Natural Science in Archaeology

8.2 Clays 185
clays have been used for writing tablets, figurines, pottery, crucibles, ornaments,
tile, brick, plumbing fixtures, foundation blocks, adobe, and related building materials.
In this chapter, the ceramic and related uses of clay are considered.
The primary raw materials for ancient ceramics were local clay-rich sediments
and soils for the paste and coarse sedimentary particles, comminuted shells or grog
for the temper. Clay deposits are of two general types: (1) primary deposits formed
in situ by the weathering of bedrock such as granite or shale, and (2) secondary
deposits formed by river (fluvial) or lake (lacustrine) deposition. These are often
referred to as transported clays.
In soils, primary deposits of clay minerals form as part of the natural chemical
weathering of the parent bedrock. In soils, clay particles (less than 0.002 mm) are
mixed with larger particles and a variety of other chemical precipitates and biogenic
material. Pedologists call a soil clay if it contains 35–30% clay. Clay minerals are
low-temperature hydrous minerals stable at the earth’s surface. Clays are the products
of the chemical weathering of silicate rocks containing a significant amount of
Al 2 O 3 . High-alumina minerals, particularly the feldspars, weather directly to clay.
Feldspar is the most abundant mineral in the earth’s crust, so sufficient parent
material is available nearly everywhere. Feldspars weather to kaolinite or smectite
depending on environmental conditions. The residues of the most extreme weathering
are quartz and alumina minerals such as gibbsite [Al(OH) 3 ]. For example, on
the island of Hawaii, the basalts weather to smectite on the dry, leeward side of the
island, and to gibbsite on the wet, windward side. Under intermediate conditions,
kaolinite formation is favored. Because the decomposition of silicates is often not
total, primary clays contain fragments of the parent material. These minerals are
principally quartz, feldspar, and mica. The other major source of clays is through
soil-forming processes. Soil clays tend to be impure mixtures of two or three clay
minerals plus mica and chlorite as well as quartz silt and sand.
Secondary clays were transported and deposited by fluvial, eolian, lacustrine,
glacial, and marine processes. Glacial clays are usually coarse and unsorted, with a
high percentage of impurities. Lacustrine clays are often high in organic matter.
Fluvial and eolian clays are most often fairly well sorted and contain fewer
impurities. Each of these geologic processes deposit clay of different grain sizes –
and, therefore, different plasticities. In Italy, an argillite formation outcropping near
Venosa has been used since ancient times as raw material for ceramics, including
bricks. This argillite is mostly clayey silt containing calcite, quartz, and dolomite.
To produce bricks with increased strength, the argillite was mixed with volcanoclastic
material (Summa 1996).
Precise identification of clay minerals requires XRD methods. The clay minerals
are not equally stable in all surface environments. This topic cannot be considered
here, but see Krauskopf (1967:176–203). Many other sheet silicates are found
mixed with these clay minerals in many deposits, but they all promote poor plasticity.
The addition of fine-grained organic material and many varieties of plastic and
aplastic tempers can improve plasticity, strength, and firing properties.
Clay is a very adaptable material. The earliest use of clay appears to be in the
upper Paleolithic when cave dwellers drew designs in wet clay on cave walls. Next