This article will cover the basic concept and steps necessary to build your own cob oven. This particular build incorporates the rocket system, however this is not a necessity and you could opt for building a traditional cob oven, in this case you can skip some of the steps and simply construct a solid base on which your oven will be build.
If possible, help others build their own cob oven first to gain experience, or ask an experienced oven builder to help you to achieve the best results. Consider making a “practice oven” first to familiarize yourself with the process, especially if you haven’t worked with cob before. If you aren’t satisfied, you can start over and reuse all your materials as long as you have sand, clay, and straw available to mix up fresh cob.
Wood-fired cob ovens can be built to whatever size best suits the user. Smaller cob ovens are good for cooking bread and pizza and require less time and wood to fire. Larger cob ovens are better for all-day bakes and big backyard parties.
The first step in building a cob oven is to select a reasonably flat spot to begin construction. It’s important to have good access to load wood, move coals, sweep ashes, as well as to conveniently insert and remove food.
To incorporate a rocket system in the cob oven a J-tube design must be built into the base of the oven. The floor of the oven must be designed with vents to allow the hot gasses to enter the oven.
As we used recycled firebricks we decided to finish the ovenfloor with a layer of reinforced (steel wire mesh) refractory cement.
Non Rocket Oven
Construct a solid, fire-proof foundation or platform at a height that will be comfortable to work at. The platform doesn’t have to be perfectly flat, but it should be as close as reasonably possible. Good choices for the platform include cob, compacted sand and rock, or (recycled) bricks.
Once the platform for your cob oven has been built to the desired height, cover it with a thin layer of sand. Some builders first put in a layer of empty bottles below the bricks and in the layer of sand to increase insulation, making the oven slightly more efficient. When using high quality fire resistant bricks this is not a necessity however. Stack high-density firebricks tightly over the sand to cover the entire floor of the oven.
Constructing the Oven’s Dome
Cover the brick with several layers of old (news)paper to provide an even surface to build the oven form. Pile damp sand over the (news)paper and sculpt to the dome to the desired shape of the oven’s interior. Plan for the opening of the oven to be about 60% of the interior height. Pack the sand tightly and cover the finished form with a few layers of damp (news)paper.
If you wish to install a chimney place a piece of tube on the sand dome where you want it to be. the installation of a chimney is not a necessity. If you decide to do so be sure you have a way of closing it as you will need to do so in order to retain heat in the oven.
The form is now ready to be covered by the first layer of a sand and clay mixture. Combine your sand and clay with just enough water to form a thick paste consistency. Begin covering the newspaper and sand form with this mixture until a depth of about 5 till 8 cm has been achieved. The depth of this layer (and all subsequent layers) will vary by the size of the oven. The larger the oven, the thicker each layer should be.
For the next layer mix up a batch of cob using the same ratio of sand, clay, and water but add as much chopped straw as possible while keeping the mixture pliable. Once the cob is thoroughly mixed, add a second coat to the oven until you’ve achieved a second layer with about the same thickness as the first. Allow the oven to dry out completely before adding the final coat.
For added efficiency you might add perlite to the second mix to enhance the insulation properties of the oven, this will help to retain the heat longer.
Fashion a door for the oven that will fit the oven opening as tightly as possible. The handle should be made from wood, or any other non conductive material so that you don’t burn yourself when you remove the it from the oven. The door or plug can partially block the cob oven’s entrance to regulate the incoming air and exhaust. This will give you some control over the fire and the amount of heat generated. Once the fire is nearly out, completely sealing the door will help retain heat in the oven and maintain a constant temperature.
Once all layers are dry, remove the sand from the interior of the cob oven along with all the newspaper you can. Start a very small fire inside the front of the oven. As it gets going, slowly push it back deeper inside the oven. The initial fire will burn off any newspaper stuck to the interior of the oven and drive any lingering moisture out of the oven.
Once the cob oven has dried completely, you can apply the final coat. The outer coat helps to protect the oven and can make it more attractive. You can use cob for additional thermal mass and strength, or use sand and clay for a clean, smooth finish. It is up to you if you want to leave the oven plain or incorporate decorative elements. Draw designs before the clay hardens; add tiles, glass, or beads to give your new oven an artistic flair.
Our oven will be finished with a final layer of lime and then lime washed to offer it some protection from the weather, you also could opt to build a roof over it as the cob will not withstand rain very well.
Now that your cob oven has been built, dried, and you have built a test fire, you are ready to break it in. Before starting a new fire, clean out the oven and add kindling and wood for a new fire. Get the wood going and push it back into the oven for 30 minutes or longer depending on the size of the wood. Once the oven is hot enough, use a rake or hoe to move the coals and ashes away from the center of the oven. Tie a damp rag onto a pole and use that to clean the bricks if you will be putting bread or pizza directly on the firebricks. If you are using bake ware, this step is not necessary. Adjust the door as needed to maintain the desired internal oven temperature. Enjoy your new hand-built, wood-fired cob oven