Natural Science in Archaeology

11.6 Weathering and Decomposition 273
the desired shape, they can be fired without excessive shrinkage, and they have
the necessary mineralogy to transform to a fired material with sufficient compressive
strength and durability. Brick clays include quartz as a major constituent. Most
brick clays include illite and kaolinite and may include smectitie, feldspar, and chlorite.
In firing, the first process involves driving off any absorbed water; then comes
(1) the dehydration of clay minerals, (2) loss of the carbon dioxide and hydrocarbons,
(3) the transition of alpha-quartz to beta-quartz, (4) various solid state mineral
transformations, and (5) partial melting at grain boundaries with the formation of
new minerals, including mullite. I have witnessed brick making in the lower Yellow
River area of China where the raw materials are silt-rich and clay-poor, resulting in
a poor grade of brick. Modern brick is 60–75% clay and up to 10% sand. The first
fired bricks were made by the Babylonians about 4000 BCE.
Mosaic. Mosaic is the art of decorating a surface with designs of closely set small
pieces, called tesserae, of colored stone, ceramic, or opaque glass called smalti.
Smalti is made from silica, lime, soda, potash, alumina, lead oxide, boric acid, and
colored with cobalt, copper, chrome, nickel, iron, silver, and manganese oxides. The
earliest mosaics were in buildings in Sumer. The Romans perfected the craft and
imported marble, lapis lazuli, malachite, turquoise, jasper, and especially travertine
from Tivoli for floors and wall art. Pliny (NH 36) discussed the mosaic technique.
11.6 Weathering and Decomposition
Weathering is the breakdown of rocks and minerals at or near the earth’s surface
into products that are more in equilibrium with the conditions found in this environment.
Chemical weathering involves the alteration of the mineral constituents of
the rock. Physical weathering is the breakdown of minerals by entirely mechanical
methods that lead to the rupture of the rock.
A number of different processes can result in chemical weathering. The most
common chemical weathering processes are hydrolysis, oxidation, reduction,
hydration, carbonation, and solution. Hydrolysis is the weathering reaction that
occurs at the surface of minerals reacting with water; it results in the decomposition
of the rock by forming new minerals. Oxidation is the reaction that occurs between
minerals and oxygen. The net result of this reaction is the removal of one or more
electrons from a mineral, which causes it to become increasingly unstable. Reduction
is simply the reverse of oxidation. Hydration involves the rigid attachment of
H+ and OH- ions to a reacted compound. Carbonation is the reaction of carbonate
and bicarbonate ions with minerals.
The processes that may cause physical weathering and mechanical rupture
are abrasion, crystallization, thermal insolation, wetting and drying, and pressure
release. Crystallization can cause the necessary stresses needed for the mechanical
rupturing of rocks. There are primarily two types of crystal growth that occur; they
are ice and salt. Upon freezing, the volumetric change of water from liquid to solid
is 9%. The crystallization of salt exhibits volumetric changes from 1% to 5%.