Chemistry Reference
In-Depth Information
TABLE 27
Ancient Sodalime Glass: Typical Composition
Glass component
Relative amount (%)
Silica (glass former)
60-75
Soda (modifier)
30-15
Lime (stabilizer)
15- 8
Iron oxides (generally added unintentionally)
10-less than 1
Sodalime glass, made of sand, soda, and lime, has been, with only slight
compositional variations from place to place or time to time, the most
common type of glass made and used throughout the world since its dis-
covery and until the present time. Although an infinite number of variations
in the relative proportions of the three components is possible, a balance
between them is essential. An excessive amount of modifier (soda) makes
the glass chemically unstable and easily soluble in water. Glass that contains
too much stabilizer (lime), tends to
devitrify
, that is, to change from the vit-
reous to the crystalline condition, and becomes very susceptible to
weather-
ing
and
decay
. Most types of ancient as well as modern sodalime glass thus
contain relatively uniform proportions of the components, generally about
75% sand, 15% soda, and 10% lime (see Table 27).
In addition to the three main components of glass (silica, soda or potash,
and lime), other materials have been, intentionally or accidentally, added to
the mixture of raw materials when making glass. The addition of
magnesia
(composed of magnesium oxide), for example, which is often naturally
mixed with limestone, decreases the solubility of glass in water and there-
fore increases its durability. Also the addition, mostly unintentional in antiq-
uity, of
alumina
(a mineral composed of aluminum oxide), which is a natural
component of some sands, improves the chemical durability of glass and its
working properties during shaping and forming operations.
Potash Glass
Glass made of sand, potash, and lime, known as
potash glass
, hardens faster
and at a higher temperature than does soda glass. Because of these proper-
ties, potash glass was much less widely used than soda glass for making
molded glass objects (Brill 1979; Olin and Sayre 1974).
Lead Glass
Lead oxide is a glass modifier that yields a very special type of glass known
by a variety of names, including lead glass, flint glass, lead crystal, and
