The last installment covered rendering the earthbag wall and creating the suspended concrete slab. Now the fun begins, the construction of the pizza oven out of very simple easy to source materials; clay, sand, straw, and water. Together these become cob.
There are 10 steps to building the pizza oven.
- Work out your clay to sand ratio
- Work out the size of the inside of the oven
- Build the entrance from cob
- Make a sand mould of the oven
- First layer of cob
- Insulation layer of cob
- Final layer of cob
- Remove sand
- Fire oven
- Cook and eat pizza
The sand to clay ratio when making cob is important. Too much clay will cause cracking and too much sand will cause crumbling. My guide to this and building cob pizza ovens has been a series of you tube videos. The one regarding clay ratios is here
Once you have worked out your clay and sand ratio its time to work out the size of the oven. There are some tried and tested rules which help with working it out. They are:
- Door width should be 50% of the diameter of the oven
- Dome height should be 70 – 75% of dome diameter
- Door height = 63% of dome heighSo in the case of this pizza oven I am making it 900mm in diameter internally which makes
- Door width 450mm
- Dome height 630mm (70%)
- Door height 397mm
golden rules of pizza oven dimensions
Building the entrance arch and doorway. It might sound like a funny place to start but it works. My research on the net led me to a series of blog entries “The Year of Mud: Natural Building Workshops” by Ziggy. They clearly explain the sequence of building an entrance, making cob, right through to the finished oven. Thank you Ziggy for such a clear step by step blog.
Here are the links to those blogs. Below is a photo gallery of our pizza oven build.
cobbing door arch
sand mould finished
sand mould and cobbed arch
all hands to deck 1
all hands to deck 2
first coat cob finished
insulation coat finished 2
insulation coat finished 1
insulation straw and clay slip
plaster coat 1
Although Ziggy’s blog gives all the information you need to create a great pizza oven I will add a few things here. I soaked the clay which I sourced from a nearby pile that had been dug out for installation of an underground water tank. It was put into a plastic dustbin and water added and left for a few hours or preferably a day.
Sand Mould. I used builders sand to create the mould over which the pizza oven was constructed. Cut a stick to the right height of the oven and place it where you want the highest point to be. I placed concrete blocks underneath the concrete slab to help take the weight as the sand adds quite alot of extra weight.
First layer of cob. There are lots and lots of videos on you tube showing how to make cob. One I find very instructive is “How to build a cob oven” by Build naturally. How to make cob is in part two. Click here
We had a lot of fun treading the cob together and applying to the sand mould. Don’t forget to put a layer or two of wet newspaper on the sand before adding the cob. This will aid in removing the sand later on.
Balls of cob
all hands to deck 2
Insulation layer. After completing the first layer we left it until the next morning to firm up. Then we added the insulating layer. This is made from mainly straw with a clay slip (clay mixed with water until runny) added and mixed in the wheelbarrow.
insulation straw and clay slip
insulation coat 1
The insulation coat adds a layer with lots of air pockets that keeps the heat that has been sucked into the thermal mass layer in the oven.
Final layer of Cob (Earth plaster) To protect the insulation layer we added a layer of earth plaster. This is basically clay and sand at the same ratio as the cob with chopped straw added. We used a lawn mower with a catcher to cut up the straw. this gives strength to the plaster but also makes it possible to make it smooth.
Removing the sand. After a few days to start drying it was the moment of truth. Would the pizza oven stay together after removing the sand. I used a short handled shovel and a trowel to remove the sand. It was very easy to see when I had got to the newspaper layer and saved damaging the cob.
Sand inside pizza oven
newspaper visible inside pizza oven
sand removed from inside
Firing the oven. To help it dry put and to see if the oven draws properly a SMALL fire was lit. This was done several times to ensure that the cob dried out slowly.
Pizza oven first firing
Cook and eat pizza, Now we were itching to see if the rocket fired pizza oven would work. A door was made from recycled floorboard backed with some cement sheet and a thermometer inserted. The rocket stove was fired up and the oven started to heat up.
It took a while and it was noted that the steel plate that the pizzas were cooked on needed to be lifted up more to give a greater draw to the rocket stove. Also the door started to char and when it was removed a light winds turned the embers into flame!! Not such a good start. However a pizza did eventuate. The next one would be cooked with a better door and with more of a gap between the floor and the cooking surface to enable the rocket stove component to work more efficiently. See part five for final rendering, finishing of shelves and the new door!