Chemistry Reference
In-Depth Information
crystal glass. Although the intentional addition of lead to glass has a long
history, the use of lead oxide as a modifier in early antiquity seems to have
been rather limited. Lead oxide was generally introduced to the glass melt
during the manufacturing process either directly, as one the natural oxide
minerals of lead, such as the mineral
litharge
(composed of lead monoxide)
and
red lead
(composed of lead sesquioxide), or indirectly, as one of the
natural salts of lead, as, for example,
white lead
(composed of basic lead car-
bonate) and
galena
(composed of lead sulfide). During the heating process
required for glassmaking, all these were converted to lead oxide. Lead glass,
as well as lead glaze have a low softening temperature and high density.
They also exhibit a characteristic brilliance and, when struck, emit a metal-
lic ring, properties that have been much appreciated since ancient times,
particularly by the ancient Romans, for making high-quality decorative
objects.
Colored Glass
Most of the properties of glass are determined by the nature and relative
amounts of the glass former, modifier, and stabilizer used for making it.
Some of its properties, however, are determined by the minor and trace con-
stituents that together usually make up less than 5% of the total mass of
glass. The color of glass, for example, is generally due to the presence,
whether intentional or otherwise, of relatively small, often trace amounts of
metal oxides. Heating a mixture of pure silica, soda (or potash) and lime,
for example, yields glass that is essentially colorless. The addition to such a
mixture of oxides of some metals, even in very small amounts, causes the
glass to become colored.
Iron oxide
, for example, the almost universal and
ubiquitous coloring agent of practically all ancient as well as modern ordi-
nary glass, produces a typical and characteristic, usually unintentional,
greenish glass. When yellow-brown sand, the color of which is due to iron
impurities, is used for making glass, the iron in the sand causes the glass to
acquire a green tint. The tint can be easily seen, even in apparently colorless
ordinary glass, when a sheet of glass is viewed edgewise. Its occurrence is
due to an optical effect caused by iron ions. The iron impurities occur in glass
as ferrous and ferric ions (see Textbox 6); the ferrous irons impart to the glass
a blue tint, while ferric ions make it yellow. The combined effect of the two
ions is the ubiquitous
bottle green
of ordinary glass. Less than 1% of iron
oxides is generally sufficient to give glass the characteristic green tint that
was in antiquity, and still is, regarded as aesthetically deleterious to most
types of glass.
As early as Roman times, it was found that the green coloration due to
iron impurities could be offset by adding to the glass, while still in the molten
