An oil drum (barrel) and a bit of metal pipe is all you need to make a really simple but effective pizza oven. It may not look very fancy but it works well and it will cook a whole lot of pizza. Also, instead of waiting for days/weeks for your clay oven to dry enough to use, an earth bermed barrel oven can be made and used on the same day. Which is really handy if you have a few people coming round.
The soil/earth/stones are vital to keep the heat in – but you could use sand, clay or bricks too. (Don’t use bark mulch or anything similar as it will catch fire for sure.) On our oven the soil ended up being around 8” thick at the top – which was good but more than was really needed. (We would have built the whole thing into a bank, which would have saved a lot of work, but there wasn’t one handy.) I would suggest you aim for 6” minimum.
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The most impressive thing to me was how fast they built this pizza oven and how much food they made. They fed a large group of about 20-30 people and did everything within a few hours.