Creating A Mold For Cob

I'm wanting to start experimenting with some of all these things I've been reading. I'd like to create a rocket stove using cob. Nothing permanent I just want to kill 2 birds with one stone by building a rocket stove and experimenting with cob. I'm wanting to make it a square pillar type structure 14 inches by 14 inches and 16 inches high. I want to build a box-like mold to form it. When I fill the mold with the cob and the cob dries, will the wood mold damage the cob as I'm removing it?

Hello Brandon. I'll first say that I'm not sure if it will or not. But, it might shrink when it dries and pull itself away from the wood on it's own...maybe. I suppose what you could try is lining the inside of the box. Maybe with plastic wrap or foil? I don't think I'd use oil on the wood as it might have an affect on the cob's surface, and therefor, give you a false reading. Anyway, these are just some quick thoughts regarding your question.

Yes, you can mold cob. Many methods from standard molding to "slip form" modalities. Too lengthy a process to really get into on this post. You may find older post here at Permies with ideas. Empirical experimentation on your part will teach you more than any of us could in ten pages. Read, research, test, and find some local folk "mess'in with cob. Also research related methodologies in "earth architecture," such as: Clay-Cob-Adobe-Bousillage-bajareque Architecture, just to name a few traditional approaches. Human have been playing in clay (and poo) for about a million years...your own blood and bones has as much to teach you than any of use...you just have to learn to listen to the whispers around you...

Good luck, take picture (we like pictures) and you will get plenty of "feed back" from this "peanut gallery."

Regards,

j. (volunteer peanut)

Make the inner mold "sacrificial." Use wood and/or cardboard and burn it out.

When you go about filling a mold with cob, chances are pretty good that you will cross over into making "rammed earth" as distinguished from "cob". It's not a massive distinction, in that a mix that will work for cob will almost certainly work for rammed earth (the inverse is not quite as true). Thing is, once you start pushing your material down into that mold, you are no longer "cobbing" so much as "ramming".

And I can tell you that having built a wooden box, with a wooden sacrificial burn tube mold that went in the box, and loaded it up with my clay/sand/perlite mix (I was going for straight rammed earth with this, so no straw as you would plan for cob, packed that stuff down in there to be sure it filled all the gaps and so on, once it had a couple of days to dry, so that the exposed top surface looked to be dry, I had no problems removing the wood from the earth mixture.

Your mileage may vary, of course, but here's one report of doing pretty much what you are talking about, with no problems removing the mold.

My problems have come later, as my material is short on clay and crumbling much too easily.

Thanks!

Peter Ellis wrote:When you go about filling a mold with cob, chances are pretty good that you will cross over into making "rammed earth" as distinguished from "cob". It's not a massive distinction, in that a mix that will work for cob will almost certainly work for rammed earth (the inverse is not quite as true). Thing is, once you start pushing your material down into that mold, you are no longer "cobbing" so much as "ramming".

And I can tell you that having built a wooden box, with a wooden sacrificial burn tube mold that went in the box, and loaded it up with my clay/sand/perlite mix (I was going for straight rammed earth with this, so no straw as you would plan for cob, packed that stuff down in there to be sure it filled all the gaps and so on, once it had a couple of days to dry, so that the exposed top surface looked to be dry, I had no problems removing the wood from the earth mixture.

Your mileage may vary, of course, but here's one report of doing pretty much what you are talking about, with no problems removing the mold.

My problems have come later, as my material is short on clay and crumbling much too easily.

A quick follow up question: The recipe I am looking at is the typical sand/clay/straw mix - no perlite. Do you think this mixture will work ok for a rammed earth rocket stove?

I think that all the folks with way more experience than I have seem to tell us that we need insulation around the burn tunnel. If I understand that correctly, then I think more insulation is needed than you get with a standard cob recipe.

Peter Ellis wrote:I think that all the folks with way more experience than I have seem to tell us that we need insulation around the burn tunnel. If I understand that correctly, then I think more insulation is needed than you get with a standard cob recipe.

Do you know if the insulation's purpose is to prevent cracking due to overheating or just to make the fire burn hotter?

The insulation is to keep heat in so the fire runs hotter but it also helps the sometimes exposed areas of the feed tube not get dangerously hot.

Well I did my little cob project this weekend. I would have taken a picture but I was super muddy and didn't want to mess up my camera. One thing I can say for sure is that little stove is freakin' heavy! It's only 14"x14"x14" and I could barely lift it above my knees. Thanks for all the help here!