Stropping a blade on a clean (without abrasive) substrate achieves FOUR results:
1) REALIGNING THE EDGE
Although a straight razor is made of hardened steel, the edge is flexible and malleable. Below is an example of a relatively large ‘ding’ in the edge. I received this blade from a custom razor maker with this defect and can only speculate as to the cause. The blade was stropped 50 laps on clean linen, 100 on clean horse leather and then 10 on clean linen (to remove residue from the leather). The same location on the blade was imaged, clearly showing the blades’ edge has been realigned.
Realignment more commonly occurs at much finer scale; the above series of images was chosen only for the clarity of showing the same location on the edge before and after stropping.
Burnishing refers to the movement of metal, distinct from abrasion and the removal of metal.
The burnishing effect is most clearly observed at an area of the edge that was previously chipped. This leaves a near square corner, and the easing of this corner occurs with metal pulled up towards the edge. Burnishing also occurs on the bevel face, smoothing asperities by plastic flow (this can be observed in the series of images shown below, where both abrasion and burnishing occur).
Abrasion (the removal of metal) occurs by two processes; nano-scale abrasion and edge chipping. The series of images presented below were taken from the same location on a razor (as witnessed by the x-shaped pattern in the top right of the images). The softening and loss of scratch definition occurs primarily by surface abrasion in the last 2 or 3 microns of the edge, increasing both keenness and sharpness.
Abrasion by micro-chipping is manifested primarily through the removal of weak or damaged metal at the edge and occurs more rapidly with stropping on linen than leather.
Manufactured razor blades are typically coated with a lubricating coating to reduce the force-to-cut and improve the apparent “sharpness.”
For SEM imaging, this coating blocks the view of the metal edge and I normally remove it prior to imaging stropped edges (as, for example, in the images above). It is, in fact, extremely difficult to remove. (In an optical microscope, these coating is transparent, although they may impart color due to interference effects). The effectiveness of this coating in lubricating the passage of the blade and thereby reducing the force-to-cut is yet to be determined.