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Coarse to medium. You're trying to create air space within the material to trap the heat, so powdery stuff isn't preferred.
Yes that's true, you don't want powdery, but the fine vermiculite makes a more workable mix although it takes more water. I doubt whether ther is any measurable difference between fine med or coarse in the final product, after all a bucket of small marbles weighs the same as a bucket of large marbles. The spaces maybe bigger but the volume of the spaces is the same. Try 50% perlite 50% vermiculite it makes a good workable mix with minimal cement needed to hold it together
Last edited by david s; 01-05-2013, 08:49 PM.