Cracking cob oven?

So I got Kiko's book a little bit ago, last weekend I decided to start.  I laid the foundation, the fire bricks, the sand form then added the 4" thick thermal layer.  At this point I stopped.  I used 1 part clay to 2 parts sand.  I live in NC so our soil is very high clay content.

Fast forward 3 days there are 1/4" cracks on the top of the thermal layer and it is still drying.  Is this a lost cause or should I not worry? 

Any help would be awesome!

Photos from build:

laying fire bricks


sand form, covered with newspaper


laying thermal layer 4" thick 1:2 clay:sand


Finished thermal layer before it dried

i was told by someone who has built quite a few ovens that a foundation that is not solid enough will cause cracks when it shifts in drying, or when the earth itself heaves from temperatures or weather. he told me that a strong foundation is critical for a long lasting cob oven. not sure if thats whats going on with you though just thought id put it out there.

It's been 3 days... have you taken the sand out yet? if not, your thermal battery layer shrunk more than the sand form would allow...

Scott Alexander wrote:
It's been 3 days... have you taken the sand out yet? if not, your thermal battery layer shrunk more than the sand form would allow...

No the sand is still in there, I was under the assumption that you keep it in there until it dries all the way.  I could very well be wrong in this. 

Regardless, does this mean im out of luck and have to start over?

It sounds like too much clay.  If you have a really "fat" clay, the ratio should be more like 1 clay to 3 parts sand.

I would use it an see what happens - it is probably o.k.  If it starts spalling off onto your food, you know you need to try again...

Soil is correct, the foundation must be pretty solid, but, unless you are on very shaky ground, and/or there has been much settling of the earth that it sits on, I wouldn't be concerned about the stability of the foundation. This is an issue that most likely wouldn't show up for a couple months, and would only get worse.

Ardilla is also correct, you need to ensure that the sand/clay ratio is correct. In fact that was my first thought, but then I wrote my reply with the assumption that the original poster actually read Kiko's book and followed his instructions on how to determine the correct sand/clay ratio ... it totally possible that this is why there is cracking in the oven.

My reply was taken from a conversation with Kiko about when to remove the sand. When you remove the sand is dependent on how wet your mud was. The correct answer is, "when the roof of your dome will support itself" when, exactly, that is becomes a difficult question that relys heavily on how wet your material was when you put it on. And what makes the question tricky is that if you take out the sand too soon, you get a collapse, if you wait too long there may be cracking from the shrinkage (because as clay drys, it shrinks). One thing to keep in mind with this delema is that the insulation layer helps give a little extra structure to help keep the dome up

Do you need to start over? I don't know, you havn't posted a picture of what you have NOW. do you think it would collapse if you cleared the sand? if so, it would probably be best to start over. If you do start over and you think that the mixture was correct, it can be re-constituted (just add water )

Scott Alexander wrote:
Soil is correct, the foundation must be pretty solid, but, unless you are on very shaky ground, and/or there has been much settling of the earth that it sits on, I wouldn't be concerned about the stability of the foundation. This is an issue that most likely wouldn't show up for a couple months, and would only get worse.

Ardilla is also correct, you need to ensure that the sand/clay ratio is correct. In fact that was my first thought, but then I wrote my reply with the assumption that the original poster actually read Kiko's book and followed his instructions on how to determine the correct sand/clay ratio ... it totally possible that this is why there is cracking in the oven.

My reply was taken from a conversation with Kiko about when to remove the sand. When you remove the sand is dependent on how wet your mud was. The correct answer is, "when the roof of your dome will support itself" when, exactly, that is becomes a difficult question that relys heavily on how wet your material was when you put it on. And what makes the question tricky is that if you take out the sand too soon, you get a collapse, if you wait too long there may be cracking from the shrinkage (because as clay drys, it shrinks). One thing to keep in mind with this delema is that the insulation layer helps give a little extra structure to help keep the dome up

Do you need to start over? I don't know, you havn't posted a picture of what you have NOW. do you think it would collapse if you cleared the sand? if so, it would probably be best to start over. If you do start over and you think that the mixture was correct, it can be re-constituted (just add water )

I have read Kiko's book and followed his directions on determining the correct mixture.  After several test blocks I found that this 1:2 ration produced the best test brick (strongest, least shrink, etc).  As for removing the sand, the inside of the thermal layer can still be dented (test kiko describes) so I am still waiting to take the sand out.  Here are some photos of the cracks, they don't look so bad at second glance. 

Let me know what you all think



 

oh, with the space left on the sides of your foundation, I didn't realise that the thermal layer was on already. it's probably fine. In fact, these cracks will get larger when the oven is hot.

The weekend is here and you have a new oven, sounds like a good excuse for a fire to me

no help with the cracks, but I'll take that dollar!!

I'd think you are fine. maybe another light coat over the cracks. From what I understand, it's going to crack anyway from heat later.

umm you might check that thermal layer mix again. in a good mix you wont get cracks. that mix looks way to clay rich to me. I missed where you are in the world but from the look you have a real high clay content and i would bet money its a high shrinkage clay.
off the top of my head i would say you need to go three to one or three and a half to one.
dont skimp on the mixing

Until you take the mold out you wont know if the cracks go all the way through (though ill bet they do given the wall thickness). Also if that oven is in full sun you would be better to keep it shaded and good air flow rather than sun heated.
just my 2cents.

Ernie is right..  When I first started mixing mud, it was hard to believe that adding that much sand was a good thing.  There is a certain sound it makes I have learned to listen for.  I'm in Ohio and I have some serious clay, high shrink rate, hard as rock, and river sand is heavy.

I'm with Ernie also.  We have very clay-rich soil here, and I think we ended up using a 4 sand: 1 clay-soil mix for our thermal cob layer. 

With the cracks already nearly surrounding your dome, I'd be concerned that the whole ceiling could collapse onto your fire or food one day.  And smoke and heat will also use the cracks to travel up and weaken the next layers of insulation, and  the edges of the crack will erode, dropping grit into your food.

It will be much less work to re-do the dome at this point, than to see it through only to have it collapse later.  You can re-wet the cob slightly, add in another 1-2 parts sand, and use the same cob to re-build your dome.
But you are rightfully proud of your work so far, the proportions look great, and it would probably work for a few firings if you want to continue as a learning experience.  With that low base, it looks like more of an experiment than a permanent kitchen fixture, so you can get away with playing around and testing things out.
  Our ovens tend to get used for massive pizza-parties, so we want them at a convenient height, and relatively flawless. 

One other comment - it looks like you did a pretty good job of keeping the layers even, and building up like a coil pot instead of like a scree-slope.  But from the handprints on the final layer, somebody may have pushed in toward the dome at the end to 'even out' the thickness.  Pushing cob inward usually results in it bulging outward again, and tends to develop cracks later.  Instead, support the sides, and push down to set the cob in place.  Cob rarely bulges back up. 
As much as possible with a dome, work in rings or layers with flat tops, as if each layer was a course in a brick oven.  You want the last few layers coming together in the top like a keystone arch. 

We had difficulty keeping the cob from bulging on our oven, so we worked two layers simultaneously: the thermal core, and a thick layer of insulation to help keep the core in place.  We used chopsticks with marks on them to keep the thickness even, placing them along the layers, or poking them in and trying to stop when we got to the plastic covering of our sand form.  It worked pretty well, and we filled in the holes afterward. 

We put up pictures at http://picasaweb.google.com/eawisner/FrogOven#

Unfortunately the keystone concept doesn't come across very well, we were concentrating on the cob at that point and nobody used the camera.  But here's a basic keystone diagram: http://upload.wikimedia.org/wikipedia/commons/d/db/Keystone.svg

The idea is to tilt each layer inward, so that the next layer has no chance to fall outward, only down and in.  When you add the final 'keystone' at the top,  it locks everything in place, and nothing can fall inward.  (Don't match the Wikimedia diagram's curve, your arch shape is much better, just use the idea of the angled joints.)
You can make a cob 'keystone' as a round plug, kinda like an igloo, but what we did was to work the top of the dome in long wedges (like segments of an orange, or a bunch of bananas) so our cob 'keystone' was shaped more like a banana.

Of course, with cob, you work each layer into the next.  But if you think of each layer as some wet adobe bricks, then any cracks that might form between layers due to jiggling or drying will not be structural problems.

Hope that's not too much information for your current goals - good luck and keep it up!

-Erica Wisner
www.ErnieAndErica.info

I haven't built an oven, but a trick I learned to keep cracks from forming in floor I built seemed to work pretty well.
I used a heavy wooden paddle and a very dry cob mixture, as I added material, I would repeatedly "spank" the area to activate the clay and make it bond with itself.  I also went a little heavy on the fiber ratio (I used fine cedar saw dust).  The combination of less water, heavy spanking, and lots of fiber yielded a very nice surface with minimal (hair line) cracking.