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May I ask you the following question, since you are the author of that FAQ? (Well, there will be a question in the end, pardon the rhetorical questions along the way.)
When the razor is flat on a hone after a successful sharpening session, we agree that the entire edge (the very tip) rests on the surface of the hone, right?
After that, damages from shaving, stropping, etc. happen. You want to resharpen. You bring the razor and lay it flat on the hone. How far do you estimate is the edge removed from the surface on the average - a lot, a little?
The reason I am asking is the following. By my reckoning, convexed or concaved bevel does not matter as much as how far the edge has receded (= bevel has become thinner). If the edge has receded significantly, it may require quite a bit of honing before edge hits the surface. However, I never see visibly (not Veerhoven-precise, but estimated from 45X scopes) significantly thinner bevels before I need to resharpen. And, in few half strokes, the edge is usually on the hone's surface. Once on the surface, the abrasion it is subjected to is much fiercer than a mild stroke on glass. That is one thing.
Another claim about drawing over glass is that it will get rid of "bad" metal at the edge. My sense is that a proper hone directing highly abrasive material toward the edge, although at an angle, is quite capable of doing the same thing much better.
Bottom line: I can see that a highly receded (from diamond pasted strops, dished hones and the like) edge can possibly mislead a novice who might test the edge without paying attention to proper markers on the hone (such as undercutting slurry, etc). I suspect that would be a very small fraction of cases today. Historically though that would have been more common. Witness all the dished hones. And, we see all those scratch marks on the side of those hones too.
So, what do you think, do we really, really need this drawing on glass, or is it a ritual from the past? However inadequately I may have articulated this, Ralfy, I am more trying to understand the idea, and not at all trying to contest the possible usefulness of your FAQ.
I may be getting this all wrong. Or, sharpening too early. Let me know what you think.
You are wrong . Just because you have successfully sharpened a razor doesn't mean the edge of the razor will rest flat on the hone when laid upon it. Parts of the edge certainly will, but the razor's edge will no more be perfectly flat than the stone will be perfectly flat. So, no to that.
Furthermore, the downstroke on glass is nothing more than a technique to eliminate "false positives" along the way. For example: you start honing a new razor and you don't know the history. Say you start honing it, and you test the SAH test and it works perfectly. However, despite your best efforts, you can't "finish" it. It well could be that your razor had been honed with a layer of tape previously and you haven't even begun to affect the true shaving edge yet. After employing the down stroke, you don't have to question it. Once it starts shaving arm hair again, you definitively know the bevel is freshly cut and ready for refinement.
No, it's not "necessary", but it also doesn't do any appreciable harm. If you take a razor that passes HHT, dull it with one stroke on glas, give it a full stropping session, you'll be stunned at what happens when measured on on HHT...
The FAQ article is a product of our hardworking FAQ-team. This crew is actively searching through all the old threads, looking for frequently reoccurring questions and compiling an answer from the replies. The team is led by our good dr Ralfson, and he "posted" the end result of this particular FAQ-entry. That's how his name ended up underneath the text (we'll correct this later, to make it reflect that the answer is distilled of the forum)
I think the text is very feature complete, and explains the reasons for pre-dulling very well. if you only sharpen your own razors, you will know the state of your razors upon rehoning them, and you may find no advantage in this practice. But is you're like me, sharpening razors you don't know anything about, it is a very useful practice. More than half the razors I receive for sharpening, have enough convexity in their bevel, to make the amount of work required for bevel correction unpredictable. (One of these days, I'll shoot a picture with the microscope, to illustrate such a case, and add it to the FAQ-article). I find my pre-dulling "ritual" very useful. And I also find it useful, in a situation where we are trying to talk a member through a honing job in a thread he started, that we can confirm the state of his bevel. If he says he pre-dulled and can shave arm hair, we know that his bevel is good.
As Paul already pointed out, even if one dulls on glass without necessity, it is not going to introduce any extra challenges for honing, and hardly any extra time (maybe 60 seconds, for a slow person). I recommend pre-dulling to anyone who ever wonders: "Was my bevel good enough to continue with the rest of my honing efforts?". People who never struggle with that question won't need the pre-dulling approach. Or they are already using it.
Indeed the Credit belongs to the team, of which I am a mere small part, I must just echo the point that establishing a sound bevel, is absolutely vital when it comes to successfully honing any razor, without that firm foundation there is no way you can be sure to get consistently good results.
I think I understand this better now. The exchange is emblematic of the burdens/difficulties in communicating 'technical' ideas purely with words, no pictures, and without intermediate back and forth clarifications.
I tried to create some graphics, but gave up after it started taking too much time.