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Inherited cob oven with new house...major issues..  RSS feed

 
Brian LeDuc
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Hey everyone, I recently purchased a new house that came with a cob oven.  When I started cleaning it out I realized it wasnt ash that it was full of, but rather the interior walls of the oven?  The consistency is like wet quickcrete sand tube sand--coarse, dense, but far from "brick" or "Hard Clay" Like.  Upon further inspection, the interior walls just feel like actual eart hand not hard at all.  They also seem to have fallen away from the outher hard cob layer and could be easily pulled out in dense chunks of material.  Am I screwed?  does this oven need to be wrecked and rebuilt?  Something is definitely not right.  Hell it even looks like there may be insects that have burrowed in the mud like interior walls.  I fired it up to see if anything would harden, and it really didnt.  This layare is about 4" thick from what I can tell.  Thanks in advance for any help and advice!!!
 
Glenn Herbert
gardener
Posts: 2180
Location: Upstate NY, zone 5
74
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Is this cob oven indoors or out? What is the climate like?
It sounds like the inner layer is what I have read about as a sand-rich "thermal cob" layer, and if formulas were followed blindly with inferior clay to start, there might not be enough clay in the mix to stand up to use. There probably is no reasonable way to refurbish the inside of this oven, as thermal cob has no fiber (which would make it lighter and more insulating as well as stronger).

The only possible but likely not practical way I can think of would be to cut a hole in the top of the oven so you could rebuild a core form (a mound with the inner space shape you want) and pack new thermal cob tightly inside between form and existing outer cob layers. I wouldn't recommend this but it might work.

Cob is completely recyclable, so you can demolish the failing parts of the oven and rebuild the dome with only labor and little new material required. I recommend Kiko Denzer's book Build Your Own Earth Oven.
 
Brian LeDuc
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Glenn-

Thanks for the reply and extra info.  The oven is outside, in Vermont, so its humid and the weather is rapidly changing.  The material itself is rather granular and not "clay" like at all.  Yes, I agree the rebuild would be quite difficult.  There is and area on the top where the maker left an indentation--not sure if they debated a chimney and left it there for an easy addition or what.  I'm thinking my fix will be:  Recycle material from the inside, add more clay to give it better hardening and adhesion.  make up the cob clumps and rebuild from the door.  Insert a large inflatable exercise ball and inflat with compressor to support from the inside(like revers paper mache).  Deflate after a few days and fire it up.  I will also patch the crack in the outside shell at the same time.  What do you think?  Attached are some pics for reference.  Thanks again!
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Brian LeDuc
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Ps.  that is my 15 month old sons hand not mine..and yes, that is a giant bush of poison parsnip to the left....  it's on the list to take care of,
 
Glenn Herbert
gardener
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Location: Upstate NY, zone 5
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Nice looking oven and base... but an unroofed cob structure in Vermont is not a long-term solution. No matter how well you waterproof the exterior, you will get some moisture inside that will freeze and degrade the dome. If you want the oven to stay, you will need a small shed roof over it. It doesn't need to be big, just enough to keep rain off and allow ventilation.

Rebuilding the dome as you describe sounds feasible. A traditional cob oven where the fire is built inside the dome does not use a chimney at the top of the dome; that would let the heat out too fast and bring lots of cooling air in at the door. The earth oven book gives good proportions for dome and door, which need to be followed for best function. An inflatable ball would unfortunately not give the best shape for an oven dome. What you would want is more like a half-ball on a flat base. Building up a temporary form of damp sand (over a core of rocks, bricks or wood for speed) would be the best method.
 
Brian LeDuc
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Thanks!  Yeay, I agree with the roof addition.  I'll put together a nice post and beam after fixing it up by fall.

As far as the ball goes, I was thinking if I had a large enough exercise ball, the bottom half would stay flat, and in essence I would really only be using half the ball itself for the shape--I completely understand what you are saying though and it may be a wast of time. 

I'm going to order the suggested book today and peruse for some details for the rebuild.

Cheers!
 
Desiree Fleck
Posts: 11
Location: Ozarks, Missouri
goat tiny house toxin-ectomy
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Great advice here already, probably not much left to say. We recently did a cob oven build using a sand form with cinder block and brick core for the dome shape. It worked beautifully. We have our ovens under a roof for protection. One of ours has a stovepipe and the other does not. The one without holds heat much better, but it's also much bigger. The one with a stovepipe, we are able to close the flu to hold in heat, but it's so small that we have to keep stoking the fire during use. That's one reason we built a larger1 one. Good luck to you and enjoy!!
 
Brian LeDuc
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Thanks again everyone.  so as far as the cob being reusable--is this true for the outer hard layer too?  If I decide to just rebuild I can knock it down and soak in water and reuse?  Thanks in advance!
 
Glenn Herbert
gardener
Posts: 2180
Location: Upstate NY, zone 5
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As long as it is cob with no cement or other "setting" additives, yes you can recycle it. If the outer shell has withstood rain and snow for any amount of time, it may not be straight cob.
 
Ardilla Esch
Posts: 225
Location: Northern New Mexico, Zone 5b
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Brian LeDuc wrote:
Thanks again everyone.  so as far as the cob being reusable--is this true for the outer hard layer too?  If I decide to just rebuild I can knock it down and soak in water and reuse?  Thanks in advance!


There was clearly a problem with the mix used that caused it to spall so badly.  I wouldn't just rehydrate the material and re-use.  You can probably re-use the material after making proper adjustments like adding clay or sand as appropriate.
 
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