I have been interested in making a cob pizza oven for a number of years now and now find myself with an abundance of time (but very little skills) to do so. I don't have access to stone or bricks for the base and also feel this would be a bit out of reach for me skill-wise but I do have some 33 inch care tyres that I am keen to use if possible.
The easiest plan I can think of is to use x6 33 inch tyres stacked on top of each other and filled with gravel/ rammed earth and then topped with a layer of cob, beer bottles and insulation, another layer of cob, sand, firestones and then the cob dome. If the cob/ insulation dome had walls that were approx 7 inch thick then the internal diameter of the oven would only be 19 inches which i am concerned could be too small as I haven't seen any reports of anyone making one this small.
An alternative plan would be to use 4 times the amount of tyres (x4 33inch tyres on each layer and built up to be 6 layer thick) which would be A LOT more work and take up A LOT more space but would allow me to have a much larger dome (would probably go for an internal diameter of maybe 27 inches but this could be flexible).
Some specific questions... What's the minimum practical internal oven diameter? What's the ideal cob/ insulation thickness? Are tyres a reasonable idea?