Honing on lather

What's to say? It's something I don't regularly do, but on a few occasions, where I just kept struggling in an attempt to finish an edge to the keenness level I desire, I've had some success with it. But I never ran a carefully setup series of experiments to determine whether lathered honing offers any real advantages on a Coticule. Quite frankly, I don't think I ever will conduct such test. Lathered honing seems dependable too much on the "draw" of the particular Coticule, which on its own is influenced by the bevel width of the razor and its grind. Instead of looking at it as something with a steady outcome, it's more a strategy to keep in your honing arsenal, that might allow you to prevail when other strategies have abandoned you.
For me, all this starts with the hanging hair test (HHT), source of ongoing controversy, that I find so indispensable for my honing. As you all know, when honing on a Coticule the sole goal you need to worry about is getting the edge as keen as possible. Other edge qualities come automatically with it. Keenness keeps improving all the way till the very last honing efforts. At the final honing stage this evolution that can be readily  monitored with the HHT. There is a pretty solid correlation between HHT-results (varying from 0 to 5, according to our official HHT-guide) and the performance of an edge honed on a Coticule. When finishing, it often suffices to follow standard proceedings in order to obtain the desired results. Nevertheless, there are those times when the edge ends up close, but not quite there yet. When that happens, it's time to try something different. Something that succeeds where standard methods cease to deliver. Honing on lather is one of those alternative options.

Barbers did it.

Allegedly, in the heydays of barbershops, barbers put lather on top of there hones for touching up and finishing the edges of their razors. I suspect the practice started out on the so-called barberhones, likely to reduce the speed and aggression of these hones to some extent. I can't imagine the urgent need for that on a Coticule, but lather, being readily available in the barbershop, was likely to find its way to the honing surface anyway. It smells good, and adds a nice draw to the honing stroke. In general, people are more inclined to add to a procedure instead of leaving something out, certainly when the addition has that "it won't hurt to try"- appeal.

But what could it possibly do?

In absence of trued empirical data, we can only theorize.
It seems logical that lather covers the abrasive particles of a hone with a cushion, thereby reducing contact friction and abrasive efficiency. While that may at first glance sound like a bad thing, it isn't necessarily so. Near the very end of the sharpening process, where we aim to refine an already very refined edge as keenly as possible, it pays to have a honing action that cuts as gentle as possible. For those who would like to observe this effect to its fullest, set a bevel on a DMT-E (1200 grit), put on a coat of thick lather and finish with 50 of your lightest laps. Strop and test shave. You'll be surprised.

Nonetheless, I don't think this first principle applies to Coticules much. They are already quite slow when used with water only, and I consider it unlikely that we should expect any advantage of slowing them down even further.

But a second principle, almost contradictory to the first, might help us more. On razors that carry a wide, singular bevel, suction between the bevel face and the smooth honing surface can become a significant factor. The air seal that creates this suction can show serious fluctuations during the honing stroke, due to interference at the very edge, while it skims over the surface of the Coticule. The steel being so thin at that region, can easily start to flutter up and down, as it tries to cope with the forces implied by the fluctuations in the air seal. Granted, this may at first seem far fetched and purely speculative, but clear evidence of something like this taking place can be readily seen at the picture of a magnified edge shown left. A small strip near the very edge carries all kinds of stray little scratches.

I have only witnessed this on wider bevels, and when it happens, the edge will stubbornly refuse to gain that highly desired final bit of keenness. Diverting to a Unicot procedure will solve the issue, as the layer of tape added to the spine, will increase the bevel angle sufficiently to break the seal of the suction and literally "lift" the interference. But we were talking about lather. A nice creamy lather can alter the circumstances enough to effectively dampen the interference. While you're at it, reversed honing, with the edge trailing, can have similar benefits, so there we have another valid strategy for tacking these kinds of finishing issues. It can be done on water, on lather or even on a dry surface. Whatever gets the job done.
In any case, these kind of air-seal issues are highly influenced by the width of the bevel, but even more by the intrinsic properties of each Coticule. Some Coticules exert draw easily, others not at all. The suction by itself can be a blessing, when interference remains absent, or a cause of frustration when the air seal provokes fluttering. Because of this unpredictable behavior, it makes little sense to run generalizing experiments and distill solid recommendations.

At the end of it all, honing on lather is just one of those options you can store at the back of your brain, for those times when an edges refuses to bow to your willpower, and your pride keeps you from searching that pasted stop, stored underneath all the other more or less usefull items that somehow keep accumulating in your honing drawer.