Recently there was a thread on the Wicked Edge forum started by Josh (Razoredgeknives). He noted that stropping with 14 micron diamond paste on leather after the 1000 grit stones did not remove the scratches left by these stones. According to the grits comparison chart the 1000 grit stones are equivalent to 7 micron. The scratches made by the 14 micron diamond paste were supposed to be larger.
Tom (Jendeindustries) responded with a series of photographs in which he showed that the abrasive effect of the diamond paste on leather was far less than what one would expect on the basis of this chart: it took hundreds of stropping strokes for scratches to appear and even then they were much smaller than 14 micron. His pictures did however show the burnishing produced by the “stiction” of the leather, which one would expect when stropping with leather.
I wanted to see whether I could reproduce Tom’s results and to test a hypothesis that might explain it. Leather is quite textured and the diamonds from the paste might get hidden in the leather. This might cause the abrasion to be reduced.
Stropping on leather
I clamped a Laguiole knife made of 12C27 steel, with a hardness of 57-58 HRC, in the WEPS. I knew from previous experiences that this steel is relatively easy to sharpen and abrasive compounds should leave their marks quite soon. I reground the edge to 20 degrees per side using the 100 grit stones. I then further sharpened this edge using the 200, 400, 600, 800 and 1000 grit diamond stones. Below is the result at a magnification of 400x. I set a scratch pattern as horizontal as possible.
I then stropped the knife using 14 micron diamond compound on leather. I used up-down strokes almost perpendicular to the strokes I made using the 1000 grit stones, so the scratches should also be perpendicular to those made by the stones.
In this way I only stropped a patch of the edge (for efficiency and to make it easier to reproduce). So the number of strokes I mention in this post is the number of strokes per patch. It is at least the number of strokes you would need per side if you stropped the entire edge.
Below is the result after 50 stropping strokes. Some burnishing is already visible, as well as tiny scratches by the diamond paste. (As in the other pictures, there appear to be more vertical scratches on the right side of the picture than on the left side. This is not the case. It is an optical distortion caused by the lighting of the microscope.)
I continued stropping and below are the results.
After 500 stropping strokes the burnishing of the leather was clear. There were also many tiny vertical scratches, caused by the diamond paste. These were, however, not nearly as wide or as deep as one might expect from a 14 micron paste.
Stropping on balsa
I wanted to test whether the lack of abrasiveness of the diamond paste was due to the fact that the diamond particles got hidden in the leather. I therefore prepared two balsa strops with the 14 micron diamond paste. There is little space between the filaments of the balsa in which the diamond particles can hide. Balsa is used even for stropping at the submicron level.
I started stropping with the balsa loaded with 14 micron diamond paste. First the patch of the edge I stropped before stropping.
I did a total of 500 vertical stropping strokes.
We see that the loaded balsa strops, like the leather ones, resulted in a lot of tiny vertical scratches. The diamond paste on balsa may remove slightly more material than on leather, but after the balsa strops there are not many more vertical scratches, nor are these scratches wider or deeper than after the leather strops.
I wanted to answer two questions:
- Is it true that the abrasive effect of the 14 micron diamond paste on leather is far less than what one would expect on the basis of the grits comparison chart?
- If so, can this be explained by the diamond particles from the paste getting hidden in the leather?
The answer to the first question is definitely yes: the 14 micron paste causes only tiny scratches and the observations in this post confirm those by Josh and Tom.
The answer to the second question is no. The diamond paste on balsa seems about as abrasive as on leather. After the balsa strops there are not many more vertical scratches, nor are these scratches wider or deeper than after the leather strops.