Sunday musings on micro convexity and the pasted strop

[kohalajohn]

I was enjoying reading in TheScienceofSharp, about the idea of micro convexity being induced in the edge. The theory is that there is an important difference between pasted stropping on a flexible strop, such as cotton or even leather, and a rigid surface such as balsa.

When stropping on a pasted cloth strop, the front edge of the strop will curve up to hug around the edge. This creates micro convexity at the edge, whereas a flat sided V would be preferable. A flat sided V can rest flat against the skin. When micro convexity gets too "fat" then the bulging belly forces you to use a more obtuse angle to make the edge reach the base of the hair.

A second problem is that when a pasted strop curves up around the edge, it will push parts of the edge over, creating a burr. Most of the burr is subsequently removed (hopefully) by finishing on a leather strop, but it would be better not to have a burr in the first place.



I think that's why the traditional pasted cloth strops are not a forever solution. Eventually the protruding belly of the micro convexity grows so much that it gets in the way. Then you have to hone it on stone to remove steel, to remove the belly, and turn it back into a flat sided V.

And I think this explains why the pasted balsa works on a forever basis.

And I think this is also the reason why you have to use such light pressure on a pasted balsa strop. If you press hard, you're pressing the edge down into the soft balsa. You're making the soft wood immediately in front of the edge, curl up and hug the edge. You're making the flat wood behave like soft cloth, hugging the edge and creating micro convexity, and you're right back at the days of pasted cloth strops. That's good for several months, but you're slowly creating a bulging belly that will eventually have to be ground away by honing.

So. The act of shaving causes edge damage at a microscopic level. A leather strop will clean up the ragged edge a bit, but by doing so you are losing microscopic particles from the edge. After a week or two of mere stropping on unpasted leather, the tip of the edge is retreating down, sinking down like the head of a frightened turtle retreating into its rounded shell.

Honing on an aggressive stone, if done after every shave, would keep a flat V at the edge, but you're removing far too much metal, far too quickly. The edge would retreat far too fast, eating down into the bevel and upsetting geometry.

So what's the answer?

The oldest way was to let the razor degrade for a couple of months and then hone it back on a 8k level coticule or jnat. After two months, a lot of steel needs to be removed, so an aggressive 8k level is good.

The more modern way was iron oxide pasted strops. Much more gentle, so you could do it frequently, but since the edge sunk into the soft substrate, even if said substrate was glued to wood, it created micro convexity over time.

Another way was a super hard black Ark. A very very slow honer, I find I have to use it weekly. But no micro convexity.

And of course the most recent way is a .1u diamond pasted balsa. Even less aggressive then a black ark, meaning you have to use it after every single shave. But .1u removes so very little steel that all it does is undo the tiny bit of damage that the shaving has caused.

It's like a time machine. After the shave, a .1u diamond pasted balsa honing (yes I called it honing. In this context I'm using the verb to explain what it accomplishes, rather than the fact that you use a stropping motion to do it) takes you back ten minutes, to the moment just before you started to shave.

If they make a diamond paste more fine than .1u, it would not be a good idea. You'd have to strop two hundred time, and would just get the same effect, with a lot more unnecessary work.

And if I used a different wood, that also would not be a good idea. Anything harder than balsa, the diamond particles would not bury themselves halfway into the wood, and would roll around on the surface, which is not good. Anything softer than balsa, and the razor's edge would sink down into it, and you'd get micro convexity.

So .1u diamond paste is the correct maintenance abrasive. And balsa is the correct maintenance substrate. (And an acrylic base to protect your fingertips is a nice idea)

 

[rbscebu]

@kohalajohn I think you have everything right.

 

[Slash McCoy]

I was enjoying reading in TheScienceofSharp, about the idea of micro convexity being induced in the edge. The theory is that there is an important difference between pasted stropping on a flexible strop, such as cotton or even leather, and a rigid surface such as balsa.

When stropping on a pasted cloth strop, the front edge of the strop will curve up to hug around the edge. This creates micro convexity at the edge, whereas a flat sided V would be preferable. A flat sided V can rest flat against the skin. When micro convexity gets too "fat" then the bulging belly forces you to use a more obtuse angle to make the edge reach the base of the hair.

A second problem is that when a pasted strop curves up around the edge, it will push parts of the edge over, creating a burr. Most of the burr is subsequently removed (hopefully) by finishing on a leather strop, but it would be better not to have a burr in the first place.



I think that's why the traditional pasted cloth strops are not a forever solution. Eventually the protruding belly of the micro convexity grows so much that it gets in the way. Then you have to hone it on stone to remove steel, to remove the belly, and turn it back into a flat sided V.

And I think this explains why the pasted balsa works on a forever basis.

And I think this is also the reason why you have to use such light pressure on a pasted balsa strop. If you press hard, you're pressing the edge down into the soft balsa. You're making the soft wood immediately in front of the edge, curl up and hug the edge. You're making the flat wood behave like soft cloth, hugging the edge and creating micro convexity, and you're right back at the days of pasted cloth strops. That's good for several months, but you're slowly creating a bulging belly that will eventually have to be ground away by honing.

So. The act of shaving causes edge damage at a microscopic level. A leather strop will clean up the ragged edge a bit, but by doing so you are losing microscopic particles from the edge. After a week or two of mere stropping on unpasted leather, the tip of the edge is retreating down, sinking down like the head of a frightened turtle retreating into its rounded shell.

Honing on an aggressive stone, if done after every shave, would keep a flat V at the edge, but you're removing far too much metal, far too quickly. The edge would retreat far too fast, eating down into the bevel and upsetting geometry.

So what's the answer?

The oldest way was to let the razor degrade for a couple of months and then hone it back on a 8k level coticule or jnat. After two months, a lot of steel needs to be removed, so an aggressive 8k level is good.

The more modern way was iron oxide pasted strops. Much more gentle, so you could do it frequently, but since the edge sunk into the soft substrate, even if said substrate was glued to wood, it created micro convexity over time.

Another way was a super hard black Ark. A very very slow honer, I find I have to use it weekly. But no micro convexity.

And of course the most recent way is a .1u diamond pasted balsa. Even less aggressive then a black ark, meaning you have to use it after every single shave. But .1u removes so very little steel that all it does is undo the tiny bit of damage that the shaving has caused.

It's like a time machine. After the shave, a .1u diamond pasted balsa honing (yes I called it honing. In this context I'm using the verb to explain what it accomplishes, rather than the fact that you use a stropping motion to do it) takes you back ten minutes, to the moment just before you started to shave.

If they make a diamond paste more fine than .1u, it would not be a good idea. You'd have to strop two hundred time, and would just get the same effect, with a lot more unnecessary work.

And if I used a different wood, that also would not be a good idea. Anything harder than balsa, the diamond particles would not bury themselves halfway into the wood, and would roll around on the surface, which is not good. Anything softer than balsa, and the razor's edge would sink down into it, and you'd get micro convexity.

So .1u diamond paste is the correct maintenance abrasive. And balsa is the correct maintenance substrate. (And an acrylic base to protect your fingertips is a nice idea)

I have tried finer diamond. I tried .025 u, or 25nm. A quarter the size of the .1u. A few razors gave me a sharper edge after about 1k laps. Most did not improve in sharpness. And most seemed to lose a bit in comfort. The .1u is in its own sweet spot.

 

[kohalajohn]

Yes, I remember when you tried that.

But you know, if a smaller grit did create more sharpness, I don't think I'd want it. Not yet anyway.

The very first shave with a Feather AC Professional is exercised with great caution. For me, anyway. While a sharp Method Edge on a traditional straight will slide over the curves of my chin, I have seen a new Professional dig in, as if the skin showed it zero resistance. There are some curves on the face where you just can't pull the skin flat and tight.

Maybe with some more years of perfecting technique, then I'll feel that there's no such thing as too sharp. But right now I'm in the camp of folk who believe there is such a thing as too sharp.

The problem with coming to sr shaving later in life, as I did, is erasing a lifetime of muscle memory from the damn cartridges and the bad habits they cause.

When my boys start shaving I'm starting them on straights. They can do what they like later on, but it's like how you should start driving stick first. You can change to automatic transmissions later on, but you'll always be able easily drive stick if you started that way.

At around age 50 I changed over for both. Traded in my cartridges for straights and traded in my automatic transmission Audi for a stick shift 911.

A lot of cuts. A lot of stalls on hills.

But worth it.

 

[bluesman 7]

So. The act of shaving causes edge damage at a microscopic level. A leather strop will clean up the ragged edge a bit, but by doing so you are losing microscopic particles from the edge. After a week or two of mere stropping on unpasted leather, the tip of the edge is retreating down, sinking down like the head of a frightened turtle retreating into its rounded shell.

I've often thought that a bevel angle on the acute side helps in this regard. With an acute angle the retreating edge still has a small edge radius width. This is all at a microscopic level of course. In other words an acute bevel angle has to wear down farther than a more obtuse angle to reach the same edge radius width.

 

[kohalajohn]

I've often thought that a bevel angle on the acute side helps in this regard. With an acute angle the retreating edge still has a small edge radius width. This is all at a microscopic level of course. In other words an acute bevel angle has to wear down farther than a more obtuse angle to reach the same edge radius width.

Yes, I think that's correct.

One of the many reasons we prefer acute angles.

With an obtuse bevel angle, the problems from a retreating edge are accelerated. With an obtuse angle you are already forced to angle the edge more aggressively downward into your skin, which can scrape skin and be uncomfortable. And as the edge retreats downward, if you have an obtuse angle anyway, your razor will be rapidly more obtuse. Those fat shoulders get in the way, forcing you to angle your razor more and more, to get the edge to reach the hairs. Pretty soon you'll be feeling raw skin and aftershave burning like fire.

It will feel like your razor has gone dull, and to some extent that's true, but in addition, your geometry is getting screwed up.

I imagine that with hard swedish steel and a acute 16 degree or so angle, you could go for a long time between honing.