The primary goal of stropping is to reduce the edge width by increasing the bevel angle near the apex (micro-convexity). Abrasive particles (paste, spray, etc) are applied to the strop to increase the rate of abrasion. The edge geometry that is achieved will depend on the type and size of abrasive, the strop material and the pressure and number of stropping laps applied. Although simple in principle, consistent results can be difficult to achieve due to the number of variables and their effects.
Abrasive stropping differs from conventional edge-leading honing in two ways; the trailing edge direction and the compressible nature of the substrate. Sharpening on a hard hone or stone in the edge-trailing direction provides the simplest case of pasted stropping; where compressibility does not play a role. As a starting point, I will discuss a comparison of edge-leading and edge-trailing honing.
When honing a blade in the edge-leading direction, steel is removed in two ways; abrasion of the bevel surface and the ‘breaking’ or ‘micro-chipping’ away of the apex. This micro-chipping process prevents the formation of a burr, but also limits the level of keenness that is achieved. In the edge-trailing stroke, the occurrence of micro-chipping is reduced to the point where a burr or foil will almost inevitably be formed. Two examples are shown below; honing on the Chosera 1k and on the Shapton Glass 16k. In both cases, the blade was honed normally with edge-leading strokes and then given 20 pairs of edge-trailing strokes approximately 10cm long.
Comparing edge-on view images, at the same magnification, shows the dramatic difference between edge leading and edge-trailing strokes.
The cross-section images confirm the formation of a foil edge in the edge-trailing case.
A comparison of the cross-sections confirms the formation of a burr in the edge-trailing strokes on the Shapton 16k also.
In summary, edge-trailing strokes can produce a keener edge than edge-leading; however, they generally result in the formation of an undesirable burr or foil.
In part 2 of The Pasted Strop the effect of a compressible substrate will be discussed.