Natural Science in Archaeology

202 9 Pigments and Colorants
This can easily be seen in comparing finely ground smalt to coarse smalt. The
coarse smalt is darker and has a much denser color than the fine smalt. Conversely,
grinding may be inadequate to disperse the pigment evenly throughout the medium.
This results in a weaker color. Before modern manufacturing methods, pigments
were ground by hand with stone, mortar and pestle, or slab and muller.
Hiding power (also known as opacity) is the property of a pigment to obscure the
surface it is covering (Gettens and Stout 1966). This characteristic is related to the
opacity of the particle and its ability to absorb or reflect light. In white pigments,
the measure of hiding power is the particle’s ability to reflect all wavelengths of
light. The hiding power of a mineral pigment is directly proportional to the refractive
index of its grains; the higher the refractive index, the greater the hiding power. The
difference between the refractive index and the surrounding medium of the pigment
further influences this characteristic of the pigment and the medium.
An important characteristic of pigments is permanence. Permanence (also known
as light-fastness) is the ability of a pigment to resist fading due to photochemical
deterioration. Pigments with poor light-fastness are said to be “fugitive”. Photochemical
deterioration can produce by-products that remain on the surface. These
can sometimes be identified through analytical methods (Corbeil and Helwig 1995).
However, most mineral pigments are permanent.
Chemical stability refers to a pigment’s level of reactivity to moisture, light,
air, acidity, and alkalinity. There are very few pigments that are completely inert,
and by-products of chemical reactions may be present in archaeological contexts
(Gettens and Stout 1966). For example, white lead turns black when exposed to
atmospheric acidity. An understanding of the chemical stability of mineral pigments
and the possible types of reactivity to which they are susceptible is important when
interpreting the archaeological record.
Pigments often have names that correspond to a precise chemical composition;
therefore, it is important to avoid using epithets indiscriminately because they may
be confused with a specific compound. For example, “lemon yellow” should be
used only to describe barium chromate. This pigment is not likely to be found in an
archaeological context since it was not synthesized until the late eighteenth century
(Feller 1986).
A word of caution is merited regarding the use of traditional terminology. Historical
references to pigments use traditional nomenclature. The traditional classifications
of pigments are somewhat unscientific and are often based on the ancient
geographical source of the material or on the processing method used, rather than
on the geology or mineralogy. Correlating historical references with archaeological
materials may shed light on geologic sources and materials. However, there
are overlaps in the use of traditional terms. These terms are often confused, misapplied,
or include more than one group of mineralogical and chemical compounds.
In archaeomineralogy, pigments should be defined by specific chemical and mineralogical
composition. Munsell produces a number of color charts designed for
matching pigments and colorants.
Other types of colorants of interest in archaeomineralogy include those used
in the manufacture of glass and ceramic glazes. There were three ways color was