I've finally finished the building of my wood fired pizza oven. It took a while to put everything together however the final outcome has been completely worth it. I started by laying a concrete slab 155cm x 175cm x 10cm. The materials were sometimes tricky to find however generally I found them at hardware and oven refractory places. I placed 12mm rebar steel placed 25cm apart within a frame made out of pine. The rebar needs to be lifted off the bottom gravel with some stones or pieces of brick. I mixed concrete at the ratio cement : sand : crushed rock as 1:1:1. I used this mix as it was recommended to me however upon investigation it appears the recommended ratio is 2:2:3 or 1:2:4 (Cement:Sand:Aggregate) depending on the source. I think it's best to go by what the instructions are on the cement bag. In any event even with my mix I haven't seen any cracks or had issues. I used about 8 bags of cement all up. It's a lot of work mixing the cement by hand so some sort of mechanical cement mixer is really necessary.
Throughout the build I referenced various blogs and forums found via google for troubleshooting issues. I particularly benefited from this wiki started by members of one of the wood oven forums. Pizza oven wiki
Step 1: Lay the Foundation Bessa Blocks
I allowed the concrete to set over a number of days by covering it with plastic and wetting it every 6 hours with a hose. Apparently if the concrete takes longer to dry the end result will be harder. After setting I laid out bessa blocks dry to establish the form of the base of the foundation. I left space at the front top level to place a concrete lintel. I placed 12mm steel rebar in the centers on the bessa blocks and poured concrete to fill the walls. I felt this method would be easier than laying the blocks with mortar.
Step 2: Add the Concrete Lintel
After pouring the concrete into the bessa block walls I set about adding the concrete lintel at the front to finish the table like surface. Alternatively I could have used more bessa blocks and steel lintels to hold the blocks elevated. The end goal was to complete a level area that I could then lay the oven foundation concrete.
Step 3: Oven Foundation Concrete
After the foundation wall concrete had set I built a frame to contain the oven foundation concrete. I used pine underneath and to the sides, with cement sheeting as the base. Again I laid steel rebar and poured a concrete layer 10cm thick. Again I let the concrete cure under plastic.
Step 4: Foundation Insulation
As the purpose of the oven is to retain heat it is important to have good insulation below and above the chamber. On top of the foundation concrete I laid a final foundation of vermiculite concrete (1 part portland cement to 5 parts vermiculite) 10cm high. The mixture is not strong however the vermiculite adds a lot of insulation properties that help retain heat in the oven.
Step 5: Hearth Bricks
I was still concerned about how much insulation I had under the base of the oven so I purchased some fire place insulation board (50mm). After cutting this to the size of the oven (42") I started laying fire bricks in a herringbone pattern over the base. I didn't need to use any sand on the base as the fire place insulation is flat. Additionally it is recommended not to place sand or mortar between the firebricks otherwise this can contaminate the pizzas.
I purchased 250 refractory fire bricks for the hearth and the dome. I used an angle grinder with a diamond blade to cut the firebricks where necessary. I soaked the firebricks in water before cutting to reduce dust and to lessen the heat generated.
Step 6: Dome Leveling Tool
Building the dome is potentially the most risky part of the construction so I wanted to be sure I had everything round and level. I built a tool out of some hinges and steel rod from the hardware store that would allow me to ensure all the dome bricks were laid at the same distance from the center of the oven, and all equally level. I cut a piece of pine board into the shape of the center of the oven and attached the leveling tool to the center. The wood board would be built into the oven however on first firing of the oven the board would be burnt up so I wasn't concerned.
Step 7: Laying the Dome Firebricks
I began cutting and laying the fire bricks to make the dome of the oven while keeping everything level with the tool. The fire bricks were mortared in place using a refractory mortar mix as regular builders mortar is likely to crack under the oven heat. The mortar mix was 3 parts sand : 1 part Portland cement : 1 part lime : 1 part fire clay. For the entrance to the oven I built a frame out of pine and laid bricks around this support.
Step 8: Finishing the Dome
I continued laying fire bricks for the dome a layer at a time and allowing for a gentle curve on each level. At the top most brick levels I was leaving 3 hours between levels so as to allow the mortar of the previous level to set partially.
Step 9: Entrance and Dome Cladding
After laying the dome firebricks I finished laying the entrance and flue with further firebricks. The flue needs to be placed on the outside of the oven chamber so as to allow the oven to be closed with a door if necessary. I clad the outside of the dome with a vermiculite cement mixture (1 part Portland cement to 13 parts vermiculite) about 5 cm thick. The insulation allowed me to create a smooth surface and add additional insulation properties to the oven.
Step 10: First Firing of the Oven
After cladding the oven I allowed it to cure and dry for 7 days. I was really concerned that if I didn't allow the oven to dry completely that when I fired it the first time cracks could form. On the 7th day I slowly started building the heat within the oven over 2 days with small fires. Eventually I built up the heat to 800 degrees Celsius and tried my first pizza! :) It cooked in about 90 seconds and tasted fantastic. From a cold oven the temperature generally takes about and hour and a half to get to the temperature required for cooking pizza. After reaching the temperature however it lasts for ages.
I'm yet to build a door as I haven't found the need just yet. Due to the insulation the oven retains heat so well the door wouldn't add a great deal. I might in the future try some longer duration cooking such as roasts, bread or drying fruit that would probably benefit from the door.