In the previous installment of this series, it was shown that (sub-micron) abrasive stropping produces convexity in the last few microns of the edge; this result was defined as micro-convexity. In this article, the role of the strop material is examined and insensitivity to the number of laps (stropping strokes) is investigated. The challenge of avoiding a foil edge burr with diamond stropping is also discussed.
In the first example, we start with a razor honed with a DMT EF (1200) plate; the nearest to a “toothy edge” that we have observed so far. The blade was stropped 3 times (return strokes) on a 10 inch long balsa wood strop loaded with 0.25 micron mono-diamond. The effect is dramatic and clearly shows how rapidly pasted stropping is accomplished.
It is interesting to note that the distance traveled by the blade in those three laps (approximately 75cm) is 3 million times the diameter of the abrasive particle (0.25micron). This is approximately the number of steps required to walk from New York City to Miami. Whether we define three laps as “not many” is a question of scale.
This three-lap result may suggest that if pasted stropping works so rapidly, too many strokes must be avoided; however, this is not necessarily the case. Below is shown a set of cross-section images following 10, 100 and 500 return laps on a chromium oxide loaded horsehide hanging strop. The effect is again rapid, with the edge already micro-convex after only 10 laps. The degree of micro-convexity appears to reach an equilibrium in the range of 30 laps (not shown). Very little change is observed between 100 laps and 500 laps. A slight foil burr is formed at high stroke counts, and this would require additional stropping on an appropriate strop to clean-up and maximize keenness. A blade stropped on chromium oxide generally benefits from further stropping on finer abrasive or a just a clean leather strop to enhance keenness. Nonetheless, there is no runaway convexity occurring with large numbers of strokes in this case. This result will not be surprising to anyone who maintains their straight razor with daily application of a pasted strop.
The next example shows the effect of 250nm mono-diamond on balsa wood following honing to the 8K level in a Shapton glass stone progression. The Shapton 8k blade (edge leading) is shown in the Honing Progression entry and has a typical edge width of 150nm. This particular razor has a 16.5 degree (included) spine-edge angle prior to stropping. The image shows that the micro-convexity introduced by the balsa strop affects only the last 0.3 microns of the apex. Nonetheless, the edge width is reduced to the 50nm level as a result of that micro(nano)-convexity. Minimal convexity results in this case because the balsa wood strop is rigid and nearly incompressible.
Essentially opposite to balsa wood, Nanocloth is very soft and flexible (although mounted on a hard, flat glass substrate). The images below show that the amount of micro-convexity introduced is only slightly greater than from the balsa substrate. This observation indicates that micro-convexity is not simply a result of strop compressibility.
In the next example, we use a strop that is both flexible and resilient, latigo (bovine) leather. The Shapton 8k honed blade was stropped on the hanging latigo loaded with 0.25 micron mono-diamond. This pasted strop produces significant micro-convexity and as side-effect also produces a foil-edge burr. Although it may appear to be insubstantial, this foil is sufficiently robust and flexible to negatively impact the shaving efficiency of the blade. Shaving with a razor with such a foil edge burr generally results in extreme levels of skin irritation.
The razor was then given another 90 laps on the 0.25 micron diamond on latigo for a total of 100 laps. The foil is diminished in thickness and length, but it still present along much of the edge. Shaving with such a razor would cause skin irritation, while breaking away the burr, and leaving less than optimal keenness. Further stropping could bring the blade to comfortable shaving geometry.
For comparison, a razor was again honed to the Shapton 8k level but this time stropped 10 laps on 0.5 micron mono-diamond on a latigo strop. The result is similar to that observed for the 0.25 micron case; however, steel removal occurs more rapidly. The increased rate of abrasion with particle size is expected; however, there is no significant difference in the edge geometry. This effect requires further study, but it appear that the abrasive size has little impact on the resulting geometry. Clearly the material of the strop plays a far greater role.
In the final example, a blade was honed to the Shapton 8k level and then stropped on a hanging denim strop loaded with Thiers Issard Pate a Rasoir, a coarse (10 micron) aluminum oxide abrasive. Not only is there significant micro-convexity, but the entire bevel has started to convex as evidenced by the increase in angle away from the apex. The significant metal removal rate provides another example of the role of abrasive particle size in the stropping process. The edge width is approximately 100nm with small burrs along some parts of the edge. Additional stropping on linen and clean leather will typically remove these burrs and improve keenness.
The blade shown above was then stropped 10 laps on the same 0.25 micron mono-diamond on the latigo leather strop that produced the foil edges shown earlier. The burr formed in this case is extremely subtle compared to the burr produced on the blade that was not first micro-convexed. This observation provides further evidence that the foil edge burrs produced by the latigo strop are a result of the geometry transition from triangular to micro-convex. Very fine, flexible burrs can be difficult to remove without sacrificing keenness, so avoiding burr formation is always preferable. One effective approach is to first micro-convex the apex with a more aggressive strop, with fabric and coarser abrasives for example.
The physics underlying the formation of micro-convexity can be visualized with the image below. In this model, the lower piece of foam represents the strop and the upper piece of foam represents the blade. To the left of the apex, the strop uniformly compressed by the blade. To the right of the apex, the strop is uniformly uncompressed (relaxed). The transition between these two regions creates a greater pressure on the blade near the apex. The increased pressure results in faster abrasion rates until the apex geometry reaches the equilibrium profile. This equilibrium shape can be visualized in this example by the line of the paper between the two foam slabs. Once the blade has been convexed to that equilibrium geometry, the pressure will be equal along the bevel all the way to the apex and abrasion will occur uniformly, maintaining that equilibrium (micro-convex) profile. For this reason, micro-convexity is introduced rapidly and that geometry remains essentially unchanged with further stropping.
In summary, examples have been provided to evidence the following observations:
Abrasive stropping rapidly micro-convexes the apex. Additional stropping, up to many hundreds of strokes does not substantially increase the micro-convexity.
Bevel convexity, as distinct from micro-convexity, is not apparent in sub-micron abrasive stropping, but does occur with larger abrasive particles. The quantity of metal that must be removed to convex the entire bevel will result in blackening of the strop, something generally not observed with sub-micron abrasive strops.
The apex geometry is determined primarily by the strop material rather than the abrasive size.
The transition from triangular to micro-convex bevel geometry can result in the formation of a foil edge burr for resilient (springy) strop materials. This is almost certainly the reason some people are unsuccessful with 0.25 micron diamond spray. If the apex is convexed on a more aggressive strop first, the edge can be finished on that resilient strop without foil edge burr formation.
These observations suggest that a successfully designed stropping progression should involve different strop materials rather than different sized abrasives on the same substrate. This is partly why the traditional combination of linen and leather are so effective at maintaining a straight razor.