Build an earth oven in your backyardJul 12th, 2011 | By Editor | Category: Skills
by Chris Elder
People have been constructing earth ovens for thousands of years. The process for building these ovens, also called hornos, has remained unchanged, except we presently have a few more tools at our disposal.
The most abundant and most commonly used building material on earth IS earth. Clay is the key ingredient for that mixture of earth used in constructing cob houses, earth plasters, adobe bricks, and earth ovens. When beginning construction of your earth oven, try to find a patch of clay that is dense and hard when dry and sticky when wet.
To start building your earth oven, you need to choose your site. Where is a convenient spot for cooking? Is it near your kitchen or outdoor eating area? Are there any good areas already offering protection from the elements? Where is the woodpile located? Is the site level?
Once you’ve determined the best site for your oven, it is time to build your base. The economical base can be built from any available rock you have access to. For example, urbanite (chunks of old concrete) is a readily available source of building materials that offers an artistic, environmentally minded, and usually free option. Build your base up to the level that is most comfortable for you, with 36 inches above grade being a recommended level to start. Your foundation needs to protect against moisture from the ground, support the weight of the oven, and keep the structure rigid. A standard oven base width is 48 inches, and all specifications in this article are based off of this initial measure.
Insulating the sub-floor comes next. There are several options for sub-floor insulation materials. The least expensive option is using empty glass bottles mortared with a mixture of sawdust and clay. Mineral insulants such as pumice and perlite are also good options and available at local landscaping or building supply stores. When building this insulation layer remember to make a containment ring around the insulation material. This containment ring should be as tall as your insulation layer is thick, and can be constructed with a basic mud mixture of one part clay soil, two to four parts sharp sand with straw or sawdust added for extra structural strength.
The next layer before the hearth is known as the heat sink layer. This should be made from a dense material. Sand has traditionally been used, but a mud mixture like what will be used for the walls of the oven is possibly more dense with fewer air pockets than sand. Construction of this layer would be aided by first building a containment ring around the whole layer. Next add the earth mixture, making sure it is big enough for all hearth bricks to fit on top. The area between the earth mixture and the containment ring can be filled with more insulation materials. Remember that when finalizing this layer the top should be screed off to be as level as possible, as the fire bricks will be placed directly on top.
Setting the floor bricks is the next step. Firebricks are the norm for earth oven construction, but regular house bricks, especially pre-WWII brick, also work well. Pizza stones or unglazed saltillo tiles can also be used. When using bricks remember to place all writing and chips down so as not to create any pockets when ash and grit can hide. Generally a thin layer of sand can be helpful in getting the bricks to come together to create a level surface. Remember not to wiggle a brick once it is set.
Now we get to make the void of the oven. Using a string tied to a pencil draw a circle of the oven in a smooth line all the way to the outside edges of your hearth bricks. Once you begin to build the sand form it is important to allow enough time to cover it with the first layer of oven mud, as a cold joint might invite future cracking. Start piling moist sand out to the mark, keeping the walls near vertical at first. Keep the edge where the sand meets the bricks defined using a mason’s trowel or comparable tool. Typical dome height is 16 inches, as much higher might create cooling eddies and much lower may make it difficult for the fire to burn well. A trick for monitoring height is to insert a 16-inch twig into the center of the form once it is partially built. A finished dome should be solid, well-packed, and even. Add a layer of wet newspaper to help with later removal of the sand.
We have finally made it to the oven mud part of constructing your earth oven. The recipe is one part clay building soil, two to four parts coarse concrete or sharp sand, and water. Add enough water to make a firm dough. Using a tarp is possibly the easiest and most fun way to make your oven mud mixture. Pour the sand in a circle around the center of the tarp and dump the building soil in the center. Add water as needed. Now jump on the tarp, barefoot or with boots on and mash it up. Pulling on one side of the tarp can help re-center the materials for further mashing. Once the mix is ready, press handfuls around the base of the oven. Remember that this mud is what holds the hearth bricks in place. This layer should be 4 inches thick. Keep going until the whole form is covered.
Cutting out the door comes next. The door height should be 63 percent of the height of the sand form. For a 16-inch form, this would yield a 10-inch tall door. Door width is typically half the oven diameter, or 11.5 inches. The door cut should angle in like cutting the lid off of a jack-o-lantern. Use a spatula or mason’s trowel to carve a rough opening and then dig the rest out with your hands.
Layer two is the insulation layer and is simply a 4-inch thick shell of sawdust-clay or straw-clay mix on top of the dense oven mud. You can let this layer dry or continue on to the final finish layer. There are many options for the finish layer. A basic plaster recipe consists of one part clay soil, three to four parts sharp sand, just enough water, and one half part fine fiber (chopped straw, manure, cattail). This layer does not have to be very thick, and more serves to seal and beautify your structure.
The last part needed is an oven door. An oven door is usually constructed out of wood and should provide a tight fit with your oven body. Cooking with and using your earth oven is another article altogether. Good luck with the mud.
Chris Elder is a farming and food enthusiast and lives in Bellingham with his wife and two daughters. He is one of the founders of the Bellingham Urban Garden Syndicate (BUGS) and continues to grow the organization into a community asset. He farms in the county and in the city, and is a strong supporter of experimental agriculture.
You can use a variety of materials for construction, many of them from reusable or local sources, including but not limited to:
• Clay or sand
• Old concrete, rock or rubble
• Sawdust, wood shavings, straw
• Building or stone bricks
• Empty glass bottles
• Pumice or perlite
• For a more comprehensive guide on how to build an earth oven, check out “Build Your Own Earth Oven” by Kiko Denzer with Hannah Field. The book also contains information about cooking and roasting, including breads, pizzas, vegetables and meats. Kiko Denzer will be one of several presenters at the Kneading Conference West in Mount Vernon Sept. 15-17. For more information, visit kneadingconferencewest.com.
• Visit cobdesigns.net for summer cob workshops with instructor Carrie Lewis, of Bellingham, that cover the basics of cob mixing, building and more.