I couldn't find any, and am cheap, so I said "screw this malarkey", and I jimmied up a substitute. By volume : - 50% waste straw, lawn clippings, or hay. This is not only air insulation, it's rebar to the mix. - 20% bentonite clay. It's literally cat liter. I put a bunch in a 5 gallon bucket, added water, and waited 3 or 5 days. Bam, instant-ish clay putty. - 20% sand. I dug mine out by a van, down by the river. Seriously, it flooded, and I got a bunch of rando sand. - 10% ash. super insulation. Can't burn what's already burnt. I was using cigarette ashes, wood ash, anything will do. Wear gloves, because caustic lye is formed when ash touches water, and it will disolve your skin. I got a chemical burn from this once. I rammed the clay with the sand, added ash, rammed it all into a wad. They I broke the wad up into bits, added compressed straw equal to the volume of the wad. Just eyeball it. If it's off by 2% your house will burn down, so no big deal (that's a joke, it's really not critical at all and is impossible to standardize in practise). The resulting glop was like a hair wad pulled out of a shower drain, but it worked like frozen peanut butter... with hair in it. I made a coil like structure like Caddo Indian pottery, and made a furnace this way. The bottom of the inside of the furnace was the same stuff, just as a cow patty type plug. I've made bricks, liners, all with this recipe. Let it still for 5 days to air dry, then fire it like a little camp fire (with no blower) and let the thing air coo. Repeat twice, and then use as normal. Whole ass furnace cost me $5, has lasted 6 months, and I've accidently melted steel once. Answer from crumpledcactus on reddit.com