It is easy to renew the surface of a flat hone/stone, and checking for flatness is much easier than trying to reset a large convex surface
When the stone is appropriate to this type of use, then yes, it is easy to maintain it flat. Your coticule is way more friable than an arkansas stone. It is a "tweener" stone and can be used flat or curved. Many more friable synthetics cannot be used curved and have to be flat. If you tried to curve them (I have tried everything), the curve makes the lapping paste just push away from the contact area. The small contact area erodes the soft stone very quickly making more paste. And the stone does not work well or hold its shape and will eventually conform to the shape of the blade and start working again, but you just wasted time and stone. It's with the ark stone in particular, and perhaps some other harder stones like slate, and the hard sintered ceramics which really don't work as sharpening stones when they are flat lapped. Some folks have granted them a mythical position of "finishing" only, and some people use them flat and can still sharpen stuff on them because they unwittingly have developed the habit of favoring the edge just slightly. But they could do much better in either case.
- there's no easy way to check such a surface for correct radius throughout - unless you do like Jared and make a concave master to use with wet/dry.
Despite Jared being very careful to precision grind his lapping plate into a perfect sphere, it is not necessary just because he thinks it is. Well, from a marketing standpoint it is actually great, and I'm sure his customers are in awe to receive such a perfectly spherical shaped stone.
I understand completely that if you put a razor on a stone, the curve/radius of that stone affects the sharpening angle, just a tad. If you increase the radius, you will start hitting more on the apex. And if you decrease the radius, you will start hitting more on the back of that bevel. But the differences here are minuscule*. The main reasons to curve in all axes is to reduce the area of contact and ensure the entire surface is being of the stone is being used, one spot at a time, to retain a regular surface with relatively sharp particles. And IMO one of the other practical purpose of curving the stone in the long direction as well as side-side is wear pattern. That is my experience. You want the stone curved enough to be able to handle normal wear without tending towards developing dishing. Is does not have to be spherical all over, and I actually wouldn't want this on a long rectangular stone. I would prefer it have a tighter radius side to side and a larger radius end to end. Hence to end up with something more like a section of a football. That's just my personal preference.
Flat stone, anywhere you put wear on it, ba-ding, you have a dish, and the wear accelerates. Curved stone, anywhere you put wear on it... it's a little flatter there, but still no dish. And where you put this flattish spot, on either end there's now a bit more radius and those spots will wear faster, returning the flat spot back to curved. Self-maintaining, glaze never happens, stone cuts more and burnishes less, no wasted hours of trying to flatten a stone so hard it doesn't want to be flattened.
Yes I know that guy, IIRC his name is Jared
This guy's demeanor is awesome. He's just so disarming. AFAIC, he lulled Dovo employees into giving up trade secrets, then they later got him to take down the video.
*The important thing is the the stone will cut more and burnish less. Wherever it does grind the bevel it will be able to cut. And this means it will be able to reach the edge, even if it has to cut a bit more from the back of that bevel (because maybe you used a flatter stone to set the bevel) it will cut that down and keep cutting until it reaches the apex. If it doesn't hit the exact spot on every pass, it is of no consequence, just that when it does touch the apex it is not burnishing and decreasing the durability and sharpness of the edge. When I first started to use these stone, I followed the info available to me. If you use it the way most people do, this just doesn't work. It only can add a slight microbevel, at best, by lifting the angle and touching only a small area of steel to the stone. If you ran the blade over the stone and weren't hitting the apex due to difference from the pervious coarser (actually doing-something-useful) stone, then the apex would never hit no matter how much you slide the blade over the stone. Most all you could do is eventually grow a burr and still have a bad edge. This is especially true of the harder black and translucent stones. Once you curve the surface, they are no longer a mystical burnishing/finishing (the blade was razor sharp , already, but can you feel the difference after watching me do 1000 passes? No? Well, I can!) stone. They sharpen things. They will put on that razor edge from nothing, if you have the time. And no that is not the case if this stone is flat; given infinite years of labor, it will still not produce a very sharp edge from dull... well, unless you finish with more obtuse "micro-bevel" passes to cut off that burred and mucky steel from the edge. IME, you need to raise the angle about 30-40 degrees for this to work on a flat ark stone.
There's nothing "wrong" with doing this microbevel, but it is necessary due to the burnishing going on. When I first learned to get a sharp edge with an ark stone by doing this high angle microbevel, I thought it was wonderful. I mean, I finally got a razor edge off this stupid stone. Maintained my kitchen knives on this flat ark stone and it is all great, at first. But fast forward 4 months later after you've repeated this procedure 4 times, and all your knives are still razor sharp?... but that edge starts to flop and fold over, and now you need a proper sharpening stone. One of the first things you might have noticed is that the burr isn't being removed by the high angle microbevel passes, anymore. Not as easily. The distinction between burr and edge is blurring, and you have a burr-edge. Used the way everyone else uses it, the ark stone is extruding a bunch of weak metal into the apex through burnishing, so it is good for "light maintenance" until the edge turns into a limp biscuit. With the curved stone, the edge is sharp at any angle you want to sharpen at, and the burr just falls off on your palm, and it's a hard, durable, new edge everytime. To some of the most prominent sharpening and vocal students/enthusiast/experts on the web, it is considered normal to have to grind the edge at 90 degrees and start over, every now and then. Some claim they do this every time they sharpen; and I've even heard a strange technique called plateau sharpening. But I'm telling you, if your stone has a high cutting:burnishing ratio, this is not necessary. It will remove material without extruding as much new burr, revealing a fresh edge.
**There's really no other good way around this. For the "hard" stone to be able to cut that steel without burnishing so much, it has to have widely dispersed cutting points with some more breathing room between them. But the finer the grit, the closer those points get to each other. Diamond plates can be engineered to do this to some degree, by altering the distribution of the abrasives. This is similar reason we don't flatten stuff to a fine finish with a giant belt sander. We plane them with a blade that rotates in an arc and cuts a tiny section at a time. We run them through drum sanders and surface grinders and such. Tiny areas of cutting.
Also, IMO, the true hard (surgical black and translucent) ark stones still burnish a bit more than ideal even when convexed. For a hard hone, sintered ruby is a bit superior in this grit/fineness range. But the true hard arks are pretty close to a no hassle sharpening stone when convexed.
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BASIC INFORMATION (that in hindsight I should not have assumed is commonly known.)
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It's not like you burnish the steel and make a burr, and then at some point you've burnished all the weak steel away from that surface. Initially, most of the metal may be the "looser" or "unimportant" stuff, but removing it exposes the important structures. These come tumbling down, and the burnishing continues. That metal just keeps oozing/flowing out past any acute edge connected to this area being burnished (the contact patch), namely the apex of your blade. If the contact patch ends over a flat surface or end in an obtuse angle, any of the burnished metal flowing there will stop, because it's supported there; it has more interaction. It has buddies saying "hey don't jump off that bridge, yet, cuz we're having a party." But over the acute edge, the burnished metal just happily flows right over.
So it's important to understand this bit:
the burr is not a bit of metal leftover after grinding in the apex. It is not formed through a subtractive process. It is being constantly extruded. It grows. These pressure points of the abrasive pushing into but not cutting do some kind of japanese pressure point kung fu to the steel, and parts of the metal temporarily act like a fluid. To get a sharp edge, you want a stone that will cut the burr away faster/better and grow new burr less quickly. If you have too much burnishing and too little cutting you cannot get down to "good" steel unless you go 90 degrees to wipe the edge off and start over, and you can still have issue by the time you form a new edge (and this is where Cliff Stamp's "plateau sharpening" theory comes in to save the day; the theory is to cut off the edge at 90 degrees, then sharpen it only until the edge becomes nearly sharp, but stop before the burr gets all the way to the apex. But if you use the proper technique, of which there is more than one, you can forget anyone ever came up with this. When it comes to the methods that aren't lapping, this essentially boils down to controlling surface area of contact when on the edge. Make the bevel smaller or reduce area of stone touching the bevel).
The burr could be considered as a glacier flowing over the edge of a cliff. If it flows faster than it is ground away, it will obscure the riverbed below, and the apex you make is made out of the glacier. If you can remove it faster than it grows, you can grind an edge into the riverbed, proper. This burnished, fluid metal is covering the entire surface of the blade, both sides. It's just where it is allowed to flow past the edge, there's no "solid" steel underneath it, anymore, and it's all burnished metal or burr.
The surface of steel always has this crappy stuff on the surface that is not as strong. This why you can't completely remove the burr. When you expose that new edge, the surface will still be relatively weak. But you have the minimum thickness of this weak stuff. Unchecked burnishing at the edge will thicken this layer of weak metal.
It's not that this info isn't out there. It's just the signal to noise ratio isn't very good. Due to internet experts like me, except some of us got it right and the rest got it wrong. When everyone has a say and everyone has feelings, truth is a popularity contest and everyone gets to be right.
It is obvious to me what most people are doing with the flat lapped arkansas stone (and/or flat sintered ceramic stones). You have 3 basic categories of time-wasters.1.
The "finisher." The "finisher" makes an actual sharp edge on a friable lapping stone. This slightly rounds the apex and produces a razor sharp edge. They then "finish" the edge on the flap lapped hard stone. This burnishes the bevel, refining the scratch pattern. And because the apex was slightly rounded by the lapping stone, they can stop before pushing burr all the way to the apex and/or they are putting some lapping compound on the stone. The notion that the scratch pattern on the bevel is what makes a blade sharp is ridiculous. The apex is what makes a blade sharp.
1.b. or he uses another stone or abrasive to form a slurry on the stone.
This is the kind of guy that talks about lapping hundreds or thousands or times on a razor that is already sharp. His favorite 2 words are "JNAT" and "slurry." Indeed, there are people who believe a slurry is the only way to get a blade sharp, despite people have sharpened knives, machetes, swords, and axes with files for centuries. If you sold this guy a block of glass, he would find it exquisite. As long as you charged him $200.00 and told him it was a rare natural stone from Japan.
2.
The "microbeveler." This guy takes the already sharp blade, and he burnishes the bevel to polish out the scratches. But he doesn't stop before the burr gets to the apex. So he does a high angle "microbeveling" pass on the flat lapped stone to cut the burr back and reveal a good apex.
A good example on YT is Richard Blaine. I believe he sells kitchen knives and sharpening accessories. He demonstrates this way of using a flat ark stone. Ironically, he demonstrates the difference between the "western stroke" (more of a drawing stroke) and the "eastern stroke" (back and forth shush-shush slave labor on a sandy stone). I believe the difference stems from using hard/file stones vs lapping stones, the former benefitting from a drawing motion over a smaller area. But he fails to make that connection, and his version of a "western stroke" has the rotation but has only minimal drawing motion.
3.
The unintentional curve guy: This is the guy that may or may not lap his stone truly flat. But when he sharpens, he is intentionally or subconsciously favoring the edge of the stone when on the straight part of his knife and is drawing the blade across the edge of the stone. When he gets to the belly/curve, he starts to lift to get onto the flat of the stone and that's fine, too. Then this person will typically periodically re "dress" the stone with a diamond plate, to take out the dip/wear and this actually maintains a slight curve to the stone. This is the guy that is actually sharpening the most efficiently of the 3. But he erroneously thinks the stone must be "flat" and that flatness does him any favors, and he is wasting a bunch of time to maintain the stone in this "flat" shape. He has learned how to get the best edge using the wrong paradigm for the stone, because everyone else is doing it.
Good example on YT is Jeff Jewell. He is just some guy posting videos with no financial incentive, AFAIK.
The first two guys are just wasting time. And if this is the only stone you had, you could not maintain an edge doing just this no matter how delicately you use the blade and/or how often you touch it up. The burr would eventually push out into and become the edge. You would have to use an actual sharpening stone and/or to cut the edge off at 90 degrees, periodically. The third guy sharpens great; he is just delusional.
The reason I use a strop touched with compound following sharpening of my razor is not to remove the burr. I don't need anything special to do that. It is to polish over the microserrations on the apex. These microserrations are a natural result of sharpening on the curved hard hone, and in most cases they are advantageous. On a razor, they can cause razor burn, cuz you actually angle the edge towards your skin. Right off the stone (convexed translucent arkansas or ruby), I can strop (a razor or any thin knife of decent razor steel) a few times on the edge of my hand and get an amazing, very close, very nice shave without cutting myself. By most objective criteria, this could be the best edge, possible. The closest shave with essentially just a single pass without missing any hairs. But 3-4 days of this in a row and some areas of my face have been thoroughly exfoliated and can start to get a bit of sting or burn. A few swipes on a strop with compound guarantees this doesn't happen. Any stone that burnishes too much will obscure these microserrations; this is where you get a razor sharp edge, but after little use it can't cut a tomato.*** Only a stone that has a high nuff cutting to burnishing ratio can form/reveal these microserrations in the "hard edge." The other reason to use a bit of compound is to remove the new schmoo that forms on the edge over time. A bare leather strop just pushes the schmoo back into place rather than remove it. The blade will be re-aligned, but it will require more frequent stropping as you go, because the edge will get weaker each time and get more quickly out of shape. Before figuring out all this stuff I used to have to re-sharpen my razor more frequently. The more you learn, the less work you have to do. On most blades, other than razors or chisels and plane blades, I don't use a strop with compound. The reason I use compound on chisels and plane blades is because those tools are only sharpened on one side and the flat/unsharpened side tends to develop a bit of back bevel through wear. It's not always worth it to me to grind this tiny wear-bevel all the way out every time I sharpen these tools. I hone the flat side but maintain within a couple degrees of flat. The apex might be only 80-90% complete (still sharp enough for most things), so there are usually parts of the burr that don't come away with a bare strop. Micro chips/scratches that go to the apex get filled in with burnished metal, so the burr has spots of thicker root that doesn't separate, easily. Compound gets in and cleans the burr out of the low spots in the apex, so that the burr will come away.
***This is exactly the kind of edge you make on a flat-lapped arkansas stone. Stropping with compound makes such an edge actually sharper. In this case, the strop is removing some of the burnished goo to reveal the hard edge, warts (natural serrations) and all. And the edge gets some bite and lasting sharpness. In the case of sharpening on a curved ark stone, the main reason to strop is to smooth the edge. The strop doesn't make that edge appreciably sharper, cuz it's already good to go. Except for shaving. If you were to try it, it just might change your perception of what the burr is, and it might give you better insight into what the hocus pocus you might have learned from others is actually doing and why it is "necessary" in the first place. When I say "you," I don't mean you, eKretz, but anyone else that might be reading this. There is some case where the smooth burnished bur-edge is desirable. Some of the eye surgeons of old experimented with burnishing steel scalpels and claimed this decreased healing time and reduced scarring of the cornea. And that's fine, cuz an eye surgeon doesn't cut very much, if he's doing it right.
But for most things, including even a razor, any of this "smooth peanut butter" filling in the valleys in the apex just reduces the useful life of the edge. If the apex isn't perfect to begin with, the flat lapped ark will make scratches and voids/chips disappear - by filling them in with goo. This is mostly cosmetic and it degrades the edge, in a way. Keep on sliding that bevel over the glassy flat stone, and you will just extrude more burr over the apex. The convex ark will keep on cutting until those defects are removed and replaced by the finest defects it can leave behind. Once it has cleaned up all the apex and reached most of the original low spots, any tiny burr fragments that are still clinging to the apex will fall off if you even look at them wrong.
Lapping stones can automatically do what the strop with compound does. You shouldn't need compound if you use a coticule, properly. That is a stone that just happens to have the properties to give a good shaving edge, right off the stone, when used right. But for maintenance between sharpenings, you might find a bit of chrome ox will extend the life of your razor better than a plain strop. The plain strop will tease off a fine burr immediately after sharpening. And it will realign the edge as it takes damage through use. Add a bit of chrome ox, and it does these two functions plus is removes new schmooey metal that forms over time and becomes part of the edge.