Cob oven Cracking

Shalom all

We are in the process of building an out door cob bread oven.

We have laid the hearth, and put up a sand dome mold, covered with wet news paper, we put 4 inches of cob on the out side of the sand dome.

This was done last sunday night 7-1-2013.

With in 2 day we have major cracking almost 7/8 of an inch.

With in 7 days you could see down to the news paper on the sand dome.



Last night we took the sand out to see how bad the cracking was. Its to bad to keep so this morning we broke it down to start over.



Some question for the more experienced cobbers out there.

Would more sand in our mix prevent the cracking?

Would more clay in our mix prevent the cracking?

Was 4 inches of cob to much?

Would 2 inches be better?

Our temperatures on the days it was drying was in the high 90's to low 100's did it dry to fast?



Also we are in Kooskia Idaho, if any one wants to come play in the mud with us there welcome, we tend to play at the river in the hot of the day so we would be glad to put you up on the farm for a couple of days, hell we might even fire up the sauna.



Thanks

Jim

Hebrews West Farms

Cracking is an indication that you need some more fiber in the mix to add tensile strength. That whole thing in Exodus about the Israelites being forced to make bricks without straw? Well, you can't make bricks without straw, they end up cracking. If you've ever played with papier-mache, it doesn't crack when it dries, because paper is a fibrous material that holds together. Another example is the addition of fiberglass to concrete mixes to improve the tensile strength. It doesn't take much fiberglass added to concrete to improve the strength dramatically since concrete by itself has almost no tensile strength (it's all compressive).

Try adding straw or grass clippings or shredded up newspaper or dryer lint or anything else that resists when you tug on both ends. There is an optimum length to the fiber that you add; lengths over 2" are no longer beneficial and may be detrimental as they can induce a fracture along their length. As long as the fiber bits are 1" or less in length and well mixed, they will resist being pulled along their long axis and will strengthen the mix so that when it dries, it doesn't pull apart.

First off thanks for your reply
Would you still us a straw or binder in the first layer it will end up being the fired roof. What are your toughts?

Shalom Jim,

John is absolutely correct on this point, cobb must have fiber and I would also point out that you may have a type of clay with more expansive (contractive) characteristics. With this type of clay I would say fiber length of 30mm is a minimum. I would also suggest, in your case, a strong fiber from perhaps cut up sisal or hemp rope. What is your mix recipe? Are you sure you have proper clay content and not too much silt? Also, for those temperatures you need to use a wet blanket and dry much slower. Traditional built "fagon" and other indigenous ovens were often built over weeks if not months. With the larger tribal/clan ovens, the slower cure times can render a much better baking oven, with much better quality in the build.

Regards,

jay

Would you still us a straw or binder in the first layer it will end up being the fired roof.

Fibrous organic material that gets hot carbonizes, that is, it goes from being cellulose to being graphite or carbon. Rather than think of it as crumbly carbon that is easy to crush between your fingers like biochar, think of graphite golf clubs -- flexible, yet able to deliver quite a smack to the ball. So there is no problem having it in the fired roof.

It doesn't have to be much, just a few percent by volume. You will learn how much from experience with your mix.

Thanks again John and Jay



We were using 1 part sand to 3 parts clay, I have a test bach going right now with 1:1, 1:2, and 1:4. I will see how they do. Not sure if our clay is the right clay is sure builds up on your feet in the garden and the kids make pots out of it all the time.

Our oven will measure 3 feet by 6 feet. I called the feed store and they have hemp rope (that a lot of cutting) any idea how much fiber to add ?



Jim
Hebrews West Farms

I know that I have heard of several different ways to determine this. I too would like to know what methods others use. What my grandmother taught me was after your clay is mixed spread out about 200 mm thick in your mix area, cover this with just enough "fiber material" to appear covered but can see clay. Fold into mix. Make a ball and split. The fiber should hold the halves together to a point and should be a minimum of 20mm to 50mm apart. Hope that helps.

Hmm... yes fiber can come in handy in providing tensile strength and rigidity against cracking, but this doesn't seem to be the issue with the oven for me. In the several earthen ovens I have built, I completely leave straw or manure or any other fiber out of the dense oven mud layer that lays directly on the sand mold, and have no problems with cracking. Your clay ratios seem way too high for the amount of sand you are amending with. Try 2 or 3 parts sand to 1 clay. As your clay dries, it shrinks. Having a lot of sand in your mix will keep the clay from shrinking and consolidating back into a thin mass, and thus prevent cracking. IMO, adding fiber to this first layer will increase insulation, especially with the temperature the earth ovens reach as it chars the carbon fibers into a light, airy masses. This will increase your firing time and the fuel load to bring your oven up to a good baking temperature, and it will not retain its heat as well as a heavy layer of thermal mass. From what I've seen, it is best to have 3-8" (depending what you will be mostly baking) of dense, fiberless cob for your primary baking layer, followed by a fluffy, fiber heavy insulation layer. Have you tried again? What did your tests tell you? John and Jay, have you built ovens with high fiber content against the sand mold? If so, how long do they stay at a good baking temperature? I could very well be wrong, its just my $0.02 based on what I've learned about these mediums.

Hi Jim,

Well I have done it several ways, including lining with soapstone, which is my favorite. As far as, not adding straw as a binding agent, that would not be something I would recommend or try. It would seem it has worked for you though. I would just ask the age of your ovens and size? That type of mix would seem to present a very strong likelihood of delamination and thermal spalling, as sand does not expand and contract as much as clay, but is not going to bind the matrix in the same way as a fiber material would either. As for thermal mass in the first layer being lower because of the fiber content causing too much insulation, the clay without fiber compared to the clay with fiber is going to be present marginal differences at best, not really effecting the thermal mass coefficient that much, unlike that of a soapstone layer, which has a very large coefficient. I would love to read about other formulations that others have used, like yours, and the history of application with it, including longevity of oven. Thanks for the information and questions.

Regards,

jay

Jim Black wrote:
We were using 1 part sand to 3 parts clay

Sure you don't mean 1 part clay, 3 parts sand? That other ratio will certainly crack on you unless there is lots of sand already present in it.

Here where I live, the normal builders cob mix is a ratio of 1:1 (soil:sand). Our oven core mix is around 1:4.
Don't add straw to the internal core, it won't help make the inner core stronger, if anything it will just make the mix more insulative and crumbly when fired. Instead you want a high thermal mass mix, with LOTS of sand in it that is also as dry as possible while still workable. If you take a handful and pack it into a tight little ball, it should hold together well, but if you pinch it hard in your hand it should "pop" apart, or if dropped it should NOT splat (like clay or liquid) but shatter.
Working with this mix is very unlike regular cob. Work around the sand form, compressing (hard) down into the cob mix, NEVER push towards the form. Maintain even height as you build, working around the form as you go. Keep a sharp outer edge, use one hand to compress and the other to "trap" against as you go, this will help keep the oven thickness uniform. As the oven gets higher, the "working face" will need to tilt so that it remains perpendicular to the sand form; this is IMPORTANT as it will properly distribute the pressure of compacting the mix and will help prevent collapse. You MUST use a good deal of pressure as you work, make SURE that each handful is pressed tight and will not move further (pressed by hand, NO SLAPPING) before moving on. To facilitate this, set up something to stand on so that you will always be working below your waist.
There is not really any point to waiting to cut open the door and dig out the sand. The entire pile will dry from the inside out, if you leave the sand in you will be waiting for a VERY long time depending on conditions. If you've done well, followed my directions above and REALLY compacted the mix, you will be able to dig the sand out immediately. If the oven body collapses, then you have failed to compact it well enough, which can be a common thing when you have a lot of people working on it; workshops and workparties need very close attention by one "director" type. If one small area is not compressed properly, it will give way there first and then travel to the rest of the dome. Be diligent, check people's work constantly and all will be well.