[Cob] cob oven cracks

[Monica Proulx]

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.