Cob Pizza Oven

A clay oven is a great feature that allows you to bake your own delicious pizzas, breads, cakes, and whatever you can think of. Charlotte and I just built one for the Wangunsari Permaculture Project in Bandung that turned out beautifully – so if you would like to build your own here is how we did it!

Materials

  • Some gravel (optional)
  • Materials for the base (such as large rocks)
  • Clay
  • Bricks (you will need roughly 40)
  • Sand
  • Straw (wheat straw, rice straw, or whatever you can find)
  • A tarp (preferably one you don’t mind getting dirty)
  • Some sheets of old newspaper

Step 1 – Designing your oven

Before you can start building you will need to make up your mind about where you will place your oven and how large you want/need it to be. You should make sure to place it in relative location to where you will eat your pizza and/or bake your bread so probably close to your terrace or the kitchen. Evidently the size of your oven will determine how many pizzas or breads you can fit in at once – keep in mind though that a pizza in a properly heated cob oven only takes about 2 minutes until it’s ready! Usually for non-commercial use an inner diameter of roughly 80cm should do the trick. Plan in another 30-40cm to account for the oven walls (15-20cm on each side) and you will know how large your base needs to be.

Step 2 – Building the base

Cob Pizza Oven BaseOnce you’ve decided on a location and size it’s time to build the base. Dig a roughly 20cm deep hole into the ground that has the shape of your base (so probably round), fill in some gravel (if you have some, if not don’t worry about it) and start piling up your rocks for the base to form a circle. As the outer walls of your base build up you can fill in the middle with spare rocks and gravel. Keep going until your base is stable and roughly hip high (or however high you want your oven to be, just keep in mind that we’ll be adding another 5-10cm of clay and a layer of bricks on top of th base) and more or less even on top.

Step 3 – Testing, mixing, and stomping the clay

2015-09-11-12.17.22Now it is time to get your hands dirty! You will need to mix your clay with sand, straw, and water in order to get the cob mixture that will become your oven. What is the ratio between clay, sand, straw, and water? That depends on the clay content of your clay: if you have a fairly high clay content you will be using roughly 50:50 or 40:60 clay to sand, and if your clay is rather sandy already you will be adding less sand. If you are unsure about the clay content of your clay you can test your clay, but in my experience cob is a fairly forgiving mix to work with and as long as you don’t use pure clay or almost no clay you should be fine! So, put out your tarp and throw a good amount of clay, a good amount of sand, and roughly the same amount (volume, not weight!) of straw on there. Pour a little bit of water on top (not too much at first) and start stomping!

Charlotte Stomping to Techno MusicYou will be stomping for a while now (Charlotte and I put on some techno music to keep us motivated!), how long exactly depends on the amount you are tackling (doing smaller amounts at a time is a lot easier). Keep adding water if you feel like your mixture is too dry and keep folding the cob like a dough by pulling in one side of the tarp at a time. Your mixture should not be too soggy nor too dry. To test for consistency smash a handful of cob on the ground: if it splatters it is too moist and you will need to add more sand, if it does not flatten from the impact it is too dry. Keep stomping, folding, and adjusting the moisture until you have one solid mass with no pockets of sand and straw left – now your cob is ready!

Step 4 – Building the oven surface

Surface LayerYou will use your first batch of cob to build a flat surface onto your oven base. To do this, form roughly hand-sized balls from your cob-mixture and start smashing them onto the top of your rock base. Once the entire base is covered in layer of cob you can use your fingers and knuckles to compact the clay into a uniform mass, smashing on more clay in uneven places to make the whole area as level as you can. The surface should remain nice and wobbly (from your fingers and knuckles), however, to give grip for the next layers. Once you are happy with your surface layer you can make the edges look pretty already (see the picture on the right), then it’s already time to add some bricks.

Brick LayerIf your cob surface is already quite dry you may want to sprinkle some water on it first, then you can start laying out the bricks. Make sure to make your brick base large enough to cover the entire inner diameter of your dome, as this is the surface on which your pizzas and breads will be baking. Also account for the area where the entrance will be and extend it outwards (see picture on the right). Add one brick after the other, always making sure that each brick sits tightly on the cob layer (stuff some more cob underneath if a brick does not sit properly). Once all bricks are in place you can spread a very soggy clay-sand mix onto them to fill in gaps in between bricks.

Step 5 – Building the arch

The ArchThis is the tricky part! Think about how high you want your door to be (it should be roughly 70 percent of the inner dome height of your oven) and build yourself a temporary construction that will hold your dome while you build it: we used two bamboo slats, which we stuck in between the two lowermost bricks of our dome, but you can also cut out a support construction from wood – get creative! Once your support is in place you can start building up your dome, using cob in between the bricks as mortar. This is not as easy as it may sound, just be patient and keep on trying!

Step 6 – Building the dome

2015-09-14 14.23.59Once your arch is up and you have removed the supports once again you can get started on your dome! You will need to pile up moist sand on your brick layer behind your arch and shape it the way you want your dome to look: the sand represents the dimensions and shape of your inner dome. Make sure to account for a chimney at the back of your sand-dome and to connect the arch with sand as well. Once you are happy with your sand-support wrap it in newspaper, making sure that no newspaper overlaps onto your bricks at the bottom and onto your arch (if newspaper gets in between your dome will not hold onto the base and the arch).

Building The DomeYou will need to stomp more cob now, just like before, and then start applying the first layer your sand-dome. Begin by smashing cob onto the edges of the bricks that should be sticking out under the sand and slowly work your way upward (don’t smash the cob too hard, otherwise you will make dents in the sand!). Aim for about 6-8cm of thickness for this layer, make sure to leave the surface nice and wobbly after pressing in the clay with your fingers and knuckles, and don’t forget to connect the arch to your dome! Once your first layer is finished let it dry a little over night and take out the sand the next morning.

2015-09-16 15.13.43Now it is time for the second layer: stomp more cob and apply it onto your first layer. This time you can smash it hard on the surface as your first layer should be quite solid already by now. Aim for another 6-8cm here, make sure all your edges look pretty and cover all dents with some extra cob. Your oven is almost ready! You need to let it dry for at least a few days now (some people say 3-4 weeks, but in my experience 6-10 days is plenty). Once the cob has dried out you can start lighting a small (!) fire, which you keep going for several hours. If cracks appear you can simply seal them with a clay-sand mixture. Light a slightly bigger fire on the next day, an even bigger one the day after, and after 3-4 days of fires, gradually increasing in size, you can finally light a big fire for 1-2 hours and start baking!

Step 7 – Start baking

Freshly Baked PizzaTo heat up your oven you will need to have a big fire going for at least 1-2 hours. Once the oven is hot and there are no more flames, push the ambers to the back and the sides of your oven and clean remaining ashes off the brick surface with a besom (do not use a plastic broom as it will melt!). You can now put in your pizzas!

To get you going with your new oven we will also share our secret recipe for an incredibly tasty pizza with you. Be warned though, the dough will take at least 2 days to mature, so plan ahead! For about 4 large pizzas you will need the following:

Ingredients

For the dough
  • 1kg of wheat flour
  • Some yeast (1 pack)
  • Some water
  • 2 tbsp. of salt
For the tomato paste
  • 200g of tomato paste
  • 4 cloves of garlic (chopped or pressed)
  • Italian herbs (a handful or so)
  • 1 tbsp. of salt
Toppings of your preference

Try spinach, feta, onions, gouda (you’ll have to briefly cook the spinach first) or champignons, bell-pepper, capers, gouda.

Preparation

  1. Mix 1/2kg of flour with your yeast and approximately 700ml of water (use a kitchen machine or a mixer) to get a very liquid dough.
  2. Cover the dough with a kitchen towel and let it rest for 1 hour.
  3. Now mix in another 1/2kg of flour as well as the 2 tbsp. of salt.
  4. Cover your dough with parchment paper and let it rest in the fridge for at least 2 days (you can keep it for up to 2 weeks).
  5. Once your dough is ready you can prepare the tomato paste by combining all ingredients.
  6. Roll out the dough on lots and lots of flour (preferably directly onto a baker’s shovel), spread your tomato paste on it, add your favorite toppings and off goes your pizza into the oven.