Natural Science in Archaeology

194 8 Ceramic Raw Materials
Chinese provinces (Guo 1987). In north China, a different, high-alumina, raw material
was used in the development of porcelain. The basic constituents were kaolin
with admixed quartz, mica, and carbonates. Both raw materials transformed into the
dense porcelain at between 1200 and 1300°C.
Rich deposits of what is termed porcelain stone were discovered in the southern
provinces of China in late Shang times. Chinese porcelain stone is a rock composed
mainly of quartz and sericite (fine-grained hydromica). Porcelain stone sometimes
also contained small amounts of feldspar and/or kaolinite. Throughout the long
development of Chinese porcelain, the potters experimented with a variety of raw
materials to achieve special effects. Without any knowledge of the chemical elements,
potters learned which manganese and iron oxides would color porcelain
glazes and that high phosphate raw materials encouraged bubbles in the glaze (see
Scott and Kerr 1993).
Protoporcelain was produced as early as the Shang Dynasty (seventeenth to eleventh
centuries BCE) in China. The earliest protoporcelain has a lead-free glaze and
a fairly vitrified body, suggesting the use of high firing techniques. Protoporcelain
requires suitable raw material as well as a high firing temperature. A recent paper by
Chen et al. (1999) explores the potential of chemical composition in characterizing
protoporcelain sources. Their results suggest a centralized source of production of
protoporcelain during the Shang Dynasty, perhaps at Wucheng. No dramatic change
occurred in making proto-porcelain during the following Zhou Dynasty (eleventh
century to 221 BCE) (Medley 1976). Europeans did not develop a true form of porcelain
until early in the eighteenth century (Kingery 1986b).
8.7 Glass
Glasses are solid, noncrystalline silicate bodies. The rapid solidification of a silicate
melt forms glass. To understand the raw material requirements for glass, it is
necessary to review the chemicals needed to make glass. Considering only ancient
glass, the most important constituent is silica (SiO 2 ). Pure silica will make a glass,
but only at exceedingly high temperatures. One of the most efficient and easily
obtainable additives to reduce the temperature of fusion is Na 2 O. A second compound
that serves well as a flux is lead oxide (PbO). Potassium oxide (K 2 O) and
calcium oxide (CaO) also serve to reduce temperatures of fusion. Ancient glass
found on archaeological sites may appear varicolored, because the inclusion of
manganese imparts a lavender hue after long exposure to the sun. Over very long
periods, glasses may crystallize, because they are unstable compared with an
assemblage of crystalline minerals of the same total composition. This process is
called devitrification. A good introduction to glass and other vitreous materials,
especially colorants, is given by Biek and Bayley (1979). The compositions of
ancient Egyptian, Roman, European, and Syrian glass are presented and discussed
by Freestone (1991). Analysis of window glass found at Pompeii indicates it was
composed of 69% silica, 17% soda, 7% lime, 3% alumina, and 1% iron oxide with