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			Building an Horno: the Adobe Bread Oven MICHAEL MOQUIN Learning how to build the adobe bread oven (or horno as it is called in Spanish-speaking parts of the world) is an ideal introduction to sun-dried mud brick construction, mud mortar and mud plastering. These are the three basic skills needed for any adobe construction. In addition, one also may gain experience in small dome (3'-6' high) construction, which could lead to larger, more skilled adobe projects. The process of making the adobe bricks defines this method of earthen construction. In contrast to cob construction, where walls are directly hand formed by shaping thick layers of mud; adobe walls, domes and vaults are made of individual sun-dried mud bricks, bonded together with mud mortar. One of the attractive advantages of building with adobe is that suitable material is most often right under the workers' feet. In New Mexico and other desert states, such a soil is mostly sand (70-90%) with varying amounts of clay and fine silts totaling from 10% to 30%. The individual sand grains (coarse and smooth) provide the "structure" which is bound together by the very fine and sticky clay particles. An ideal proportion for adobe bricks and mortar is 70% sand and 30% clay. Another advantage of working with adobe is the relative speed in which house walls can be built, seven courses a day for walls 10" or 14" thick, and three courses high each day for walls 20"-28" thick during sunny hot weather. For strength and durability, as well as aesthetics, the adobe brick and mud plasters should have a low number of small shrinkage cracks, which often occur during drying. The New Mexico adobe code specifies that each brick may have no more than three cracks, and no crack can be greater that two inches in length or one-eighth inch in width. One of the simplest yet most most important field tests for characterizing the adobe soils used to make bricks or plaster is to perform the "jar test" by filling a clear (glass) container half way with dirt, then fill it the rest of the way with water. Shake it well and then let the material settle out. The largest particles settle first, such as gravel and then coarse sand. After about an hour, further gradation results with layers of fine silt and then clay at the top. By visual inspection one is able to estimate the relative proportions of clay to sand. After having made a few test adobes and doing the jar test, you will know if you need to add amendments such as straw or more sand, if there is too much cracking, or more clay if the adobe is too sandy and weak. The standard sized adobe for building in New Mexico is 10" wide, 14" long, and 3.5" thick (the width of a wooden 2"x4" form). These are usually produced at the local "adobe yard," though many are made by owner-builders. However, the adobe walls of the horno (5"-8") are only half the thickness of adobe home walls. And the only way to procure such a diminutive adobe unit is to make it yourself! Crafting the Horno There are two ways of forming each adobe for the walls of the horno: with a wooden form, or directly shaped by hand. A slurry of mud is placed in a form and left to set for awhile. The form is then removed and the adobes left to dry in the sun. By use of the form (approximately 4"x7"x 2.5"), each adobe will be the same size (with more complex angles if desired), which allows simple mortaring and assemblage for making the round shape. This formed adobe would ideally be pie shaped (and truncated at the narrow end). The hand-moulded adobe is actually the simplest and most ancient method, (dating back at least 10,000 years to in the Middle Eastern town of Jericho.) This creative and fun method works best in rainy areas. Not only can you shape an adobe that will be thinner and thus dry much faster, it will also bond better with the mortar and stay in place (as on an incline at the top of the dome). In reality, there are many ways of building an adobe oven. Following the simple rule of using materials that are closest at hand, you have freedom to create in adobe and stone. To successfully construct the horno, it should be built in phases to allow drying to occur for stronger support of the next overlaying courses. An elevated foundation is needed to keep the oven above the potentially wet ground, and as a convenience when cooking. This can vary in height from one to three feet, with the width of the oven being 2' to 6'. Located away from ponding areas (standing water) with some drainage, stones or adobes are placed in mud mortar at ground level to begin the massive base. Sometimes for the taller bases, this beginning phase should take two work sessions (for a longer drying period). Now the small dome of the oven can be started on the top of the platform hearth. The shape of the floor plan is circular with an opening for the fire wood, bread and pies. The dried adobes are placed in mud mortar, positioned and leveled. The layers of mortar can vary from 1/2" to 1" thick to accommodate irregularities in the adobes. There are two ways of making the inward curving walls which close together at the top of this small dome: by laying the adobes flat and corbeling inward, or by inclining adobes at an increasing angle as the horno rises up and closes at the top. I find that when teaching at elementary schools, either method will work, though the corbeling method is usually simpler. By laying each course of adobe flat in mortar but projecting 1"-2" over the course below it, the small dome is formed as the courses close in on themselves. This mud oven should be built in two or three sessions so as to allow drying. The horno opening will close in the shape of an arch, but will slope inward with each succeeding adobe course. Sometimes you may need to use a stone lintel (flat beam) to bridge the gap at this point. At this stage the oven shape is weakest, so it is important to stop building for a few days so it can dry better. It is now time to lay the last (3-6) adobe courses to close the top of the oven into a dome. Near the top, on the side away from the prevailing wind, put an opening the width of a soda can (laid in the adobe but later removed) to allow for the fire smoke to escape. As these small walls go up remember to mud plaster the interior walls while you can more easily reach them. To complete the building of the horno, it must be mud plastered on the outside. Again, it is best to wait a number of days for the adobe work to dry. Begin by shaping the adobes and dried mortar to a pleasing shape. A stick wrapped with expanded metal lath makes a good shaping tool. Brush off the loose adobe chips and dust to insure better bonding with the mud plaster. Wet the surface (sprinkle water with whisk broom) to further ensure a good bond of the mud plaster to the adobe walls. The mud is applied by lightly throwing it on, and then after an area is covered, sprinkle water on the plaster and then spread the surface smooth. If this first layer of mud plaster (which is about 3/4" thick) cracks too much, give the horno a second coat of plaster (1/4" thick). Before putting the fire in the horno, it is best to let all the mortar and plaster be dried. The first fire should be small and brief so as not to crack the walls of the oven. Now the horno is ready to be used. Build a large fire in the oven for about an hour. Sweep out the coals and ashes and place the bread dough inside. To enclose the heat, stop-up the smoke hole with a rag and close over the oven opening with a large stone and mud, or with a piece of wood with mud sealing the edges. The thermal mass of the adobe walls will store enough heat to cook the food for 1 1/2 to 2 hours. Your bread is ready! Michael Moquin is a preservation craftsman, a mud plaster artisan and the editor of the Adobe Journal. PO Box 7725, Albuquerque NM 87194
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