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Sasquatch 07-12-2010 09:12 PM

curing the cob oven
 
We've built a very substantial 36" dome oven of dunite grog and clay and have let it air dry for the past 48 hours. The dome is 3 to 6 inches thick and feels quite hard. I'm forcing myself to wait another 24 hours before I excavate the sand and styrofoam form but I'm unsure about the best way to continue curing the oven.

I've searched these forums and Googled for other's experience on curing cob but it seems to range from "go for it!" to suggestions to air-cure for two weeks and then build a series of small fires.

I'd love to be baking tomorrow evening but considering the huge effort it took to build this oven I don't want to risk damaging it.

Any suggestions?

BackyardPermaculture 07-12-2010 09:55 PM

Re: curing the cob oven
 
Well, you definitely should not cook in it tomorrow!

I have heard advice that the sand form should be left for a long time, and advice that it should be removed ASAP. I suspect ASAP is best, as the clay dome will be shrinking as it dries - so the quicker you get the sand out the better.

The curing should span a few weeks, with fairly small fires. Don't rush it. I got some small cracks; my friend got bigger ones. Your dome is thick, so will take a long time to dry out.

Have you put up any photos yet?

Cheers,
Mick

rzwahr 07-13-2010 05:09 AM

Re: curing the cob oven
 
I just built my first cob oven a couple of weeks ago. I deviated from most in that my first layer contained chopped straw. I wanted it to have the tensile strength the first few times it was fired to relieve or alleviate cracking altogether. It has worked well. I have not had any but very minor hairline cracks. I realize over time the straw will burn out, but that will take a while. I've fired it about 5 or 6 times now, with no problems. Gets extremely hot - cooks great. Now I fall into the school of waiting 48 hours or so, cutting your door and scooping out the sand. I did this with absolutely no issues whatsoever. Again I had the straw in the first layer so that probably played a factor. I fired my oven that day with a low burning fire, and let it burn for several hours. By the end of that time, the dome was mostly dried out - you couldn't see all but a small amount of moisture around the edges next to the plinth. I was cooking in it 2 days after that. So in my opinion I see no need to let it cure for as long as some folks say, but then again a) I am impatient and b) I didn't have a problem with this method. I can see how a pure sand/clay first layer may tend to crack more, which is why I chose to mix in some straw. Not a ton but enough to create strength.

Anyway - for what its worth, I say go for it. My philosophy was, if something bad happens, take a sledge to it and redo the dome. It's not like the materials are expensive. That's why we build with cob/clay now, isn't it?

-Ryan

Sasquatch 07-13-2010 09:02 AM

Re: curing the cob oven
 
Thanks Mick, it definitely hadn't occurred to me that there was any urgency on removing the sand form but that does make sense. Just as soon as I can put my boots on I'll excavate the form and begin curing with a low fire.

I'll also post some photos later -- I've documented the entire build.

For anyone else considering a inexpensive DIY 36 inch cob oven, my total cost has been $87.00

Had I planned ahead and been just a bit more patient, the cost would have been zero -- I just located a good vein of sand not far from our cabin and a friend gave me enough used firebrick for the hearth. I'm passing these materials on to a couple of neighbors who are now inspired to do their own ovens.

Sasquatch 07-13-2010 12:24 PM

Re: curing the cob oven
 
Ryan... adding straw for a more adobe-like first layer makes a lot of sense. What is the internal diameter of your oven? I've done a couple of very small ovens with the crudest adobe -- once it hardens, it is amazingly tough, and also amazingly easy to repair. I think our tendency in the "First World" is to overbuild, even with something as simple as a mud oven. But... as for knocking my oven down and building another, unless I'm quite mistaken it would take a direct hit from an artillery shell to do serious damage!

It took the better part of an hour to excavate my form of sand-over-styrofoam. I hadn't considered that the weight of the sand and first layer would compress the form so much. It took a machete, crowbar, garden spade and sharp trowel to finally clean out the form.

Once the oven was empty I was amazed at its capacity. I'd read so many posts here suggesting that a 36 inch oven is relatively small that I decided to ditch the plan to do a 22 inch oven (Kiko Denzler's suggestion). From my point of view, that of a home baker who normally makes 3 loaves every ten days, this thing looks cavernous. Nonetheless, I love it!

I have a small curing fire burning now.

rzwahr 07-13-2010 01:36 PM

Re: curing the cob oven
 
My oven is 36" wide. Dome height is 15.5". My dome "slouched" a little when I built it. Well, actually a lot. It originally was 18" high. My kids were helping me build it and I think may have gotten a little overzealous on packing the cob in place. Anyway, no worries - it works great. My door is 9.75" tall and 16.75" wide.

You can see a picture as it currently stands here:
Picasa Web Albums - Ryan

Here is what it looked like prior to building the arch:
Picasa Web Albums - Ryan

Obviously I am no brick mason. In fact, this is the first time I had ever attempted to build anything with brick. Other than playing with Legos as a kid. I also did not use any mortar, just straight cob as mortar. It is *very* sturdy.

Now, this was right after I had completed the tunnel and arch, so you can see that the arch is still wet. I fired the oven immediately after this photo was taken. It dried in about 3 hours of a very hot fire.

I have yet to put the insulation layer and final plaster on. That is coming in the next couple of weeks.

Hope this helps

-ryan

Sasquatch 07-13-2010 04:15 PM

Re: curing the cob oven
 
Ryan.... that is a very nice oven. What are you using for a door, or do you need one?

I've had a fire going for a few hours now. The upper part of the dome is steaming and is quite warm to the touch but the base is much thicker (like yours, from sagging) and is still almost cool.

Sasquatch Observatory Oven...


Carl

BackyardPermaculture 07-13-2010 07:52 PM

Re: curing the cob oven
 
Hi Ryan and Carl,

Both cob ovens looking good!
BTW, I'm still trying to decide what kind of door to use. Really keen to start baking in the thing.

Cheers,
Mick

david s 07-14-2010 03:28 AM

Re: curing the cob oven
 
Because all clays shrink it is advisable to remove the form as soon as possible. That is as soon as the dome will be self supporting. If it feels cheese hard it should be ok but you want it to dry (shrink) evenly to prevent cracks. straw and maybe manure (horse manure that contains lots of undigested straw) are good to add because they'll create some tiny voids that allow moisture to escape through when they burn away. Modern castable refractories contain fibres that do a similar job.

rzwahr 07-14-2010 08:53 AM

Re: curing the cob oven
 
I have not made a door - yet. I plan to make a heavy wood door and soak with water thoroughy prior to baking bread. Prevents burning and provides steam. I'm going to begin adding more layers this weekend, so I will let you know how it goes. I plan to make a clay/vermiculite mix for the insulation layer, then earthen plaster on the outside. We'll see how it goes. I can always take a sledge to it and start over if it doesn't work out ;)


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