The word sharpness generally refers to observed cutting performance. In the absence of the quantitative microscopic measurements used in the experiments described here, macroscopic observations allow relative comparisons of sharpness. The ability to cut hair is a common blade test. Although the test can be performed in several ways with varying interpretations of the result, the premise of such tests is that the blade must be keen enough to penetrate the hard, thin cuticle sheath and sharp enough to sever the hair with minimal force. The Hanging Hair Test (HHT) is eloquently described by my friend Bart Torfs on his website coticule.be where he discusses the interpretation and quantification of the HHT test.
The HHT is a two step process; first, catching and penetrating the hard cuticle shell and second, completing the cut. “Catching” only involves the last micron of the edge and the keenness or width of the apex. Cutting or force to cut will involve not only keenness but bevel angle (thickness behind the edge), and friction from smoothness/polish/coating of the bevel over no more than the next 100 microns (diameter of the hair). Since the hair is held only at one end, it can bend away from the blade if the force-to-cut is greater than that required to deflect the hair. Although the physics involved is complex, the test does provide a relatively simple determination of whether the edge is at or below the required keenness threshold. Unfortunately, if the apex is just 50nm (0.05 microns) thicker than the threshold, it will not pass the test, although the blade is still very sharp by almost any standard.
My own experiments have shown that an edge width of approximately 100nm is required for shaving. This corresponds to an edge radius of 50nm, consistent with the measurements found in Gillette razor blade patents indicating a manufacturing target of 200-300 Angstroms (20-30nm) for the edge radius. Unfortunately, even the highest grit synthetic stones, when used with edge leading strokes, only just reach this level of keenness. Very light strokes, or edge tailing strokes can bring the apex width down the few tens of nanometeres required; however, a triangular bevel with a 16 degree included angle is typically too weak to survive shaving a normal-to-heavy beard. Abrading the apex by stropping will micro-convex and bring the keenness past the threshold while simultaneously making the apex more robust.
There is a commonly repeated myth that a hand honed and stropped blade cannot be as “sharp” as a factory sharpened razor blade. The images below prove this statement to be false. The straight razor shown is both keener and sharper (thinner behind the edge) than the Feather DE blade for the first 50 microns from the apex.
There is a common misconception that finer grit honing and stropping correlates with sharper and keener edges. As a example, blade sharpness is sometimes (mis)described by the effective grit rating with the belief that a “12k edge” is keener than a “10k edge”. The difference between edge-leading and edge-trailing strokes, shown in the pasted strop part 1, and the results from coarse diamond plates in the diamond plate progression, have already shown how misleading this concept is.
The images below are from a straight razor honed on a Belgian Coticule, without stropping. Analysis of this particular stone reveals the abrasive (Spessartine garnet) is between 5 and 10 microns in diameter, equivalent to the grit in a Shapton 2k hone. Unlike the edge produced by a Shapton 2k stone, the Coticule-honed edge is easily keen enough to shave. Avoiding generalizations about Belgian Coticules, the results clearly show that grit size does not determine keenness. In this particular case, the apex has been micro-convexed by the honing surface, in a similar fashion to that observed for coarse diamond plates, resulting in what I have personally observed to be a nearly perfect shaving blade geometry.
In summary, “sharpness” must be measured by a method consistent with the intended purpose; it is not predicted by the size of the abrasive grit applied.