Even after screening and washing, there will still be some tiny particles that didn't get removed by either process that are suspended in the water that remains in the mix. When the water evaporates, it leaves the particles behind. Also, some of it could be dissolved mineral solids that were in the water you rinsed with. Nothing to be alarmed about

Al

I found that when I wash my turface the first few rinses would get rid of most of the dust, but after that there's always a bit of dust in the water, I think because it crumbles slightly as I mix it. Now I just fill the pot and pour water through until it comes out clear, seems to do the trick

Thanks for the info. I put my mix into the pot and sprayed it down one more time. Some more silt came out and then it went clear. Maybe I won't bother washing it when all the components are separate. It seems like an extra step I can do when i put it all together in a pot.

I also screen the ingredients, make the mix, pot or repot the plant, and water until what comes out of the drain hole is clear.

Al