In part 1 of this article, the edge geometry associated with the deliberate “raising of a burr” was described. As noted, the thickness of this burr was several microns. For shaving, we are targeting an edge width (keenness) in the range of 0.1 micron or less. If we were to “break off” this burr, the edge profile would be far coarser than is achieved with even the lowest grit stones typically used in straight razor honing. For that reason, the raised-burr is not desirable to straight razor honing.
In the typical honing progression on a flat hone with edge leading strokes, alternating sides, a triangular bevel is formed, terminating at a width determined by the level of “micro-chipping” that occurs at the apex. A burr, as defined by “metal beyond the apex” does not form; the raised-burr is easily avoided. However, other forms of “finishing” burrs can be produced and some examples are described here.
Shown in the image below; a straight razor honed to the Shapton 8k level, prior to stropping. The edge width is in the range of 150-200nm, slightly greater than the threshold for comfortable shaving.
With sufficient stropping on clean linen/leather, an 8k edge can be brought to shaving-level keenness. Few people have the patience for this. The addition of sub-micron abrasive to the strop will greatly accelerate this process; however, if done “incorrectly” can produce a very undesirable burr.
This foil-edge burr can be reduced, but not eliminated, by increasing the number of laps; shown below after 100 stropping laps.
Fortunately, this foil is fragile enough to be completely removed by a few strokes of beard shaving and the edge width after breaking off the burr is well below the threshold for comfortable and efficient shaving.
As a starting point, these experiments and the discussion have been limited to carbon steel straight razors; however, the same principles apply to knife sharpening. The primary difference being that a knife edge must be more robust than a razor and therefore have a larger bevel angle. Below, is the edge of my H1 steel pocket knife, sharpened with an approximately 1500 grit ceramic stone at 20 degrees per side.
The rough burr is easily removed by stropping on a diamond-impregnated kangaroo leather bench strop, shown below.
A close-up view reveals that a slight foil is present at the apex.
The significance of this foil edge will of course depend on how the knife is used (or abused). Shown below is the same edge following the cutting of some heavy cardboard. Clearly, burrs and foil edges are of no consequence in this case.
A similar experiment is shown below for a straight razor with a foil edge.
Following stropping, a few centimeters of bond paper were cut, removing the foil, unfortunately also destroying the cutting edge. The effect cutting of paper could be described as forming a “folded burr.” Another view is that the blade was simply used for a purpose requiring a larger (stronger) bevel angle.
In the above examples, I have shown burrs where the “unwanted metal” is beyond the apex. Another possibility is that the steel fails, or is removed during normal use. This observed loss of keenness is often attributed to the presence a foil edge or sometimes described as an “over-honed” edge.
In the example below, the razor was honed to a 20k Suehiro Gokumyo, an edge which just achieves shaving-level keenness prior to stropping. The reader will be forgiven for being unable to observe a difference from the 8k edge.
Stropping on (clean) leather produces a very subtle improvement; shown below after 300 and 500 laps.
In contrast, stropping (lightly) on clean linen causes the edge to break off, resulting in an edge width too large for comfortable shaving. Without the accompanying images, this effect would likely be attributed to the presence of a foil edge that has broken off with stropping. We should consider whether a foil-edge burr can be defined as any part of the apex that does survive the intended use.
In the examples presented here, I have highlighted some of the issues that have been observed during various experiments. More detailed discussion will be provided in posts specific to those experiments. The images presented here are intended as discussion points, with the expectation that a “part 3” will appear in the future.