small cast cob core experiment

i have been playing with my local clay for a good cob mixture. mixing and making making bricks, with different ratios of sand. i then firing them in my fire pit, to see who stood up. i settled on what seems to be a pretty strong mix, so i cast a core. its a little 4" square core made from that sign board stuff(to be burned out at start up). the thickest part of this casting is about 5" thick. its been setting for 2 days and is still wet. my question is how long should i let it dry before firing it up and baking it hard? i figured it would crack if i fired it too soon. not a lot of work or time in it but dont want ruin rite off either. still cool and damp here 50s for highs.

thanks Ron

Ronald; Did you add any perlite to the mix when making your core? How about refractory cement ? The perlite acts as an insulator making the heat go up your riser instead of heating your core up. The refractory adds strength to the whole mix, also fireclay would perform better than regular clay. My next cast core this summer will be made using all refactory with some perlite added for insulation. As to your question, don't build a roaring fire rite away. A slower heat up should resist cracking.

yes, i added perlite, straw, sand, and clay to the mix. no refractory because this is a freebie experiment and that costs too much. i used my local clay because it was free and bricks used to be made from it, at the end of my road. it is the yellow type of clay and fires pretty hard from my experiments in the fire pit. oh yeah, i wrapped some fiberglass mesh tape around it, for extra strength, before adding the outer layers of cob.


slow heat eh, so should i bake the thing by my fire pit before actually starting a fire in it?

thanks for the reply

well, i couldn't wait any longer. lol i fired it up today and ran it twice. no big compromising cracks so far. it wasn't even near dry but seemed to hold up fine. the second burn was a lot hotter than the first burn. for a 4" tube it gets pretty hot! the wood was split 2xs of pine and some yard sticks. been 30min since the fire went out and you still cant lay hands on the riser part for long. thats impressive heat retention for just the 40#s of core mass. think ill cast another one and fire it off sooner and see what happens to that. man these things are almost as fun as fishin. almost lol

Hi Ron; Yes , these stoves are fun! I have been obsessed since i first heard about them. I tell anybody i meet all about them and how cool they are ! I'm sure most of them think i'm some kind of crackpot. My cast core is an 8" and it is apx 4" thick on all sides built mainly out of fireclay & perlite with some refractory added (yes its very costly) I built it with a plywood outer form and let it air dry for a few days and then wheeled it into the backyard and fired it off. It took a few firings before it started to hold heat , my chimney for that was simply a piece of reg stove pipe set on top. After the core was fired (still in its plywood box) it was moved into place in the green house and stripped the outer form once it was in place. My riser is also a casting done using fireclay & perlite ,with a 16 gal grease barrel for an outer form and a piece of 8" cardboard concrete form (sonitube) as the inner form. This worked perfect! I built it in place on top of the cast core, the cardboard tube burned out with the first fire leaving a smooth bore insulated heat riser , that has lasted all season with no issues at all ! I wish I could say the same about the core. I have had problems with wear on my feed tube side that i patched several times this winter including using water glass to harden the surface.Not sure why i'm having problems with the core casting most likely i didn't mix as well as i thought. I will be replacing the core this summer and I will use all refractory (despite the cost)with some perlite added for insinuative value. I hope to be able to lift off the riser and set it back when the new core is in place but not sure if it will handle moving or not. Have fun Tom