Earthbag wall and cob pizza oven part four. Building the pizza oven

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.

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!

Earthbag wall and cob pizza oven part 3. Truth window and rendering the wall

Once the wall was finally built, the suspended concrete slab poured and the rocket stove built it was time to build a truth window and render the earthbag wall. I will update this post as I put successive layers of render on.

A truth window is often built into straw bale houses so that people can see how the walls were built. I decided to build one into the earthbag wall. I built a wooden frame and inserted lengths of reo bar in the back of it which were driven into the earthbags. After construction I taped up the front surfaces to protect them from any render that I might get on it. The window will have a hinged front with polycarbonate so you will be able to see how it was constructed.

truth window construction

truth window construction

truth window

truth window

For the rendering I would have liked to use a lime render but here in Tassie we have a lot of rain and it will be out in the openAfter m so after much research I decided to use a render ratio as follows

First coat:

  • One part cement
  • One part lime
  • 4 parts putty or plasterers sand (You need a sand that is angular and not rounded so that it locks together)

I soaked the lime for about two weeks in an old plastic dustbin. this ensures that the lime fully hydrates. Is is so fine that it takes time to take up the water. Wearing mask, goggles and gloves to protect me as lime is very alkaline and can burn I half filled the dustbin with water and then added the 20kg of lime. Using a drill attached paint stirrer I mixed thoroughly and then added more water as needed. Checked it every few days and stirred it well before use. Always leave an inch of water on the top to keep it well hydrated.

1st coat render

1st coat render

1st coat render 3

1st coat render 3

The first coat really started to fill in the gaps between the earthbags. I made sure that I cross hatched the surface so that the next coat would have a good key to stick to.

Second coat: I read that to help ensure that layers of render do not de-laminate that each successive layer should be weaker than the last. To this end I made the mix

  • One part cement
  • One part soaked lime
  • Five parts plasterer/putty sand

To ensure that cracking did not happen I added a layer of fibreglass mesh between layer one and layer two.  The mesh was about 200mm wide and I found this was a good width to render onto. Any big gaps between the earthbags were filled in before the mesh was put on. I kept the walls damp to slow the drying time as it was reasonably hot (even for Tassie). Note the edge of the render mesh in picture below.

2nd coat render 1

2nd coat render 1

close up render mesh

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,