Natural Science in Archaeology

192 8 Ceramic Raw Materials
and “low” temperature range fluxes. The differentiation between these temperature
ranges has a direct influence on color (high temperatures “burn off” certain colors)
and compatibility with the ceramic body. Different clay mixtures will absorb the
glaze at different rates, largely depending on how open or fine the texture of the
vessel after the first firing. High-fired glazes usually contain feldspars, calcite, dolomite
or wood ash. Low-fired glazes are usually alkaline in composition, containing
sodium or potassium.
A number of compounds can be used as flux agents including lead, tin, sodium
and potassium. These compounds have been derived from numerous mineralogical
sources throughout history. The simplest and earliest fluxes were probably derived
from wood ash adhered to the surface of ceramics fired in open pits. If fired at a
sufficiently elevated temperature, the wood ash causes the surface of the clay to
vitrify, thereby resulting in a glazed surface. Sodium and lead are the most common
elements used as flux agents. Soluble salts (NaCl), saline water or natron can be the
source of sodium fluxes for “salt” glazes. Salt glazing was known from the earliest
times. This was one of the easiest ways to make a glaze flux. Other common fluxes
are Na 2 O and PbO, although K 2 O and CaO are also used. Boron and lithium also can
serve to make low temperature glazes.
The total reflectance spectrum and luminosity of a glaze provide a useful measure
of its opacity and ability to conceal the underlying body color. Vendrell et al. (2000)
showed that tin glazes contain SnO 2 particles with a diameter similar to the wavelength
of visible light. These particles are responsible for the glaze opacification.
Lead fluxes (which were highly favored for their bright colors) have only recently
fallen out of favor since they can cause lead toxicity if used for drinking or eating
ware. In China before the first millennium BCE, lead was a common fluxing agent
in glaze on pottery. The Assyrians used both lead and tin. Knowledge of lead glazes
used in Dynastic Egypt and Mesopotamia was transmitted by the Romans to Italy,
France, Germany, and England.
Glazes may also contain metal oxides as colorants, and various alumina (Al 2 O 3 )
compounds, which help stabilize the glaze and bind it to the surface of the ceramic.
The colors of glazes result from the presence of the transition elements iron, manganese,
chromium, vanadium, cobalt, and copper. Iron is ubiquitous as hematite
or limonite. Black manganese oxides are fairly abundant in small quantities at the
earth’s surface. Copper is also fairly widely distributed as oxide and hydroxycarbonate
minerals. It is harder to determine the sources for vanadium, chromium,
and cobalt, although the common vanadium minerals are highly colored and would
attract artisans. By the fourteenth century CE, cobalt arsenide was the initial raw
material for cobalt blue in the Near East and China. Coming initially from Persia
it is likely the arsenide was roasted to provide cobalt oxide. Sodium was the low
temperature fluxing agent in Egyptian alkaline glazes that provided a rich bluegreen
glaze when copper was the colorant. With a sodium glaze manganese gives
a reddish purple color. If potassium replaces the sodium, the manganese imparts a
bluish-purple color.
Ninth to tenth century CE Abbasid pottery from Iraq used traditional
Mesopotamian alkali-lime glazes on calcareous clay earthenware. Some of these