3
THE TECHNOLOGY OF SILICA GLASS MANUFACTURE
3.1
INTRODUCTION
The first part of the present treatise, i.e. Chapter 2, presented a brief account
of the phase transformations of quartz and synthetic raw materials to silica glass.
However, from the standpoint of technology, these phase conversions are signifi­
cantly affected by the geological origin (purity) of natural quartzes and by the chemi­
cal composition of synthetic silica raw materials.
The phase transformations are further influenced by the processing technology
and the melting technology employed. These so-called technological factors have
a marked effect on the physico-chemical properties of melted silica glasses, as
demonstrated in Chapter 4.
This matter will therefore receive attention in the present Chapter.
SILICA GLASS
3.2
On heating, crystalline jS-quartz (stable at temperatures lower than 573 °C
and containing less than 0.01% of foreign admixtures) undergoes a phase trans­
formation to high-temperature α-quartz, which is stable up to 1400-1450 °C [1]
(Fig. 40).
Further heating converts α-quartz to high-temperature α-cristobalite, while
simultaneously producing a transient amorphous phase. The proportions of the
J>
0)
Ϊ
Q.
amorphous
phase
^
&
/
'
^
/
^.---^^to-cristo-
^^-~<Γ~ϋ>Φ^
!
balite
Ύ
1
1
1
i
!
'
'
i
i
|
1
i
•1400 1723
250-270 573
fCC)
Fig. 40 - Diagram of phase
transformations of crystalline
Si0 2 at normal pressure
95