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Retrofitting a cob oven

 
pollinator
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Hi,
I built a cob oven approximately a year ago, and the more it is used and the higher the temperatures, the more there is grit coming down onto whatever is cooking.
Also there are ever more cracks that expand and contract along with the goings up and down of temperature
We initially patched the cracks with more cob mix, nevertheless the same cracks keep opening each time the oven is fired.
So is there any way to prevent the grit from falling off the roof and the cracks from opening?
Thanks
 
gardener
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Hi Antonio,   It sounds to me like maybe your cob mix was not quite right. I don't know how much change you are willing to do to your oven but have a few suggestions. One could be to line the inside with dense firebrick which can easily handle the high temps and holds the heat quite well. I assume it would be a hard thing to do with your stove all built though.
Another idea that has worked well for me with my rocket mass heater is to coat the cob with linseed oil. Not sure how food safe it would be but it has definitely slowed any kind of deterioration down from within the inside of the stove for me. Perhaps another type of drying oil could work that's more suitable for working around food. If you did go this route, I would strongly suggest using the natural linseed oil (not double boiled which has the chemical drying agents in it) and for sure giving it a few firings to cook off any volatiles off.
 
Antonio Scotti
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Thanks Gerry
Yes the mix wasn't probably all the best it could have been, since I didn't have enough time to do appropriate tests....and probably not enough experience to correct it on the go...

As a last resort, I was thinking to demolish the higher part of the dome and redo it. I wonder if this can be a good idea in terms of insulation, in the way how the already build part would coalesce with the newly and fresher new top, and not let any heat escape in between the the old and new part., or if there is any risk that the new part wouldn't actually blend so well with the older part.
 
Gerry Parent
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I have done a lot of rebuilds on my Rocket Mass Heater where I had to join old dried cob with fresh cob. The bond is never as good as the original however, there are ways to help make the joint as strong as possible. 1) Make sure your dried cob gets thoroughly wet. Water it down several times and let it soak in. Just before applying the new cob, hit it again with a spray bottle.
2) A thin layer of clay slip acts like a glue of sorts to help increase the stickiness right at the joint.
3) Make sure that the joint is not smooth or straight. A rough surface provides much more surface area and bonds best.
4) Try to make your joint in an area that may not be noticeable as much. Perhaps a seam where one colour meets another or flows with a design you've made so cracks won't be as noticeable.

I would think your insulation loss will be pretty much negligible.

Another suggestion would be to just remove a layer from the inside, apply the steps above and just re-plaster a new inside skin with a known good mix? Can you lay on your back and stick your head in there to work comfortable enough?
 
Antonio Scotti
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Thinking about the linseed oil, isn't it flammable? That is: wouldn't the oil coating just burn off (and disappear) as soon as the oven is fired?
 
Gerry Parent
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I applied the linseed oil inside my rocket mass heater. Its not directly in the flame path but its pretty close and gets quite hot where it is. I haven't noticed any of it burning off and it still is providing a hard shell to protect the cob from deterioration. Here is the link if your interested and can see a bit more of where I'm talking about: J-tube-Batch-Box-Conversion
I do know that you have to be careful if you have rags of it soaked in oil and piled up that it can spontaneously combust.
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