A horno is a type of outdoor oven made from adobe or other natural clay mixtures in the shape of an elongated dome. Although the word "horno" is the Spanish name for the oven, it has been found at archeological sites around the world, including Africa and Hungary, where it is known as a kemence and is still occasionally used in the early 21st century. The hollow interior of the structure usually has a flat cooking surface on the bottom where fire or coals are stoked until hot and then removed, after which the food to be cooked is placed inside the horno. The opening to the inside of the oven can be closed for the cooking period by covering it in mud or, for more modern implementations, by positioning a door over the hole. The properties of the adobe used to make the oven allow it to retain a large amount of heat and maintain a good level of moisture within the oven despite the dryness caused by the initial fire.
There are two common ways in which a horno can be constructed. The first is to build a mold or framework from wood or another material in the distinctive curved shape of the oven. Adobe can then be pressed into the mold, allowing a seamless base to be constructed, after which individual bricks can be stacked and sealed with more clay to make a solid surface. This method can work very well but also might cause flaking on the inside of the oven if the adobe is not formed or cured correctly.
The second method that can be used to create a horno is to slowly stack adobe bricks in a circular pattern, working up from the base. As each layer of bricks is laid down, mud is used to fill in the gaps created between each brick. This method potentially takes longer to implement but can result in better heat retention and a longer lifespan than other methods.
The shape of a horno is designed to allow heat to build up inside and then be evenly dispersed throughout the interior of the oven. Uneven or more sharply cornered shapes tend to allow heat to accumulate in certain areas, potentially causing uneven cooking. The porous nature of the adobe in the walls of the oven also will naturally absorb moisture from the outdoor air and then slowly disperse it as the heat penetrates into the material, creating an environment that is well suited for making breads and other dishes that need some moisture while cooking.