Natural Science in Archaeology

190 8 Ceramic Raw Materials
(limestone, sandstone, basalt), volcanic ash, organics (ranging from straw to dung),
and grog (ground sherds). Coarse sediments for temper are normally available in
nearby streams. Fossil shells make good temper because calcium carbonate has the
same thermal expansion properties as the average pottery clay. The same is true for
marble and limestone (also calcium carbonate). Figure 8.2 is a thin section of pottery
showing the larger grains of temper against the fine-grained background of clay
paste. The temper is predominantly quartz with one grain of feldspar in the upper
right. Steatite-tempered pottery is known from the Neolithic of Yugoslavia (Kaiser
1989).
When determining whether temper has been added to a given clay, a problem
arises for the archaeologist, because many natural clays contain quartz sand, shells,
volcanic ash, and rock fragments. Four properties are used to suggest whether
coarse material in pottery is naturally occurring or purposely added. These are identity,
particle shape, size range, and ratio of coarse material to clay paste.
Volcanic by-products have been used as temper by many cultures. The Maya
used volcanic ash for several centuries as temper in their utilitarian pottery in the
limestone lowlands, well away from the volcanic highlands (Simmons and Brem
1979; Ford and Rose 1995). Volcanic sands have been used for temper in the pottery
of the South Pacific since prehistoric time. Pottery from the island of Tonga
has been a major provenance problem. The scarcity of volcanic sand on most of the
inhabited islands in the Tongan group has prompted suggestions that either temper
was imported from a volcanic island to the west or that the finished pottery was
imported from Fiji. Although the mineralogy of the tempers is compatible with
that of Tongan volcanic rocks, Tonga lacks deposits of the rounded and well-sorted
sands found in the tempers. Recent discovery of beach placer sands derived from
reworking of tephra deposits provides a satisfactory source. Using petrographic
techniques, compositional analyses of temper sands in numerous ancient sherds
Fig. 8.2 Thin section of pottery from Big Rice Lake site, Minnesota, USA