yep that they are however each has a specific insulation value. pumice being the lowest and perlite being the highest. in order to get the insulation values you have to use more materials. so you end up spending just as much to get 4 times the pumice as you would for a two 2 cubic foot of perlite.
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So are you saying to replace the "clay-stabilized perlite" as the base of the heat riser per your book, I have to use an 8 inch layer or pumice stone instead of a 2 inch one?
I have alot of light weight white round pumice stone to be used in a soil mixture (not dense crushed black or red landscape lava rock as the previous questioner). It seems very similar to perlite still probably more dense as you say.
Thanks if you can confirm, and thanks for your book,
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