How to make a cob pizza oven

In this article we will share with you the story of how we built our pizza oven entirely from cob (mix of clay, sand and fiber). We hope our experience, tips and thoughts will be a good guide for you to make your own.

Choose the right spot and size of your cob pizza oven

To make sure you enjoy maximum fun and comfort when using your pizza oven later, you need to consider certain aspects before you start building.

• It’s a good idea to think about the right spot carefully, since the pizza oven is obviously not to be moved.
• Also you might consider to put it up to a height that makes it easy for you to operate it later.
• Further, your base or counter needs to be able to hold the significant weight of your oven.
• Finally you should decide how many pizzas you want to be able to fit in and how frequent you will use it. This will help you to decide on the right size for you.

We chose to place our oven on top of a counter which we built from cement blocks. The countertop is a poured concrete slab with some steel rods in it. This way we have a good height for easy access to the baking chamber later and at the same time we created a storage compartment for firewood under the oven. On one side we added another little shelf from clay blocks and poured slabs as a space to roll out the dough and to place out the toppings.

We figured about 120 cm in diameter as a base should give us a final baking chamber of around 80-90 cm in diameter. This will be a sufficient space to place two pizzas next to each other and still have space in the back for the fire. We started with a ring of cob (looking actually a bit more like an omega) which serves as a base for the oven walls. Inside the ring would be the floor of our baking chamber. If this is your first experience making cob and you don’t know how to mix it, here is a great page of one of our friendly projects, Quinta do Vale, where you can read all about cob recipes:

https://www.permaculturinginportugal.net/cob-and-earthen-plaster-recipes/

The insulated oven floor

It is important to insulate the floor, so it can warm up faster and stay warm longer. Since this insulation will be exactly under your tiled floor of the baking chamber, it needs to withstand great temperatures and a certain pressure. Therefore there are only a few options if we want to use inexpensive rather natural or recycled materials. We chose expanded clay, which is easy to work with and cheaply available. If you have the option to choose the size of the expected clay grains, we recommend you to go for a smaller, finer selection. This way it’s easy to spread the loose material evenly inside of the ring.

We placed a layer of aluminum foil over it with the only purpose to make tiling easier. 1 cm of fine sand held the foil in place and gave us an even ground to put the tiles on top. We laid out the tiles diagonally to minimize the chance of bouncing against a step from one tile to the other, when we will slide in the pizzas later. Also the whole floor of the baking chamber is even but slightly tilted down towards the opening. Just 1cm will be enough. This will help us later to clean out the ashes. We already built an arch from solid oven stones as well that shall become our “oven mouth”.

Building the body of the cob pizza oven

Now we are ready to build up the body of our cob pizza oven. We want to create a dome with a slightly flat top. To do so we started laying out fist sized balls of cob along our ring. With every row we also filled the inside of the ring with moist sand. You can also build up the sand completely and then place the cob balls over it. Make sure to leave the opening for a chimney near your oven mouth behind the top of the arch. We chose the 110mm in diameter chimney, since we already had some pipes this size and it turned out totally sufficient. You can also choose a flue pipe with a flap. This can be useful when you also want to bake bread.

Expanded clay insulation and the final plaster

Now we apply an insulating layer. Here we mix the expanded clay directly into our render mud. It should be more expanded clay than plaster, just enough to get the clay grains coated and sticky. When all has dried a bit we can apply a final layer of fine render. If you choose only cob here as well, you will have to keep your oven well sheltered from the rain.

Now it’s time to wait. At least days (in summer), better weeks or even 1-2 months till your oven is fully dried through. Only then it’s the time for a final step. You can paint over your render with linseed oil. This will form a crust, so your cob will not crumble or dust anymore and it will also get water repellent to a certain degree. If you oil the final render, you should apply 2-3 coats one after another, to soak the oil deep into the clay. A fourth layer after a couple of days, will give extra water resistance and is recommended.

Ready to use

Last but not least you can now finally light up your oven. You would usually start with a small kindle fire close to the opening, just under the chimney and then slowly add bigger dry hardwood logs as the oven gently gains heat. When you feel it’s nicely warm all over the outside, your new cob oven is ready for pizza!