Horno Construction Techniques at La Purisima Mission State Park
By Joe McCummins
A base for the horno is constructed of adobe bricks, bricks or stone. The outside of the base is plastered. The top of the base is paved with ladrillos (floor tiles).
The diameter of the horno interior is marked on the base top. A form is constructed to create the door opening and placed at the desired location on the base. The attached photos show construction using red fire bricks. To provide the additional thickness needed for the horno, two more layers were added on the outside of the fire bricks. The first layer consisted of adobe mud and broken ladrillos. The second layer was composed of adobe mud and broken tejas (roof tiles). The use of ladrillos for the construction of the horno walls would eliminate the need for the two additional layers of mud and broken tiles. Also, the wider ladrillos make it easier to shape the horno dome.
The walls are laid up to a height that it is easy for the builder to reach inside and plaster the interior with adobe mud. This lower portion of the interior is plastered with adobe mud. A piece of plywood is cut to the diameter of the horno interior just below this point. The plywood is then cut in half. The two halves are placed against the interior wall and supported by bricks or wood blocks to support the construction of the dome. Sand is then placed on top of the plywood and mounded to the desired dome shape. Wet adobe mud is place on top of the sand. Bricks or ladrillos are laid to finish the top of the horno making sure they are placed in the adobe mud on top of the sand form. The adobe mud on the sand dome becomes the interior mud plaster, which reduces the labor of laying inside the oven spreading the mud plaster after the horno walls are finished. If bricks are used in the wall construction add the two additional layers of adobe mud and broken tiles mentioned above.
A vent hole must be placed in the back wall opposite the door. The hole is placed about a quarter of the way down from the top of the dome. The hole is about four inches square and slightly tapered from the outside. This hole helps to draw the air through the horno during firing and is plugged during cooking.
The outside of the horno is covered with a layer of adobe mud plaster. A final layer of lime plaster or white wash is added to help seal the exterior of the horno.
The dome support can be disassembled through the horno door by removing the bricks or wood blocks holding up the plywood. The two halves of the plywood circle and sand can now be removed from the horno interior. Touch up plastering and sponging (smoothing) of the interior mud plaster can now take place.
A thick (2-3 inches) wooden door is constructed to fit the door opening. Allow some space around the door edges for the wet burlap that is wrapped over the interior door surface during cooking. A wooden plug is needed for the vent hole in the back of the horno.
Allow the horno to dry. Then build a very low fire inside the horno for a couple of days to slowly dry the walls. Excessive fire/heat or heating too quickly is the greatest danger to a horno. The heat will cause moisture trapped in the walls to turn to steam. If the steam has no outlet it then cracks the walls. It takes time to heat the thick oven walls. Cracks will appear in the horno even with careful firing. This does not mean the horno is ruined but that it is used. These cracks will expand and contract with heating and cooling.
We have found that building a low fire in the horno the day before an event helps to drive out the moisture thus reducing the cracks and preheats the horno. We allow three to four hours to heat the horno for cooking. With practice and experimenting we have found that feeling the side of the oven about a third of the way up on the side gives us a pretty good idea of the horno temperature. We place a hand flat on this spot. When the wall is warm at this point, the horno is usually heated sufficiently for cooking. The coals and ash are removed from the horno. Take care when removing the coals and ash to not damage the lower interior plaster with the rake. Allow the horno to sit a few minutes without the door then, close the door and plug the vent hole. This allows the heat to even out in the horno. After, fifteen to twenty minutes open the door and use either corn husk or corn meal to test the interior temperature. The husk or meal should brown in about ten to fifteen seconds at the correct temperature. If they brown very rapidly, the horno is too hot. Let the horno stand with the door open for awhile. Retest until the corn husk or corn meal browns at the proper rate. If it takes too long to brown or doesn't brown replace the coals and add more wood until you heat the horno sufficiently. Don't rush!
La Purísima docents baking bread in a horno