What is pre-dulling a razor? What is the purpose of it?
a razors edge on glass is performed by lightly dragging the edge of the
blade once over a glass object such as the rim of a drinking glass, or
neck of a beer bottle, using only the weight of the blade and no
pressure. Afterwards the razor should no longer shave arm hair; if it
still does a second, equally light stroke may be necessary.
An edge can't develop keenness on a hone before both sides of the cutting bevel are completely flat all the way up to very edge.
It is of equal importance that the edge is free from the damage that accumulates at a microscopic level from the impact with coarse beard hairs.
By dulling the edge, and then setting the bevel until the razor shaves arm hairs, we can be completely confident that the sides of the bevel are flat, and the action of dulling on glass can also help with the removal of damage caused from the impact with coarse beard hairs.
Pre-dulling is also recommended when honing newly fabricated razors, because the bevels on these razors aren’t always as one might expect.
Also, when a bevel has been set on a synthetic hone, it can be useful to re-dull the edge before starting to work on a coticule. The reason for this is that synthetic hones, up to 2K grit, leave a saw-tooth pattern on the edge of the razor and leaving this edge as it is might not result in the typically smooth coticule edge.
Other than that, there is no special voodoo to the practice of pre-dulling a razor.
It renders a razor barely below shaving keenness. So barely that it can be stropped to shave again. But, if there is any convexity in the bevel, or if the razor was previously honed with tape, then the very edge will not regain a shred of keenness on a flat, solid hone before the bevel is entirely set. Things become easy after the edge has been pre-dulled. No second-guessing, no worrying. If the razor starts shaving arm hair again the bevel is ready for the next step of your honing procedure. It doesn't matter what bevel setter you use, or what the next hone or step in the procedure will be.
Last update on 2011-03-28 by Ralfy.