The oven floor is in two parts.
1. The cob container wall for wine bottle insulation
2. The firebricks oven floor
Build your plinth or base out of anything you want. (see previous steps) In our case we found a lot of boulders from around the area, old construction sites etc. We threw in old concrete, rocks, sand in the middle to fill it in and create a better thermal inside. We even put in a very old fire oven that we had removed from house during renovations. Once it’s full and the sand has been poured in to fill in the gaps you bring it up to a level with sand that you can then create a cob floor and wall to allow wine bottle insulation layer. You can see what we did in the photos below. The wall height should be just a bit higher than a wine bottle/beer bottle lying on it’s side. This is so when it’s filled in with sand you can form a perfectly flat layer of sand for fire bricks to sit on.
This cob layer will have a small wall built around the outside to just over the height of a wine bottle lying down. But just prior to that we sprinkled some left over concrete on top and wet it down to ensure there was a little bit more integrity and strength to the base. That’s really optional.
Then we make our first cob mixture! (See the CLAY page to learn how to get clay ready for making cob) We made it with about 2 parts soft clay, 5 parts sand, 2 part straw. What we were doing here is making it structural (straw) and insulated to help the oven floor maintain heat. The next step covers this more thoroughly in pictures.
BEST way to mix your cob is to get a tarp and throw your clay down on it, add your sand, mulch around with your feet like you are squashing grapes. Then add your straw. As you do all this you keep pulling over the sides of the tarp like folding dough. A good cob should be able to be held like a soft ball and maintain somewhat of a shape if it’s dropped. Too slushy and it just splatts.
Once we finished this first layer, we took a deep breath and started filling it up with wine and beer bottles.
Keeping it level
The other thing to keep an eye on is level. Using a piece of wood lying across from one side of the wall to the other – throw a level on there to keep an eye on it. You should be building up the wall so all sides are level – this is the stage where you really do start needing to level it out. Up until this point it’s been pretty organic and done by eye. The firebricks will need to be laid completely level (or very very slightly sloping towards where your door will be to ensure rain is not going to drain into the back of the oven).
The wine bottles & firebricks
This took us a while as we just used wine bottles that we were consuming. A much quicker way would be to rally the neighbours. Once your floor is full to capacity you then need to put in sand. We sifted sand found from a local back dirt road (the sand should be clean and fine), and poured in all over the wine bottles until we had a beautifully flat surface. Keep an eye on the level at all stages. This step needs to be done when you have your firebricks. As any rain, pressure or anything touching this level and you will need to start again. We ordered our firebricks from a place in Sydney – Sydney Fire Bricks, who were great and extremely helpful. These were the most expensive part of the whole build (in fact the only real thing we needed to buy). Here is their website to check out if you live in Australia.
As you lay your firebricks it’s very important to sit them gently on the sand, placed exactly against each other. You almost drop them on and let the slide down the edge of the one beside it. A gentle tap with a rubber mallet to ensure they are completely flush with each other helps too. But don’t stress. This is pretty hard and ours wasn’t perfect. But it was good enough for our Pizza pale to slide right across and eventually the cracks will up with ash etc. As you place them remember where your front door is going to be and think about having a protruding tile or slab coming out a little.
