There is no substance that is completely fireproof. There are some chemicals that can be used on padding materials such as foam and cotton to make them fire resistant, but many of them are now out of favor, because they can cause cancer.

Bricks? Best use fire bricks. Vermiculite? Fiber glass or 'rock wool.' Acourse they both got itchy lil fibers and splinters that will work thru the upholstery. Mebbee you won't be sitting there long enuff to catch fire. You could install an impermiable inner liner, plastic maybe? That way you'll succumb to the Organic-petro-chemical fumes and before feel the flames. A 'safer' fireproofing is to soak cloth fibers in a boric acid solution, Whoops, it's fire RESISTANT not PROOF. *********************************************************************************************************** I'm replying to your comments here cause i got too verbose for the coment box. Imagine that. Vermiculite is a mineral that when heated expands and is realitivly lite for its volume, its used in some potting soils, its light and absorbs some water, looks like little shiny pieces of gravel reminds me of mica, also used as loose fill insulation, and as a cushioning packing material, like styro peanuts. These guys sell a "Extra coarse 'non-dusty'" Vermi, 4 cu.ft. $18.50 http://airseacontainers.com/prod01_UN%20boxes_04-Miscellaneous.htm It would be sorta lumpy, like a bean bag chair, as would your peanut husks. Bean bag reminds me of some that i have seen with tiny styrofoam balls. Perlite is another expanded mineral used in potting soil, looks like lil pieces of styrofoam, kinda rough and wouldn't slip past each other as much as some things so wouldn't be as cushiony. As with lil styro needs TIGHT seams or it works out. The thing about most 'foam' is that it gives off fumes, some deadly, as it burns or even before it actually flames. That's the thing that kills most people in fires involving upholstery, even if fumes don't kill they make ya pass out or just befuddle you and then fire gets you. You don't need or want to treat your stuff yourself. You can buy it alredy done better than you could do yourself. In US all stuff for baby beds and such has to be treated with fire retardants. May be ALL cushioning has to be. You shouldn't have any trouble buying treated materials at well stocked supplier.