Natural Science in Archaeology

11.5 Mud Brick, Terracotta, and Other Earthen Architectural Materials 271
Unbaked Clay or Mud. This category includes “clay lump,” “chalk mud”,
“wichert”, “clay daubins”, and “cob” of England. These terms refer to various
clayey soils mixed with sand, straw and sometimes chalk. Unbaked clay materials
were utilized in a variety of related construction methods throughout England in
all historical periods. Clay lump is compacted into wood molds, and the resulting
masonry units are permitted to air dry. Cob walls are formed in place by the simple
process of pitching on a soft but cohesive mixture of the materials mentioned, in
layers, the wall surface being pared down with a flat-backed spade to form a fair
face as the work proceeds (Ashurst and Ashurst 1988).
The unbaked clay category also includes mud “plasters” and mortars that may
be applied over other types of earthen masonry or onto other types of substrates as
a protective and decorative surface finish. Mud plasters can be used as the leveling
or “brown” coat under a finished layer of lime plaster. Unlike lime plasters, mud
plaster does not require burning, thus saving on both fuel and labor.
The use of mud over fiber probably has prehistoric origins. A number of scholars
have speculated that the ancient dwellings of the Fertile Crescent and Nile Valley
were constructed of reeds to which layers of wet alluvial mud were applied. “Wattle
and daub” is a good example of the traditional use of mud plaster in a two-part
system. The “daub” is a thick layer of mud composed of sand, clay, straw and dung,
applied onto the wood or fiber frame called “wattle” (Kholucy 1990). This method
was known and used by the Romans as an inexpensive construction technique.
Vitruvius was critical of wattle and daub, urging his readers against its use.
As for “wattle and daub” I could wish that it had never been invented. The more it saves in
time and gains in space, the greater and the more general is the disaster that it may cause;
for it is made to catch fire, like torches. It seems better, therefore, to spend on walls of burnt
brick, and be at expense, than to save with “wattle and daub,” and be in danger. And, in the
stucco covering, too, it makes cracks from the inside by the arrangement of its studs and
girts. For these swell with moisture as they are daubed, and then contract as they dry, and,
by their shrinking, cause the solid stucco to split (De Architectura II.viii.20, pp. 57–58).
Considering Vitruvius’s comments, it is not surprising that it is difficult to find
archaeological evidence for this type of construction. Wattle and daub is usually
inferred from pieces of fired “mud” containing the impressions of organic materials.
Sun-Dried Mud Brick. Adobe is unfired earthen architectural material common
in the desert southwest of the USA. Adobe is applied as courses rather than shaped
into brick. It usually is called pisé outside of the Americas. The raw materials for
adobe wall construction between 1275 and 1450 CE at a pueblo near Albuquerque,
New Mexico, were local clay-rich soils composed primarily of Na-Ca montmorillonite,
bentonite, and goethite. During different phases of construction the source of
the raw material changed. Some soil types were preferred and some local soil types
were unsuitable (Balsam et al. 2007).
Adobe mud brick is generally composed of mud, sand, straw, and various additions.
“Mud” is a geologic term meaning a mixture of silt- and clay-sized particles.
The word “clay” is also commonly used to describe mixtures of silt- and clay-sized
particles. In addition to mud or clay, the mixture may include various mineralogical