Natural Science in Archaeology

196 8 Ceramic Raw Materials
in glass production was based on changes in manufacturing technology, not changes
in raw materials.
The Romans were the first to truly master the control of color in glass using
colorizers and decolorizers. Colorless glass was popular in the last first and second
centuries CE. Pliny (NH XXXVI) mentions colorless glass mimicking rock crystal
as closely as possible. Most Roman glasses were blue-green due to the iron content.
To make a colorless glass, the Romans had to select the raw materials very carefully
and/or add a decolorizer. Roman glasses were made from silica sand and natron
(Na 2 CO 3 10H 2 O) or trona (Na 3 H(CO 3 ) 2 2H 2 O). If a decolorizer was required, antimony
and manganese could be added to oxidize the iron. Tin-based opacifiers (lead
stannate yellow and tin oxide white) were used in glass-making in Europe from the
second century BCE. Tin-opacified glazes were introduced into the Islamic world
in the ninth century CE (Tite et al. 2008).
In Roman Palestine glass was manufactured in two stages. The raw materials
were first melted together, resulting in the production of large blocks. These
were then broken into smaller chunks and shipped to factories where vessels were
produced. The great glass slab found at Beth Shearim is an example of an unsuccessful
first melting (Weinberg 1988:25). In a program of chemical analyses of the
samples from the glass factory at Jalame in Palestine, Brill (1988) reports that the
glass has a soda-lime-silica composition with only minor potassium and magnesium
oxides. Brill suggests that this is evidence that natron was used as the alkali.
About 1680, on the slopes of the Riesengebirge in Germany, glass was decisively
improved by the addition of chalk. The chalk glass was not only clearer than the
compositions used previously, but the refraction was also changed, making the glass
more sparkling and allowing thicker glass to retain its transparency.
Glass can be colored by adding small amounts of particular elements or compounds
to the glass melt. These additives can enter the glass structure as ions, or
they can be dispersed in the glass as small metallic particles. In both cases the metals
develop an electronic structure in the glass that absorbs light of distinct wavelengths;
sometimes causing striking colors (see Chap. 3). The best-known case of
glass colored by dispersed metallic particles is that of gold ruby glass, known since
the seventeenth century. The golden color of this glass results from finely divided
gold particles. Silver particles in glass color it yellow. The red color of copper ruby
glass may be due to copper or copper oxide. Some of the ions that have long been
added to color glass are:
Ion Color of glass
Mn3 + Purple/violet
Mn2 + Brown
Fe3 + Green/brown
Co2 + Blue
Cu2 + Blue
Cr3 + Green
The colorants in some seventh century Roman silica-soda-lime glasses were
determined by Mirti et al. (2000). Iron determined the color of blue-green, green,