Natural Science in Archaeology

200 8 Ceramic Raw Materials
tiles for hypocausts (Singer et al. 1954/1956). The colors painted on a polychrome
Etruscan terracotta slab representing a warrior were determined spectroscopically
by Bordignon et al. (2007) to be from red ochre, azurite, yellow ochre, burnt umber,
and carbon black. The authors suggest that the white pigment was a kaolin.
Eventually, the decorative effects of terracotta tiles and structural qualities of
masonry units were combined by artisans. The result was the creation of architectural
masonry units that mimicked the appearance of carved stone. The creation
of these intricate architectural units required precise anticipation of clay shrinkage
and a tightly controlled firing process. A few early examples of this have survived,
including terracotta window tracery at Sutton Palace in England, dating from the
sixteenth century (Ashurst and Ashurst 1988, p. 68).
Tiles may be affixed by any number of methods, but the most common technique
is to embed the tile slab in a thin layer or “screed” of mortar. Tiles were frequently
manufactured with ribbed or textured backs to create a bond with the mortar. Color
was imparted to tiles with glazes and slips.
8.10 Refractory Ceramics
Prehistoric iron smelting placed enormous stress on the refractory material used to
contain the iron smelt as well as the smelting tuyeres. Childs (1989) conducted a
series of experiments on clay used for refractories at an Early Iron Age African ironmaking
site. She reports that modern sources were usually sandy clays collected
from upper banks along the edges of swamps or on ridges above the swamps. Such
(tropical) soils are in relatively early stages of breakdown. Another source of furnace
materials was termite mounds. Childs determined that most of the source clays
were effective up to 1500°C in an oxidizing atmosphere. A montmorillonite clay
had poorer refractory properties. The addition of broken sherd fragments increased
the refractoriness and structural stability of the briquettes tested.