Natural Science in Archaeology

272 11 Building, Monumental, and Statuary Materials
and organic impurities. Sand in the mixture helps control shrinkage. Sharp coarse
sand is better than fine-grained or rounded types (Boudreau 1971). Straw acts as a
binder, controls shrinkage and prevents cracking. Straw does not give added strength
to adobes but serves to make the brick dry and shrink as one unit. Without it, more
than one center of contraction develops and major cracks appear (Boudreau 1971).
Lime and other materials can be added to the mixture to further control shrinking
and cracking.
Adobe bricks must have a high percentage of clay. Too much clay causes the
brick to develop cracks as it dries; too little clay leaves the brick weak and crumbly.
The Roman architect Vitruvius specified the type of clay suitable for mud-brick
manufacture:
They should not be made of sandy or pebbly clay, or of fine gravel, because when made of
these kinds they are in the first place heavy; and, secondly, when washed by the rain as they
stand in walls, they go to pieces and break up, and straw does not hold together on account
of the roughness of the material. They should be made of white and chalky or of red clay, or
even coarse grained gravelly clay. These materials are smooth and therefore durable; they
are not heavy to work with, and are readily laid (De Architectura II.iii, pp. 42–43).
It should be noted that different clay minerals have different bonding properties
and that the lime content as well as the organic content affect the physical properties
of the finished product. Sand/silt/clay ratios cannot tell the full story.
In the Western Hemisphere, sun-dried mud brick is often referred to as “adobe.”
This term is Spanish, but it is derived from the Arabic “al-toubeh” ( “the
brick”). The Arabs borrowed the word from Coptic ( ), the Afro-Semitic
language of ancient Egypt. The Egyptians also used adobe extensively for the
construction of walls and fortifications (Clarke and Engelbach 1990).
The etymology of the name “adobe” hints at the antiquity of mud brick technology.
Evidence for mud brick construction has been associated with Neolithic villages
in Mesopotamia dating from about 7000 BCE (Steen 1971). Steen also notes
that mud brick has also been found in the remains of Neolithic villages in Anatolia
and Crete dating from about 5000 BCE. Mud brick has been used in Egypt for at
least 6000 years (Lucas 1989). Mud brick was used for the construction of the pyramids
at El-Lahun, Hawara, and Dashur.
Although an Old World term for adobe was transferred to the New World, mud
brick was actually known in the Americas long before the arrival of the Spanish
explorers. Prehistoric Native Americans in New Mexico and Arizona lived in apartment-style
dwellings called pueblos, which were above-ground structures originally
constructed of sticks and mud or adobe. Later pueblos were more often made
of masonry and/or adobe. Classic Mimbres pueblos were made of cobbles held
together by adobe. Later pueblos in southwestern New Mexico were constructed
almost entirely of adobe. In South America, adobe was used in the Chicama Valley
of Peru as early as 3000 BCE (Steen 1972).
Fired Brick. The change from impure clay to fired brick is analogous to high
temperature thermal metamorphism. To be suitable for brick making, fine-grained,
clay-rich sediments must have the following properties: they can be molded into