Page 144 - Materials Chemistry, Second Edition

131
2.4. The Amorphous State

borosilicate glass. The physical properties of this glass resemble fused silica (e.g.,
1
coefficient of thermal expansion: 3.3 10 7 cm cm 1 K ), except that its soften-
ing temperature is only ca. 700 C. Borosilicate glass is the variety that is sold in

™
stores as Pyrex cookware and laboratory equipment. These applications demand a
glass that resists thermal expansion (i.e., cracking) as a result of significant changes
in temperature.
It was not until the late seventeenth century that PbO was substituted for lime in glass
formulations. This “soda–lead” glass is what we know as crystal (referred to as flint
glass in pre-Civil War America), and has always been a symbol of wealth and
extravagance such as expensive glassware and chandeliers. In order for crystal to be
legally given the “full lead” designation, at least 24% of lead oxide must be present in
its structure. The addition of the heavy element lead adds significant weight to the glass,
while increasing its refractive index. This latter property results in the familiar clear,
sparkling appearance of crystal glassware. The presence of lead also makes the glass
softer than regular types that must be cut with a diamond saw. Black crystal is truly one
of the most fabulous materials for modern artistic design. The lack of transparency is
caused by a combination of additives – typically Fe 2 O 3 ,CuO,MnO 2 ,and Co 2 O 3 .
Colored glass has been used since the construction of the first churches, prior to
the tenth century. Although decorative applications represent the majority of uses
for colored glass, there are some other recent functional applications such as traffic
light signals. The colors imparted by glass are a result of dopant species that are
added during its fabrication (Table 2.13). Both transition metal ions and colloidal
suspensions yield an observable color, with the hue dependent on the concentration
used. Variation of the color and intensity is also extremely sensitive toward the
heating regime (both temperatures and exposure times) used during the glassmaking

Table 2.13. Colors of Glass Resulting from Doping
Additive Color
Blue
Co 2 O 3
Yellow-green
Fe 2 O 3
FeO Bluish-green
Colloidal Se a Red
Colloidal Au a Red
Colloidal Cu a Red
CuO Turquoise
NiO Blue/violet/black
SnO 2 White
Sb 2 O 3 ,As 2 O 3 White; oxidizing agents
TiO 2 Yellow-brown
UO 2 Fluorescent yellow/green
Orange-red
AgNO 3
Yellow (opaque); oxidizing agents
PbO/Sb 2 O 3
Dark green/black
K 2 Cr 2 O 7
Purple
Mn 2 O 3
a
With average particle diameters of ca. 50–100 nm.