Bill of materials:
100 or so Recycled bricks
250kg Pottery Clay - I got mine from a local pottery equipment supplier, who delivered to my doorstep.
4 x100 liter bags of Vermiculite - also known as Perlite - obtainable from a building supplier. £20 per bag
2 x 25kg bags of concrete mix
2 x 25kg cement mix
4 x 25kg bags of mortar mix
10 x 65mm-thick reinforced-concrete Lintels
6m of 1x4 wood for the foundation formers
Hard-core builder's rubble for the foundation
1 recycled Chimney or similar (eBay is can help you)
Thin wood slats salvaged from a wooden venetian blind (from IKEA!), or long twigs, for the oven dome former
Some odds & sods...
Plaster mixing impeller
Angle grinder with masonry disc
Air compressor & pneumatic chisel
Step 1: How does it work?
This oven works on the convection principle: A fire burns in the back of the oven. It is fed by the cooler air that flows in along the lower half of the cavity. The hot air and smoke rises to the top of the dome, and is then drawn out along the top half of the cavity through the chimney. The hot air in the dome causes heat to be radiated back to the ground and any pizza that may find itself in its way....
That takes care of the top of the pizza. The bottom layer in the oven is a material of high thermal mass (thick terracotta tiles in our case) that is heated up by the radiation from the dome and also by the conduction of heat from the fire. A porous clay-based material like terracotta is ideal as it temporarily absorbs moisture from the pizza base, which helps to give you a crispy pizza base. We intend to make proper pizzas here, not the thick, soggy things that those multinationals dish up, after all.
The fire is started in front part of the oven, and when it is ablaze, it is pushed into the back. The fire may appear to momentarily die because of the lack of oxygen, but the convection process starts to work within seconds and it grows again. The secret of keeping the fire going it to regularly feed it small amounts of wood, preferably using long tongs so that you can place the wood on top of the burning embers. You can test the convection process by dropping a few drops of water on the bottom layer of the oven: the resulting vapour should move into the oven towards the fire, not the other way around as you would normally expect.
There is no hard and fast rule of determining when the oven is hot enough. I just know it is ready for use when it feels much hotter than my domestic oven set to maximum temperature. I also measure the exhaust temperature on the chimney and when it exceeds 300 DegC (because that is how my temperature measuring device will go), then I know that the internal heat is well above that we are ready to go.
When the oven is ready, the cooking surface needs to be prepared by sweeping the any ashes either into the fire or out of the entrance using a natural bristle broom dipped in water. Then use a wet floor mop to pick up the remaining ashes off the cooking surface. You can also wrap a wet rag around the broom for the same effect.
You are now ready to bake your pizza.