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I´m very interested, what "undercutting slurry" exactly means. I still don´t get the point. I´d be very interested to see pictures of it, if that´s possible. This might be a great help for other Newbies, I guess.
You will see that on some parts of the blade the slurry covers the edge and there are some "clody" slurry puddles further up the blade. As you pull the plade over the hone, undercuting means that you scoop up the slurry and it flows up the blade towards the spine.
On the right side there is no undercutting. The blade just glides over the hone but the slurry stays below the edge as it passes.
shaved, I will agree with everything you say.
However, the undercutting really is helpful, beacause if the razor does not undercut, you can be damn sure that you're not even close to a positive HHT
But serious, I use it during the dilution phase every other dilution step or so. It will tell me quite reliably if I went too fast on the dilution or not. If so, I'll throw in another couple x-strokes until I'm there and then move on with the next dilution step.
The idea is that a keen edge will cut between the fluid and the surface of the hone. The thiner the coat of fluid, the more difficult this is for the edge. Only very keen edges can cut under a thin coat of fluid. Less keen edges, will run over the fluid, or they will need a thicker coat of fluid.
With proper experience, these kind of observations can tell you something about the keenness level of the edge.
You can also observe the contact point between the edge and the fluid in front of it, as guidance for your honing stroke. A good honing stroke first pushes the fluid in front of the heel, then in front of the middle part, and near the end of the stroke in front of the tip. That's a good stroke.