I now have 3 coticules one select , one kosher grade, and my newist vintage. i did 50 laps on each with slurry select and kosher seemed to cut the same only slightly turned creamy slurry a tiny bit grey whilst to my surprise mant the vintage which seemed the hardest turned slurry almost purple/dark grey cuts fairly quik. so now i no they do vary some are the same as each other but how do i tell which one polishes the best. I also noticed the tiny vintage when honing on water only down the edge of hone mainly in corners i had to wipe as a dirty substace very light was apearing which made me think this is actual removing visable metal with only plain water not a lot but i noticed it straight away, even though the surfice feel like glass its so smooth. never noticed this on my other yellows.