[Cob] cob oven cracks

[Doug Shaker]

Monica-

I have no building experience with cob, so take what I say with a
grain of salt.  From what I have read, you need sand to prevent
cracking, but your earth is so silty that it can only stand a small
percentage of sand before it stops adhering.  It seems to me that
the thing to do would be to add pure clay and, with the additional
adhesion, add more sand.  You can get pure clay at most building
supply stores, sold as "fireclay", for something like $6 for 50 pounds.
One bag would be enough for at least one oven.

Anyone with more experience than I (read any experience at all), feel
free to correct me.

-Doug "Anonymous" Shaker



At 02:19 PM 7/21/2008, Monica Proulx wrote:
>Thanks to everyone who answered my questions about making a lightweight
>portable cob oven recently.
>
>One more question.  How do you prevent cracks, particularly deep ones.  We
>are on our third ovens (all done with soils from 3 different locations) and
>no matter what we do we have cracks.  (more sand, less sand, lots of clay in
>soil, not much).  I have read in Kikos' book and and elsewhere that they all
>crack, yet I see many photos on the web of ovens with no cracks.
>
>Any ideas from someone who has success with no cracks would be appreciated.
>Are we drying them too fast?
>Are we not putting enough sand in?  (our soil doesn't have too much clay,
>lots of silt though, and makes great adobe bricks w/o any sand, hard as a
>rock)
>Why does the crack get bigger when we cook and then shrink?  Is it due to
>moisture still in the walls?  (I know metal expands when it is heated, but
>didn't think that earth would)
>How long before you folks fire them up the first time?
>
>We are in a very dry climate, and will cover this one while it dries &
>perhaps use a fan to equalize the drying of inside and out, and not start a
>fire in it until it is dry.
>
>Thanks so much.
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