Cob oven on expanding clay soil

I am un/fortunate enough to live on expanding clay soil. It is good when you need some clay. Bad when you want to grow things or have a stable foundation.

Lust for a pizza oven has filled my heart for years, but I never knew how to get a economical, solid foundation. Meanwhile, this 80-year-old oak stump just won't rot...

A-ha. I decided to use the stump as the oven foundation. I know it is an impermanent solution. Sometime between now and 20 years from now, that stump will rot and take the oven down. But it has been rock solid for three years now.

I built rocks around the stump and then layed down a six inch insulation layer of clay slip mixed with hardwood planer shavings (toothpick shaped fluff). Put a sand/firebrick hearth on that.

I've made a paper-mache dome and entry chamber instead of sand. One, I don't have that much sand. Two, I want to see if I can premake the form and quickly build other ovens. Three, paper mache is cool. Four, I can just burn it out.

This decision has already had consequences. I was feeling pretty good about mixing cob with the kids on a nice spring evening and getting the refractory layer built. But I ran out of cob at the top. So I mixed up a way-too-wet quickie batch and slathered it over the top. it wetted the paper mache and caused a collapse.

The rest of the oven stayed standing and was a thing of beauty, but I took it all down and will try again tomorrow with the already mixed cob (in sealed buckets for now).