ADOBE

a heavy clay soil used to make sun-dried bricks. The term, Spanish-Moorish in origin, also denotes the bricks themselves. Adobe clay is basically calcareous and sandy, a mixture of clay, sand, and silt having good plastic qualities and drying to a hard uniform mass. Its use, or that of clays with similar properties, dates back thousands of years in several parts of the world, especially areas with an arid or semiarid climate. Techniques varied, but all took advantage of the special properties of adobe. This use of earth for building construction resulted partly from the scarcity of wood as a building medium, partly from the ease of such construction, and partly from its insulation value against both heat and cold. Adobe or sun-dried bricks are found in the Old World in the dry areas east of the Mediterranean Sea, in North Africa, and in southern Spain. In the Western Hemisphere adobe appears in many of the pre-Columbian sites from the American Southwest to Peru, always, again, in those regions with a dry climate. The American Indians built walls by hand manipulation of the plastic clay into courses, allowing each course to dry before adding the next. In the 20th century, adobe also became a fashionable construction material in the American Southwest. Sometimes the bricks were shaped by hand and were more or less loaflike. Molds for shaping the bricks, probably introduced into Spain from Africa, came to the New World at the time of the Spanish conquest. The usual modern method of making adobe bricks consists of wetting a quantity of suitable soil and allowing it to stand for a day or more to soften and break up clods. A small quantity of straw or other fibrous material is added, and the materials are mixed with a hoe or similar implement. The mass is then trampled with the bare feet. When it is brought to the proper consistency, the adobe is shaped into bricks in simple molds. The molds, made of smooth lumber or sheet metal, are four-sided and open at the top and bottom. Although they vary widely in size, depending on the intended use of the finished brick, they are usually from 8 to 13 cm (3 to 5 inches) thick, 25 or 30 cm wide, and 35 to 50 cm long. The bricks are allowed to dry partially while flat on the ground; then they are stacked on edge to permit more thorough and uniform drying. The bricks are not used until thoroughly dry. This usually takes at least two weeks under average dry climate conditions. The addition of straw or similar material helps to prevent shrinkage cracks during the curing process but does not add any structural advantage to the finished product. Adobe walls are normally built on a solid, waterproof foundation of stone or concrete; otherwise the capillary action of groundwater may cause the lower courses to disintegrate. The bricks are laid in a mortar of the same material, then finished with a coat of adobe or with lime or cement plaster. With proper construction and maintenance, an adobe wall may last centuries.