I've been in the process of creating an earth oven recently. I'm basing it upon the many other sites I've found on the net - try http://handyprojects.blogspot.com/ for a great description of how to build it and links to other's sites.
As a result of my dabbling with the local clay I've decided to publish what I've learnt on the net. So... a blog is born.
Thus far I've created the supporting structure for the oven and it's concrete and clay brick base. I used concrete wall capping to create a flat, solid base on top of H3 radiata pine 100x50mm exterior wood. My choice was dictated by the better price on the capping than the concrete pavers at the local diy store. The total area of the base was one square metre.
On top of the concrete I placed about 30mm or so of a sand/clay mix just to form a heat barrier between the clay bricks and the concrete. I figure that the capping pieces will crack if they get subjected to too much heat. On top of which I don't want to burn the H3 protected framing timber. That wouldn't be too cool. Here's a thought - why don't I put an insulating layer of pumice (like vermiculite) between the concrete capping and the heat retaining layer supporting the bricks?
Anyhow - as for the clay - here in Wellington, New Zealand, we have plenty of yellow clay. I've noticed that some of the clay I've collected acts differently to others. The best so far seems to be from the side of the narrow back road heading over the hill from Plimmerton to SH1. This seems to form a very elastic goey clay when soaked with water. I've mixed some coarse sand in and heated it in a friends chimera over drinks on New Years eve - sintered quite nicely. Even has a pinkish appearance to it.
Here's a picture of progress thus far...(the strange wood uprights are to hold a roof - I can't think of what to make of yet - not sure how high the flames will reach!)