
 
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.
 
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.
 
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 .

 
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.
 
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.

i see windex a lot for stuff like this. why windex? do you think water would work fine? or, if you need surfactants for some reason, a spraybottle of water with a drop of soap in it?
 
It's slippery and clears swarf in a nice way - something about the combination of surfactants and wetting agents is just a nice combination. Same reason you can used it to slide apart two drinking glasses that stick together. Just helps things move. And as Mike noted, helps clean old oils just a bit.

Simple green also works well. I don't like hand/dish soaps for their fragrances and other additives that can be hard to remove. Windex and Simple Green evaporate if you but leave them out. Soaps leave residue.
 
I’ve been fighting dead-flatness on Arkansas stones, and went through some experiments recently. I’m copying a post I made in Badger and Blade in case this is useful to anyone else later. Warning, it’s really long. But if you have need to flatten an Ark, it may help.

——————————————————————

I’ve had this exact issue recently with two arks and a charnley (getting a subtle covex dome in the middle). I’ve been critical of all my processes, trying to isolate variables. Here’s how I’ve been tackling the issue.

Having written this, I realize it’s now a small book, my apologies. I’ve been thinking hard on this recently. I realize this may be WAY more detail than you personally want, but I’m brain-dumping as part of a couple larger conversations with other folks too.
  • Start with a new granite reference block. Not a tile or counter off-cut. A verified, machined flat block. It’s $60, and an ark is $200. But a domed ark is worth $0. Easy decision. You’re going to use it later to true-up your straight edge, so it’ll pay for itself twice over.
  • Buy good w/d paper - cubitron ceramic cuts fastest, longest in my experience on metal, wood and stone.
    • Buy a 50-pack of 60 grit, and use it like it doesn’t cost money.
  • Flatten my straight edge on the block right away. Now I have a second reference surface line for dead-flat. This helps checking the light gap between the stone and the straight edge, and helps me make sure I’m sane as the process goes on. Alternately spend $60-100 on a machinist straight edge and move on with life. They’re handy for all types of things. Keep it rust free. Get one without lines - it only has one job and measuring isn’t that job. It’ll also keep other people from ‘borrowing’ it and returning it dinged and wet (or maybe you don’t have teenagers at home like me 🤬)
    • Mine was both rusted and bowed. Life got a LOT better when I did this step.
  • Ditch the coarse SiC powder. Unless I’m removing a visible dish, I find it does more harm than good now. As soon as I get the point where I need the straight edge to interrogate flatness, it’s too late for the coarse SiC powder to add value to the process.
    • Finer SiC powder will be used later for surface conditioning.
  • Draw the pencil grid.
  • Pull out a fresh sheet of 60grit, wet the back, put it on the granite block. Time investment ~ 2 hours. Don’t drink beer, it makes your work sloppy. Put on music or a podcast.
  • Lap 10-20x with only the weight of the stone. Look for wear patterns.
    • If I have a dome in the middle (convex), then use a diamond plate to create a subtle concave in the middle. Lap in the middle, perpendicular to the direction you’d hone. Now the stone is touching the paper/granite block on 4 points rather than rolling on the dome. Verify with your straight edge - you need just a sliver of light in the middle, don’t overdo it.
    • this step hurts the most at first, but is worth it
    • The GOAL is a FLAT ark, not a LARGE ark. A non-flat ark is worth $0, no matter how large.
    • A diamond plate used as Gamma described DOES help - having the plate on top is better for some reason.
    • HOWEVER a worn plate can STILL impart a DOME since the center is more worn than the edges. Not a big deal when lapping synths, but makes a noticeable difference with uber-hard naturals that do the LAST step of razor honing. It’s just not good enough. Full stop. I’ve talked with @musicman980 about this at some length online and in person. We agree it’s a real factor. A totally fresh diamond plate may be better, but w/d paper and a reference block are both cheaper and a better long-term solution.
  • With a now DISHED stone, you can get to work
  • Make a pencil grid, lap until the pencil around the edges is removed, and a grid remains in the center.
    • Do this with fresh paper and no more than 5# of pressure. Weight of stone, plus fingers on the corners just to keep it steady.
    • Wash the paper of any particulate - this moves around between the stone and paper, abrading your lines, and washing them out, and LYING to you about where the wear patterns really are.
    • Once you lose the ‘fresh cutting’ feeling from the paper, get a new piece of 60grit, reapply the grid, and go again. This happens in about 60s for me on an Ark. Wash, rinse, repeat. Dry it, and keep the used paper in a plastic folder for lapping softer stones, or woodwork. It’s done helping your ark (but has lots of life for other tasks).
      • You NEED more pressure for it to continue to cut, and pressure leads to a domed ark. Paper is cheaper than arks, move on.
    • Rotate the stone regularly so you even out any wear patterns.
    • Move straight back and forth, in the direction you’d hone. No figure-eights.
    • Take a 5 min break every 30 minutes. Drink water, close your eyes, refocus.
  • Eventually, you’ll have only a small grid in the center of the stone after about 20 laps.
    • From here on, wash the paper often, reapply the grid often, and stop lapping when those 20-30 laps remove all the lines basically equally from the surface.
    • Knowing when to quit is key. Once a stone is dead flat, it’s easy to start introducing a dome. Leaving a hair of hollow in the middle might even be desirable depending on your personal feelings - you’ll still be able to hone razors on 95% of the surface, it’ll float over the small hollow, and you’ll be more confident you don‘t have a central dome.
  • Check your work
    • Using the known straight edge, check 8 axes for flatness with a bright light/daylight window behind the stone. Best if the straight edge balances on the stone face on its own (the reference edge itself is straight, but it may be bent like a stick of gum, which will introduce light between the stone and the straight edge even if the stone and reference edge of the straight edge are dead flat). If it balances, you avoid this problem.
      • left, center, right on the long axis
      • left, center, right on the short axis
      • both diagonals
    • The stone can be non-flat any of these ways
    • If you don’t pass, STOP, ID the issue, perhaps starting over TOMORROW or next week. Don’t work frustrated. Come back to it. A flat ark is forever, no need to hurry (I have to tell myself, too)
  • Move onto 120 grit cubitron. Time investment ~ 30-45 min.
    • Repeat the entire process, noting that you will STILL be flattening the stone at this stage, just more subtly. Don’t rush it, don’t use pressure. Use paper like it’s free. Use all the same caution, frequent checks, and light pressure you did with the 60 grit.
  • Check your work as you did with the 60 grit - all axes.
  • Move onto 220 grit. Time ~ 5-10 min
    • Now you’re basically just smoothing the surface, but still pay attention. You CAN create some doming at this point if you overwork things. Just be gentle. Use paper like it’s free.
    • Chamfer the edges at this point.
  • Move onto 320 grit. Time 5 min tops. You may now drink beer.
    • Same notes as 220 grit.
    • I usually get away with one sheet of paper, using a little pressure isn’t a game-ender any more.
    • I also use 1/4 - 1/2 teaspoon of 320 SiC powder on the 320 paper at this point to ‘frost’ the surface and try to remove the striations from the paper. Use plenty of water, make a mess and never look back.
  • Move up to 600 grit.
    • Same notes as 320 grit. Time 5 min tops
    • When I use the SiC powder, I move in large figure eights, frequently rotating the stone to evenly condition the surface. I make a mess. **** runs down the granite block. At this point my wife usually notices my increasingly beer-fueled mess and kicks me out of the kitchen, but I know that I’ve evenly conditioned the surface. I’m not really worried about creating a new surface geometry at this point.
  • Run a kitchen knife lightly over the surface to deal with any microscopic weak bits of stone created by the lapping process. No need to go hard, just touch up your favorite knife.
  • Enjoy the ark.
Hope this helps, hope it’s worth reading, but this has produced the most reliable results for me so far.

If anyone disagrees, please jump in.
 
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