Thread: How does sharp relate to smooth?

I always make a distinction between smooth and sharp personally. Though I do agree that they both need to achieve some kind of minimum level before you can call an edge properly honed.

But dichotomies like "properly/improperly" hide a lot of nuance and variation. Sharp and smooth lie on a continuum, and so their interaction does too.

So anyway, to me sharp is a measure related to the inverse of the distance between the two sides of the bevel (ie the smaller the distance, the higher the sharpness). The measure might actually be important: consider two possible measures - mean distance and maximum distance (over the entire edge, bevel width will vary so we need a summary measure unless we start delving into sharpness as a function of distance from heel or something, and that's just going way too complicated IMO).

Smooth, on the other hand, to me is a measure of the variability of the distances along the edge. Highly variable bevel distances will not give a smooth edge, whereas homogeneity will.

So, for the average distance measure things might get a little rough because as we all know with means, they are made up of, potentially, large and small values. So an edge with a highish sharpness (low average distance) could very well feel rough because that mean distance value comprises highly variable "peaks and troughs".

On the other hand, if you defined sharpness in terms of maximum distance, then something with a high sharpness is by definition guaranteed to be smooth because all other points along the edge must be less than the maximum, of course.

I don't know if that makes sense to people, but that's how I look at it. Feel free to ignore as you see fit!

James.

Originally Posted by Jimbo

I always make a distinction between smooth and sharp personally. Though I do agree that they both need to achieve some kind of minimum level before you can call an edge properly honed.

But dichotomies like "properly/improperly" hide a lot of nuance and variation. Sharp and smooth lie on a continuum, and so their interaction does too.

So anyway, to me sharp is a measure related to the inverse of the distance between the two sides of the bevel (ie the smaller the distance, the higher the sharpness). The measure might actually be important: consider two possible measures - mean distance and maximum distance (over the entire edge, bevel width will vary so we need a summary measure unless we start delving into sharpness as a function of distance from heel or something, and that's just going way too complicated IMO).

Smooth, on the other hand, to me is a measure of the variability of the distances along the edge. Highly variable bevel distances will not give a smooth edge, whereas homogeneity will.

So, for the average distance measure things might get a little rough because as we all know with means, they are made up of, potentially, large and small values. So an edge with a highish sharpness (low average distance) could very well feel rough because that mean distance value comprises highly variable "peaks and troughs".

On the other hand, if you defined sharpness in terms of maximum distance, then something with a high sharpness is by definition guaranteed to be smooth because all other points along the edge must be less than the maximum, of course.

I don't know if that makes sense to people, but that's how I look at it. Feel free to ignore as you see fit!

James.

This is a logical argument and consistent with my experience. What is missing, I think, is the variability in the shaver's technique and perception.

Having read most of the threads that Glen refers to, I have come to suspect that some (many?) people are unable to absolutely perceive the sharpness of a razor blade. My theory is that we subconsciously adapt technique/pressure/force to the blade we are using. With skill and experience, we do this successfully and most shaves are smooth.

There is a problem when the blade is 'highly variable' as James says, we compensate for the less sharp and damage our skin with the more sharp in a manner of speaking. I think this most commonly occurs for a freshly honed blade with a fragile edge. Sub-micron bits break away from the edge during the shave leaving the troughs that James refers to. My guess is that these are the type of edges that are called 'too sharp.'
With stropping, these troughs are sharpened and the blade becomes 'smoother'

I also forgot to add that if we use mean distance as a sharpness measure, that measure will not necessarily be unique. As in, many scenarios can give the same mean value - from a fairly uniform set of distances, to edges that vary wildly between peaks and troughs, to possibilities in between.

Eg: two razors with 5 distance measures (for simplicity). Units don't matter, I'm making the numbers up :

Razor A (homogenous, smooth): 1, 1, 1, 0.75, 1.25. Mean distance = 1, sharpness = 1.
Razor B (variable, not smooth): 3, 0.9, 0.3, 0.8, 0.01. Mean distance (2dp) = 1, sharpness = 1.

So there you have a mathematical (ish) theory of why one "sharp" razor feels harsh, whereas another of equal sharpness feels smooth. Either that, or I've just proven people think of sharpness in terms of the mean. Or that the mean is bad for this kind of thing....Or maybe I'm just spinning a whole lot of bollocks!

I mean, this is all theory of course. We don't wander around with nano (or micro?) callipers measuring edge distances at 100s or 1000s of points along the edge. But I just find it a useful, and fun, way to visualise it.

James.

Great question, I've thought about this a few times myself. I like Jimbo's definition of sharp - basically the smaller the angle inside the two faces of the bevel the 'sharper' a blade is? Though I thought of smoothness as something more along the lines of how serrated (on a microscopic scale) an edge is dictates how smooth the edge. e.g. a serrated blade (steak knife) and a straight blade (filleting knife) may have the same angle between bevel edges, but try running your thumb over the edges and one is going to feel much smoother than the other. So the smaller the serrations along the length of your edge, the 'smoother' the blade?

Practically this can be felt when shaving. I used to finish my razors on a Norton 8k and that's as far as I'd go. Then I discovered 0.5 micron paste and a 12k finishing stone. Neither are aggressive enough to significantly change the bevel angle, so 'sharpness' isn't altered, but I think they reduce the coarseness of the serrations along the length of the edge, making the blade feel smoother when shaving. It's worth trying this on a razor to feel the difference it makes. Both shave, but the smoother edge is definitely more comfortable.

As a side thing - I use the hanging hair test a lot. I've come suspect that a razor finds this test harder to pass when it's smoother. I think the 'serrations' grab and cut the hair easier than a really smooth blade does. So your sharp and smooth edge won't pass the test as easily as a sharp harsh razor will. Anyone else found this?

Originally Posted by SimonG

Great question, I've thought about this a few times myself. I like Jimbo's definition of sharp - basically the smaller the angle inside the two faces of the bevel the 'sharper' a blade is? Though I thought of smoothness as something more along the lines of how serrated (on a microscopic scale) an edge is dictates how smooth the edge. e.g. a serrated blade (steak knife) and a straight blade (filleting knife) may have the same angle between bevel edges, but try running your thumb over the edges and one is going to feel much smoother than the other. So the smaller the serrations along the length of your edge, the 'smoother' the blade?

Practically this can be felt when shaving. I used to finish my razors on a Norton 8k and that's as far as I'd go. Then I discovered 0.5 micron paste and a 12k finishing stone. Neither are aggressive enough to significantly change the bevel angle, so 'sharpness' isn't altered, but I think they reduce the coarseness of the serrations along the length of the edge, making the blade feel smoother when shaving. It's worth trying this on a razor to feel the difference it makes. Both shave, but the smoother edge is definitely more comfortable.

As a side thing - I use the hanging hair test a lot. I've come suspect that a razor finds this test harder to pass when it's smoother. I think the 'serrations' grab and cut the hair easier than a really smooth blade does. So your sharp and smooth edge won't pass the test as easily as a sharp harsh razor will. Anyone else found this?

Love the avatar!!!

Thanks, I liked the quote it came with too but didn't have the room to include it - Funny quotes: A true gentleman knows

Originally Posted by israfelli

Understanding sharpness in terms of what is physically happening to the edge seems natural. But if you have two razors of identical sharpness, but whose smoothness is quite different, what at the physical microscopic level is different?

Probably a semantic point but is that actually possible ? I would think identically sharp razors would be identically smooth.

A lot of this can come down to usage, perception &/or experience.

At 200x under the scope I still can't understand why some ragged looking edges shave just as comfortably as the smooth looking ones.