Laboratory Glass-Working for Scientists - Sciencemadness Dot Org

Chapter 2
PHYSICAL AND CHEMICAL
PROPERTIES OF GLASS
The Structure of Glass
A OLAS5 is a product of fusion which has cooled to a rigid condition
Without crystallizing. This definition includes a large number of
Organic glasses, and does not restrict the term 'glass* to inorganic
gubltances, which is a frequent practice in the U.S.A. This restriction
ntnu somewhat arbitrary, particularly when we consider how G.
immann established the general principles of the glass-like state by
•larch on organic glasses, and how the devitrification of technica
hues is paralleled by that of organic glasses. Tammann concluded
|tt a glass could be regarded as a supercooled liquid in which the
National movements of the molecules had been frozen (see W. E.
,, 1952). In fact, as R. Boyle described it about 1660, 'the
of the glass agitated by the heat, were surpriz'd by the cold
they could make an end of those motions which were requisite
| their disposing themselves into the most durable texture.' In
pdarn terminology, a glass is thermodynamically unstable with
to the corresponding crystal.
lalline silica (quartz, tridymite or cristobalite) in its various
lions is built up of Si04 tetrahedra linked together in a
manner so that every oxygen is between two silicons. The
ra therefore share corners. The arrangement in space of the
'a is different in the various crystalline forms, but is always
regular. A silica glass, in contrast, again contains Si04 tetrawith
every corner shared; but by slight distortions of the valency
compared with the crystal, a continuous and irregular threeional
network is built up. The orientation about the Si-O-Si
of one Si04 tetrahedron with respect to another can be practirandom.
Thus a two-dimensional picture of a silica glass would
a series of irregular rings, with an average number of about six
ra in each ring, but with the number of tetrahedra in indirings
varying from three to ten or more. The silica glass
Ultflw the condition for glass formation proposed by W. H.
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