by mannytheseacow » Thu Jan 23, 2014 1:12 pm
Welcome, alex52e. I think it's by volume. 1:1. At least that's how I built mine. Also see Colin's tiny bell RMH thread where he gets pretty explicit about his mix. I've read that vermiculite adds strength to the mix while perlite weakens it, but I can't say from experience. I don't think you'll want to use the gardening sources. Find a brick or fireplace shop that will have it specifically fire fire insulation purposes. You might even try the big box hardware stores. The vermiculite I used was extremely fine black material that also very light weight and says zonolite on the label.
"Knowledge is power. Arm yourself."