I convexed a coticule some time ago just out of curiosity. It worked fine for me but I went back to flat because I don't dedicate any of my stones to just razors.
Convex Stones...Legit or Gimmick?
- Thread starter Acmemfg
- Start date
My hypothesis is that any benefit of to an edge off of a convex stone will experience diminishing returns at the most extreme examples of sharpness.
Chandu
I Waxed The Badger.
Sharp is sharp and at some point the only difference will be seen with a high power microscopy and what shaves best, may in fact not be the sharpest edge. There are objective ways to measure sharpness and so on, but no objective way to measure shaving comfort. I think there is a big assumption by many that sharpest = best shave.
Perhaps a good thought experiment would be to pose the following question:
How does the manufacturer envision the roll of the convex hones’ roll in terms of final sharpness?
I pose this question simply because both Dovo and T.I. finish with one or sometimes two compounds. T.I. also uses convex stones as well but I’m not sure if theirs has a curve across the width. So it seems that we can speculate that both makers only expect their bench stones to get the bevel to a point at which the compounds can take over. In fact I just received a new T.I. today and their maintenance instructions indicated that the user simply maintains the edge with their green or white paste (if it’s still available).
So however keen the convex surface may make the bevel the keenness of pasted stropping is going to far exceeded that of the hones.
How does the manufacturer envision the roll of the convex hones’ roll in terms of final sharpness?
I pose this question simply because both Dovo and T.I. finish with one or sometimes two compounds. T.I. also uses convex stones as well but I’m not sure if theirs has a curve across the width. So it seems that we can speculate that both makers only expect their bench stones to get the bevel to a point at which the compounds can take over. In fact I just received a new T.I. today and their maintenance instructions indicated that the user simply maintains the edge with their green or white paste (if it’s still available).
So however keen the convex surface may make the bevel the keenness of pasted stropping is going to far exceeded that of the hones.
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And a pasted strop is concave, rather than convex.
So T.I. uses hard-backed strops. They use rough leather for both the green and white compounds whereas Dovo uses a somewhat looser pasted frame strop. But either way this will wind up slightly convexing the edge a bit.
Now Wacker uses a peculiar strop. The leather is backed by a piece of wood that has a slight arc in it that’s slightly convex. But regardless of the strop configuration the paste has the final say.
But in our discussion here the debate seems to be whether or not convex hones will objectively surpass flat stones (minus the compounds) in comfort and keenness and I’m of the opinion that such a determination is almost impossible.
The whole argument that the manufacturers are using convex hones is bollocks in my opinion.
If you look up any video from one of the German factories they are finishing on flat hones. The dovo factory appears to be using a rotating wheel to set the bevel - but NOT on the curved face. The wheel is mounted horizontally and the razor hones on theside perpendicular to the round side of the stone. This is rounded in one direction to reduce the cutting area, but not the other direction that is normally advocated by people. Then finishing is done on flat stones. Here is a link to a video of the dovo factory
So the convex thing is a fad in my opinion.
If you look up any video from one of the German factories they are finishing on flat hones. The dovo factory appears to be using a rotating wheel to set the bevel - but NOT on the curved face. The wheel is mounted horizontally and the razor hones on theside perpendicular to the round side of the stone. This is rounded in one direction to reduce the cutting area, but not the other direction that is normally advocated by people. Then finishing is done on flat stones. Here is a link to a video of the dovo factory
So the convex thing is a fad in my opinion.
Here’s a link to a video recorded at the Dovo factory a few years ago. There can be no doubt that they’re using such stones...
Thanks for sharing that one, I hadn’t actually seen it and it’s very interesting! Indeed, those stones are definitely curved. Is that along both axis of the stone or just one?
Obviously the wheel can only be curved in one direction, otherwise the razor would chatter and it would be crazy dangerous and destroy the blade.
So the bevel setting wheel is an odd duck. Jarrod had explained that the honing area was actually slightly convex (like a file in a way). And that there was slightly more relief at the outer edge than at the center to prevent unwanted contact at the heel. But it’s impossible to really see that in this video. This video is probably the best documentation of this topic that I’ve seen.
Those stones are said to be convex on both axis. 25’ radius along the length and a 6’ radius along the width. But I can’t see the curve along the length in the footage though.
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Several knife makers are using conical disk sanding surfaces. The face is turned with a 1° angle with the center the high point. This allows the knife to go past center without contacting the far side.
It also creates a slight convex surface it the direction of the tangent.
I feel like this whole subject over complicates the simplicity of steel and stone. Ruining the back cultural heritage of honing in general. Honing is simply complicated in its own right. Mucking the waters further is unnecessary to me.
I've been obsesivelly chasing stones for quite a few years in France. Purchased many worn and NOS stones, read all hone related material, forums, blogs and old papers I could find, talked with many old timers, and the only convex stones I've ever seen or heard off, are the PDSO.
You find many old writings telling how to sharpen, what stones, how to get them flat, to use oil, even virgin olive oil, but none mentions convex hones.
As for Dovos they are known arround here for being out of the box absolutely useless. Countless newbies on forums asking why they shinny new Dovo is pulling their faces off, and fortunately there is always an experienced hand to help them out. So convex may work as a production technique, but I don't see anyone raving about those edges as the absolute pinnacle of pro sharpening. No one telling newbie honers to buy a dovo as a reference edge...
Of the French razor manufacturers, TI seems the only real survivor producing new blades. There is someone on the forums that knows them. I will ask, because now that I think about, while he has said they finish on a coticule, he didn't talk about flat or convex.
In any case their new razors are not shave ready, at least compared to what most of us can do.
PSDO may be the best indication that at some point, convexity was considered a good idea. But so where drilled surfaces and many other ideas and marketing ploys we can see in any old or new catalogue.
As for a personal opinion, when Harold shared the convex thing, I flipped a PDSO ,(I only flatten one side) and covexed a purple la lune to try out. While sure you can sharpen, and it seems a bit faster, I don't find you can't push as much the edge to the absolute limit of what the stone is capable off. It makes sense since for the same pressure applied to the razor, the efective contact surface is much narrower, increasing partial pressure. As I try for the opposite, more glide and less pressure, I don't find convex an advantage for a finisher.
Finding out what T.I. is using would be interesting. It would be good to get verification from an additional source. I have no idea what kind of stone they’re using in this video but it’s directly from their own YT channel. I’m not sure if this is a finishing stone.
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Whatever it is (looks like a black ark) and it would surely be a finishing stone being as it is done entirely by hand - it is a flat stone.
It looks flat to me as well. But there is a natural stone mined in France that’s called a Pierre Pyrenes that’s estimated to be roughly 600-800# and another grade that’s roughly 1200# that’s very dark in color. Makes me wonder if he’s actually setting the bevel. I just can’t tell...
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Steve56
Ask me about shaving naked!
Yes, the title is ‘Sharpening (a) razor on (a) Pyranees stone’.
@PaulMallorca pretty much nailed it, for all the training, apprenticing, guilding, and convex stones they use the factory edges leave much to be desired, as also evidenced that the vendor of these stones also re-hones every razor that he sells. He hones well, and if he did not re-hone them he would have returns and complaints like all the shops that don’t re-hone them.
The primary purpose of a convex stone, IMHO, is to get wonky blades out the door. They’re functionally a narrow hone. Thinning the bevel behind the edge is a hollow argument, also IMHO. If you look how much thinner a 1mm wide bevel is in the center of the bevel due to the convex stone of stated curvature, it’s about 1/5000”, the depth of a mid-grit synthetic. IOW, over the width of a bevel it’s functionally flat. The curvature is however, sufficient to ride into non-straight areas of a blade, and if it works for you and you like the results, by all means go for it.
Here’s a video of Glen Mercurio fixing the classic over ground Dovo factory edge so that all the edge will hit on a flat stone.
@PaulMallorca pretty much nailed it, for all the training, apprenticing, guilding, and convex stones they use the factory edges leave much to be desired, as also evidenced that the vendor of these stones also re-hones every razor that he sells. He hones well, and if he did not re-hone them he would have returns and complaints like all the shops that don’t re-hone them.
The primary purpose of a convex stone, IMHO, is to get wonky blades out the door. They’re functionally a narrow hone. Thinning the bevel behind the edge is a hollow argument, also IMHO. If you look how much thinner a 1mm wide bevel is in the center of the bevel due to the convex stone of stated curvature, it’s about 1/5000”, the depth of a mid-grit synthetic. IOW, over the width of a bevel it’s functionally flat. The curvature is however, sufficient to ride into non-straight areas of a blade, and if it works for you and you like the results, by all means go for it.
Here’s a video of Glen Mercurio fixing the classic over ground Dovo factory edge so that all the edge will hit on a flat stone.
Funny you posted this because it reminds me of the 1st straight I ever bought. A Dovo BQ. It didn’t shave well out of the box and started to frown like a demon after honing. I got so tired of trying to get the frown out of it that I just taped some lapping film to a length of 3” PVC tubing and just honed it that way frown and all. That thing shaved better afterwards than it did when it was new!
It also seems to me that in order to keep the peak and shape of said stone you would always be wasting a large amount of stone on say a coticule since you are only honing in the center narrow area. Wouldn't it be simpler to just buy a cheap coti slip stone and hone on the top. Coticules are not the cheapest stones.
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