In my experience the dragon's tongue is more coarse than the purple slate.
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Agree the purple I have is 10k and can produce a nice edge the Dragons Tongue is much coarser. Some do get the DT and Welsh Black confused due to coloration. The DT can be darker than the "Black". Are you sure you do not have a DT instead of a Black? You may want to post a pic so AJ can have a look see.
Even if aj labelled my black stones wrong they still arnt near the labelled grit rating. Especially the purple slate which seems the least fine of the 3.
Like I said I still enjoy using them as mid range stones, I just can't get an edge I'm happy with off of them. And out of the 3 the one labelled 8k seems to give the best edge.
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Is the vendor labeling them with grit ratings?
Mine have 8k scribed on the DT 12k on the purple and 15k on the black.
I know there is no point assigning grit ratings to natural stones but it would help you knowing where to fit the stones in to your progression. If the ratings were accurate.
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I've had good luck using AJ's purple slate after a coticule. In my case, the purple slate is lapped with a worn coarse DMT. For use, a light slurry is made with a fine DMT to start, before switching to just water. I've also had nice edges from an Inigo Jones dragon's tongue lapped and used in a similar fashion, followed by a red-pasted linen strop. I haven't tried the "Welsh Thuringian" as I haven't needed to; but my understanding is that the progression is DT > purple > WT in terms of increasing fineness.
Wish there were something I could add to help, but I'm not sure what I'm doing different to get good edges off mine.
Set the bevel on a 1K stone. Then I work a blade on the Dragon's Tongue until the stria are gone. By this point it should look to be around 8K level of polish.
Beyond that there's really no need to mess with slurry, just move on to the purple, then black slates with pure water.
You can completely skip the slurry phase by working your edge up on a synthetic 8K of course. And like most naturals, I find a burnish is better. My purple and Black slates both give really good results.
Rob, 20 laps and test. What test do you use? If you did 150 laps would you have overhoned? Wasted time or steel? I'm not sure when I should be done, so I usually over do it...
personally ready to give up on natural hones. way beyond my level of expertise or interest. Lesson learned.