BC) word ḏbt "mud brick" (with vowels unwritten). Middle Egyptian evolved into Late Egyptian and finally to Coptic (c. 600 BC), where it appeared as ⲧⲱⲃⲉ tōbə. This was adopted into Arabic as aṭ-ṭawbu or aṭ-ṭūbu, with the definite article al- attached to the root tuba. This was assimilated into the Old Spanish language as adobe , probably via Mozarabic. English borrowed the word from Spanish in the early 18th century, still referring to mudbrick construction. In more modern English usage, the term adobe has come to include a style of architecture popular in the desert climates of North America, especially in New Mexico, regardless of the construction method. Composition An adobe brick is a composite material made of earth mixed with water and an organic material such as straw or dung. The soil composition typically contains sand, silt and clay. Straw is useful in binding the brick together and allowing the brick to dry evenly, thereby preventing cracking due to uneven shrinkage rates through the brick. Dung offers the same advantage. The most desirable soil texture for producing the mud of adobe is 15% clay, 10–30% silt, and 55–75% fine sand. Another source quotes 15–25% clay and the remainder sand and coarser particles up to cobbles , with no deleterious effect. Modern adobe is stabilized with either emulsified asphalt or Portland cement up to 10% by weight. No more than half the clay content should be expansive clays, with the remainder non-expansive illite or kaolinite. Too much expansive clay results in uneven drying through the brick, resulting in cracking, while too much kaolinite will make a weak brick. Typically the soils of the Southwest United States, where such construction has been widely used, are an adequate composition. Material properties Adobe walls are load bearing, i.e. they carry their own weight into the foundation rather than by another structure, hence the adobe must have sufficient compressive strength. In the United States, most building codes call for a minimum compressive strength of 300 lbf/in2 (2.07 newton/mm2) for the adobe block. Adobe construction should be designed so as to avoid lateral structural loads that would cause bending loads. The building codes require the building sustain a 1 g lateral acceleration earthquake load. Such an acceleration will cause lateral loads on the walls, resulting in shear and bending and inducing tensile stresses. To withstand such loads, the codes typically call for a tensile