In part 1 of the series, it was shown that edge-trailing and edge-leading strokes on a hard, flat abrasive hone lead to entirely different processes for apex formation and refinement. The edge trailing (stropping) stroke allows a keener apex to form by avoiding the micro-chipping that occurs in edge-leading strokes. However, the edge-trailing stroke can lead to the formation of a foil-burr while the micro-chipping that characterizes an edge-leading stroke generally limits the formation of a foil-burr.
In part 2 of the series, it was demonstrated that the use of a compressible substrate (a strop) produces micro-convexity and can avoid the formation of the foil-burr that is usually produced when performing edge-trailing strokes on an in-compressible substrate. This micro-convexity typically improves keenness and apex uniformity.
In part 3 of the series, it was shown that the amount of micro-convexity is determined primarily by the choice of strop material. It was also demonstrated that partial or incomplete micro-convexing of the apex can produce a foil-burr (as in the case of 0.25 micron diamond on leather, for example). It was also observed that the effect of stropping is rapid (requiring only a few strokes) and that further stropping (up to hundreds of strokes) has minimal additional impact.
In this edition of the series, the role of abrasive size (grit) is examined. In particular, it will be shown why stropping a straight razor with metal polish on denim is so effective, despite being seemingly contrary to the common “wisdom” of internet experts.
Some time ago, I purchased a stick of “Thiers Issard Pate a Rasoir” stropping compound. This white crayon-like stick of aluminum oxide abrasive in a waxy carrier is fairly well regarded among people who have used it on their straight razors. The product is usually described by retailers as being “10,000 grit” (abrasive particles in the vicinity of one micron). When analyzed by SEM, the abrasive is clearly much coarser than 10k grit – there were many particles in the ten micron range and I found one as large as 100 micron.
Despite being far coarser than advertised, I applied the product to a hanging strip of denim and worked it into the fabric with the spine of a razor. The razor was honed on a progression of Shapton glass plates to 16k (producing a triangular bevel) and then stropped for 30 laps on the loaded denim. This razor has a 16.5 degree innate bevel angle and after stropping, the angle measured 20 microns from the apex has increased just one degree. This increase in angle is a result of a very slight bevel convexity resulting from the use of a hanging strop.
The induced micro-convexity is evident where the angle is measured at 3 microns from the apex and observed to increase by 8 degrees from the innate angle. Micro-convexity is produced independently of the larger scale bevel convexity. The apex is relatively clean with minimal burring and keenness sufficient for shaving.
When comparing the effect of different abrasives, we need to minimize the impact of other variables. We can use the same strop material, and the same length and number of strokes; however, the downward pressure is a challenging variable to control. The three images below were presented in Simple Straight Razor Honing and demonstrate that downward pressure plays a determining role in the amount of micro-convexity produced on a particular strop.
In the experiments that follow, I have made every effort to maintain the pressure in the normal range (100-150 gram equivalent), and repeated the tests with different razors to confirm the observed trends are repeatable.
Over the past two years, I have tested a variety of other metal polishes on denim strops (Mother’s, Flitz, Simichrome, Wenol, Pikal to name a few) and all have performed and analyzed similarly. The example below shows a blade honed with the Shapton 16k and then stropped 30 laps on a Mother’s Mag polish loaded strip of denim. The result is nearly indistinguishable from that produced by the TI white razor paste.
The are two obvious questions that arise from the results above. First, why is the result so insensitive to the choice of stropping compound? and second, how is such a keen edge produced by such coarse abrasive? To provide some insight, I compared the effect of denim strops loaded with diamond abrasive ranging from 0.1 microns up to 15 microns.
Prior to performing this particular experiment, I had two experiences with very surprising results. The first involved the use of the Thiers Issard razor paste. After using the Thiers Issard pasted strop for 30 or 40 razors, the strop had become glazed and seemed to be less effective than when new. Rather than making a new strop, which is now my habit, I “re-loaded” that strop by rubbing the abrasive “crayon” lightly over the stropping area and working it in with the spine of a razor, as usual, and then stropped a C135 steel Thiers Issard razor, one that I have used regularly for several years. After 30 laps on the TI-pasted denim, I proceeded to strop the razor on the 0.25 diamond sprayed leather and the stropping both felt and sounded rough. Inspection with a loupe and feeling with my thumbpad revealed that the edge of the edge had been broken away at a relatively coarse level. So much damage had been done that reseting the bevel required a substantial amount of steel to be removed, as much as is usually required when restoring a vintage razor. SEM imaging had shown that the paste contains particles of up to 100 microns, and these are more than capable of destroying the edge of a straight razor, but had not done so prior to re-loading the strop.
The second surprising result came when I began testing coarse diamond loaded strops. I purchased a set of inexpensive diamond pastes (the ones that cost one dollar each, shipped from China). I analyzed several of them by SEM and found that the size distribution was excellent, in fact better than some products that had cost me 50 times as much.
I loaded a strip of denim with the 10 micron diamond paste and stropped a razor honed to the 8k level, and was astonished to find that the strop had essentially no effect.
It is challenging to quantify the concentration of diamond in a product, particularly in an oil-based paste. The obvious hypothesis is that this product may have good size distribution, but low concentration. Moving forward, I acquired a set of “scientific” quality, high concentration, poly-diamond sprays with sizes ranging from 100nm to 15 microns.
In the set of images that follow, a razor was honed with Shapton glass stones up to the 16k level to produce a keen triangular apex, one that would easily demonstrate the effect of stropping – the induced micro-convexity and any effect on the apex. The razor was then stropped, 30 laps, on a strip of hanging denim loaded with the particular sized diamond abrasive. As a control, the effect of clean (no diamond) denim is also shown.
In the subsequent images, stropping with progressively coarser poly-diamond spray is characterized.
The most striking observation that can be made of this series of images is that, with the exception of the coarsest (15 micron) diamond abrasive, the amount of micro-convexity produced is insensitive to the abrasive size. There is greater apex damage with the coarsest abrasives, but not an increase in micro-convexity.
Consistent with the earlier observation with the inexpensive diamond paste, the 15 micron diamond sprayed strop produced no micro-convexity. Apex uniformity decreases with the coarsest abrasives, but at much smaller scale than the abrasive size. There is no unevenness or “micro-serrations” resulting from scratches of depth comparable to the grit size.
Observation with optical microscopy (not shown) indicates that the larger abrasives fall into the weave of the fabric and therefore do not make contact with the blade during stropping, other than glancing collisions that chip the apex at the submicron scale. The same effect was observed with diamond-spray loaded paper (also not shown). In this context, the result of the re-loaded strop described earlier can be understood. The waxy carrier fills the spaces in the fabric, preventing the newly applied larger abrasive particles from entering the weave, leaving them exposed to the blade resulting in the substantial blade damage that was observed.
None of the diamond sprays produced an increase in near-apex angle as large as is typically observed with the various metal polishes. This is most likely attributable to the oily and/or waxy carrier holding the abrasive in place and presenting it to the metal more efficiently than the water based diamond sprays.