Earthbag wall and cob pizza oven part two. Rocket pizza oven

As I was planning, designing, and musing over how to build our earthbag wall and cob pizza oven I was reading articles about how much wood a wood fired oven uses. This correlates with my own experiences in building two previous wood fired ovens in our local community gardens here is Tassie.

An article in Owner Builder magazine about rocket mass heaters and stoves by James Henderson caught my eye. It was the very small amount of wood they use and the high amount of heat that they can output. More research was warranted. I found the following to be very useful.

After more looking I decided to build a concrete and perlite rocket stove to test out the concept. There are lots of videos on how to make these very cost effective, simple stoves. For instance try here. I used perlite mixed into the cement to make a lighter concrete. It worked but was very crumbly.

However after the trial I decided that I would incorporate a rocket stove as the main heating element of our Pizza oven.

Continuing to research I decided to build the rocket stove from clay bricks. Fire bricks are ideal but here in Tasmania they are very expensive so I found a source of clay bricks from Austral Bricks at Longford.

I built a trial rocket stove from loose bricks on the patio and added a length of stainless steel flue as the chimney. The idea is to have what is known as a J tube. The short upright end of the J is where the sticks are fed in and then the burn happens in the horizontal part and the exhaust and heat are pulled up the long vertical part of the J. The term Rocket Stove is because when they fire up they sound like the roar of a rocket. If you take away the chimney the fire goes out very quickly.

Having trialed the stove and finding out that indeed it heated up to over the 350 degrees C that my infra red thermometer goes up to in quite a short time it was time to build the rocket stove into the earthbag wall.

The bricks were mortared together using fire cement and then the chimney was added. The chimney has been made from a stainless steel flue riveted to a circle of cement sheet with a hole cut out of it the same diameter as the flue. Slots were cut around the flue and bent out wards.

The idea is to to get as much heat up the chimney as possible so it need to be very well insulated. To do this I made an outer container from some scrap sheet metal and riveted it to the outside of the cement sheet. Then the perlite/cement stoves I made originally were crushed (they were very crumbly) and recycled with some more cement and a little water and poured into the cavity. The chimney was placed onto the rocket stove. Chicken wire has since been wrapped around the outside and will be coated with a cob mix to make it look good.

The thinking at this point was to finish the earthbag construction and pour a suspended concrete slab around the chimney with the flue sticking out the top just under the height of the clay pavers that will be used as the base of the oven. A recycled barbecue plate with feet welded on it will be placed over the top of the flue and this will be the main cooking area for pizzas.

When I was completing the earthbag construction I made some fabric tubes half the width of the normal ones (350mm) these I placed around the edge of where I wanted the slab to be. This formed an earthbag platform on which I could place the steelwork. These earthbags I pinned to the wider ones using reo offcuts driven vertically through the bags.

The slab was poured leaving a 100mm space between the top of the slab and the top of the earthbags. This was filled with glass bottles and sand making an insulated floor to the pizza oven.

Next installment will cover the rendering of the wall and building of the pizza oven,


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