Monday, October 1, 2012

Earth Ovens: A Great Introduction to Adobe

Traditionally called hornos in New Mexico, building an earth oven is a great way to learn the basics of adobe brick construction over the course of a small 3-day project. In addition to breads and desserts, hornos are great for baking pizza and cooking meats and vegetables. I always try to use natural, local materials when building an oven and have adapted the larger oven size often found in New Mexico to a smaller design which requires less fuel and time for firing.

Materials
  1. The oven foundation is usually a simple gravel pad.
  2. The square oven base is made of sun-cured adobe bricks.
  3. The oven floor can either be made of troweled mud or fire brick.
  4. Clayey soil taken from the foundation dig is used to construct the oven dome. Depending on your soil, additional coarse sand might also be needed.
  5. Ovens are usually finished with natural earthen plasters which are strengthened with wheat paste or cactus juice for water-resistance.
  6. Chopped straw is needed for the dome rings and plaster mixes.
Tools
  1. tape measure, mason’s string, 3-ft. carpenter’s level, Sharpie marker
  2. long-handled, pointed-tip shovel
  3. wheelbarrow
  4. 5-gallon bucket
  5. 8” X 8” hand tamper
  6. mason’s trowel (optional)
  7. plaster trowel (optional)
  8. your hands (mandatory)
Instructions

Step 1: Choose a level spot away from overhanging trees and other structures. Dig a 60” X 60” square pad down to about an 8” depth using a pointed shovel. Fill this square with 3/4” gravel in 3” lifts. Be sure to tamp the gravel after each lift using your feet or a hand tamper.


Step 2: Construct a 60” X 60” square oven base using adobe bricks and mud mortar. Oven bases in New Mexico are usually 4 or 5 courses high but choose a height which is most comfortable for you. Using standard New Mexican adobe bricks (10” X 14” X 4”) you will need about 14 bricks per course. Be sure to always overlap the joints below when laying up new courses.


Step 3: After the base walls have dried overnight, fill the center of the base with rubble from the building site (gravel, sand, adobe brick chunks or rocks) to within about 4” of the top of the base. Add the rubble using 3” lifts and be sure to tamp each lift well using your feet or a hand tamper before adding the next lift. Finish this step with about 2” of coarse sand on top of the rubble below if you are planning on continuing with a fire brick floor. Note: Some people also add a layer of insulation below the oven floor. Bottles covered with a straw clay mix work well.


Step 4: Install a fire brick on sand oven floor using about 35-40 bricks. The brick floor can be flush with the top of the base or can protrude slightly higher if you wish. Use mud mortar to fill in any gaps left around the sides of the brick floor.Instead of using fire bricks, you can also simply trowel mud mortar to the height of the base to create the oven floor. Once the floor is in place, find the center point of your floor and draw a 32” diameter circle on it using a Sharpie marker or a piece of colored chalk. Note: The diameter of the oven cavity can vary but 32” works well for this base size.


Step 5: Install a sand form to create the oven cavity. Begin piling slightly wet sand in the center of your circle and compact it with your hands. Try to create a beehive shape being sure to bring at least the first 4” of the sides straight up before slowly curving inwards. The sand form should have about a 32” diameter and should be roughly 19” high. Note: The height of the oven cavity can vary but 19” works well for this base size and oven diameter.


Step 6: Create the thermal layer of the oven by wrapping a 3” thick layer of mud around the sand form. Start at the bottom and lay a complete circle around the sand form that is about 3” high and 3” thick. Continue with subsequent 3” X 3” rings until you have closed the dome at the top. The mud mix for this thermal layer should have a high clay content. A bit of chopped straw will make it easier to shape the mud ring and keep it in place. The mud should be thoroughly mixed with water but not too wet (cob consistency). You will know if your mix is too wet if the rings begin to slump as you place them. Thicken the mix with more dirt and a bit of straw if this happens. Once the layer is finished and still wet, use the tip of a mason’s trowel to scratch the surface to allow the next layer to key into it better. Finally, use a toothpick to draw the oven door outline. Good door dimensions for this size oven are 14” wide by 12” high. Once the line of the door has been marked and you are happy with the shape, use the tip of the mason’s trowel (or a knife) to cut the door shape out. Leave the cut section in place until the next morning.


Step 7: After 1 night of drying, remove the door cutout with your hands. The material can be broken up and added to your next mix. Continue by using a combination of your hands, the mason’s trowel and a small shovel to remove the sand from the interior of the oven. Be careful not to damage the inside of the thermal layer when removing the sand. Work slowly and use your hands to scrape the sand off of the interior walls of the oven. Incorporate this sand into your mud mixes for the next oven dome layer.


Step 8: Create the insulation layer of the oven dome by wrapping a 5” thick layer of mud around the 3” thermal layer. Start at the bottom and lay a complete circle around the existing thermal layer that is about 3” high and 5” thick. Continue with subsequent 3” X 5” rings until you close the dome at the top. The mud mix for this thermal layer should have a lower clay content (more sand) and lots of chopped straw. The straw will make it much easier to shape the mud rings and keep them in place. The mud should be thoroughly mixed with water but not too wet. As with the thermal layer, you will know if your mix is too wet if the rings begin to slump as you place them. Thicken the mix with more sand and straw if this happens. Once the layer is finished and still wet, use the tip of a mason’s trowel to scratch the surface to allow the first plaster layer to key into it better.


Step 9: Once the oven dome is completely dry, finish your horno with 3 coats of mud plaster applied with your hands or with a plaster trowel. The plaster mixes are essentially the same as the mud mixes used to construct the oven dome but they will be wetter so that they are easier to trowel on. I like to add lots of chopped straw to my mud plasters and also sometimes add wheat paste or cactus juice to make the plaster more resistant to rain and snow. Plan on touching up the plaster once a year to keep the oven looking nice.
Questions or more help? I offer horno construction workshops at various times during the year throughout the Southwest of the US. Please contact me to find out when the next live instruction or internet class begins. The internet version of the class contains a series of videos covering the 9 steps outlined above in more detail as well as introductory videos on selecting and testing soil and making your own adobe bricks.