[Cob] clay oven foundation

[Mary Hooper]

I have a yard full of clay that is hard as rock when dug wet and left  
to dry in the sun. We've had about an inch of rain in each of the  
past two months, so there's lots of drying time here!

If i wanted to play with the dirt i'm digging out to replace with  
soil for growing things, what would I use for a foundation for a clay  
oven? I think it would be a fun way to play in the dirt creatively.
If anyone has a link to newbie instructions, I'd appreciate it. I'm  
sure a link has been posted in the past but I don't save everything  
anymore.......
Thanks for any help.....
Mary J in the WNC mountains.