This is a dead easy and cheap oven to build. Forget the fancy kits and complicated plans, this uses a home made adobe mixture and second hand materials.
1. Theoretically you could build this on the ground, but for ease of use, we raise them about 50cm off the ground. To do so, place 4 - 6 pillars to support a metal frame.
2. Maybe over engineering but the ghost of my father made me
put star pickets through these besser blocks then fill them with concrete.
3. You need a metal frame to hold the oven up. We found a grate at a wreckers yard but a gate or old shed door frame would do.
4. Lay cement sheet over the frame.
5. Lay floor bricks. These are regular house bricks. Don't bother sourcing firebricks. They are expensive and unnecessary. House bricks are dense and will hold the heat, which is what you want. (firebricks will reflect the heat and can be quite crumbly). Lay them with the flat side up to make for a smooth floor.
You probably don't need to, but I lay them on a mortar bed.
6. The oven mouth. This is the trickiest bit but it is not really. Make a form in the shape of an arch. I prefer low and wide so not too much heat escapes but you have good shovel access when cooking. Start on the outside and build your arch up. You can use precast arch bricks or cut them but I just use straight bricks with mortar.
7. You can see here the only shaped brick is the keystone. I just chipped this to fit the gap.
8. Now you are ready to build the dome. Make a big sand castle in the shape you want. The highest point of the dome should be above the mouth arch. When its good, cover with plastic to stop the adobe mix from sticking.
9. Mixing the castable adobe. You can buy castable refractory material, but it costs. A mechanical mixer would be neat but I just use a barrow and shovel
My mix is this (roughly)
4 parts sand
3 parts refractory gravel (ie. granite or scoria,
not Bluemetal)
4 parts clay (fireclay is probably best but I use recycled school kids projects, you could use any clay you can find)
2 parts straw and/or sawdust
1 1/2 parts cement
10. The thickness of the adobe should be about the thickness of a brick or thicker. A thick layer is better than lots of thin layers.
11. You can see here the inclusion of a flue. This needs to be at the front, close to the door, (for reasons explained later).
12. I usually put a layer on the outside that has a bit more cement in it. This helps weather proof. Keep in mind that the laws of thermodynamics mean that the whole unit will expand on heating. Some cracking is inevitable and normal.
The Diagram below explains the theory behind the oven. You light a fire in it about 2 -3 hours before cooking. just before cooking, you push the fire back with a rake. The floor can be brushed clean with a damp brush on a stick. The idea is then that while you cook, the heat travels up and forward, the smoke going up the flue rather than in your eyes. With a hardwood fire and good coals you should have about 2 hours cooking time.
