Another question about combo coticules!

I picked up both of these at an auction and wanted to get some opinions on them. The stone on the right is a natural combination. The garnet matrix is pretty obviously visible. The left stone appears to be a glued combination (though I didn't show the sides). When lapped, the bottom seems to be a purplish color, as all other Belgian stones I have worked with have been. This leads me to believe that the top must be coticule, but, even under 10x magnification, I can't find the garnet matrix in it. They are both about the same size and weight, and both velvety smooth. Let me know if you guys can tell anything from the pictures, or if you have any other questions about them.

hmm... I'm not sure the right one is a natural combination of a Coticule and a Belgian Blue Whetstone (BBW). They both look as if they are backed up with normal slate. The right one could be bonded naturally to the slate, the left one could be glued. The natural combo's they sell nowadays are naturally bonded with BBW. Of course, it's hard to tell from looking at a picture only. There's an easy test however: a BBW produces a purplish slurry when rubbed with another piece of BBW (or coticule, or even sandpaper) If it's just slate, it won't produce slurry.
Some say that paler coticules are of higher quality, but I have not find any performance or result differences between the three coticules I 've experience with, and they varie from almost orange to nearly white.
I don't doubt that the right top is citicule, of the left, I'm not sure.
Does it produce a slurry?

Nice find,

Bart.

Yes, they both produce slurry while lapping. The big dots you see on the right BBW are the garnets in the matrix. Probably the biggest I have seen!

I would think you would want very tiny garnets if you want a fine finishing stone. If the garnets are large you will have a very coarse stone almost like using rough garnet paper for sanding. Most fine coticules have garnets so fine you would need alot of magnification to actually see them.

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Originally Posted by Bart

Some say that paler coticules are of higher quality, but I have not find any performance or result differences between the three coticules I 've experience with, and they varie from almost orange to nearly white.

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I agree with this. Though I've seen some variation in the quality of the (seven or so) coticules I've had, that variation has not been predictable by the look of the stone. The feel of the stone on the other hand....I have found that my softer coticules are finer.

Howard says the selling of light homogeneous yellow coticules as "extra extra" grade for a premium is a gimmick, and I've come round to sharing that opinion.