Geoscience Reference
In-Depth Information
360
the composition and condition of the bedding/jointing
materials and in the assessment of crack morphologies
(Wong
et al
., 2003).
Modern flat glass for architectural applications is
commonly manufactured by the float glass process, or
less frequently using the older sheet process, or the rolled
process. It is a soda-lime-silica glass with a typical raw
material composition (by weight) of silica sand (72%),
soda ash (13%), limestone (10%), and dolomite (4%).
Coloured glasses are produced by additions of small
amounts of colouring agents such as iron (green), nickel
(brown) or cobalt (blue).
Microscopical examination of glass can provide much
useful information. It affords a quick means of
identification of glass type through measurement of the
refractive index. Glass comprises amorphous silica that
appears isotropic under the optical microscope. Although
relatively uniform in composition it may contain
impurities and imperfections, of which the frequency,
size, and sources may be determined.
Imperfections include bubbles (or 'seeds') that may
have a number of possible sources, the most common
being gas evolved during firing. Bubbles may contain
crystalline materials formed during cooling of the glass
that may provide clues to the origin of the bubbles. Cords
are linear features within the glass that may result either
from imperfectly homogenized raw materials, dissolved
refractories or devitrified material. Figure
360
shows the
appearance of soda-lime-silica glass that exhibits
bubbles and cords. 'Stones' are solid crystalline
substances occurring in glass that are regarded as defects.
They are usually derived either from the batch material,
refractories, or devitrification. Figure
361
shows the
appearance of soda-lime-silica glass that contains a
devitrification 'stone'. These may develop as the result of
incomplete mixing of the molten glass constituents
and/or too low a firing temperature. The 'stone' shown in
Figure
361
contains an aggregation of tridymite crystals
(see
362
).
360
Section through glass showing cords, bubbles,
and a devitrification 'stone' (centre); PPT, ×100.
(Courtesy of Barry J Hunt.)
361
361
Section through glass showing a devitrification
'stone' associated with a fine crack; PPT, ×150.
(Courtesy of Barry J Hunt.)
362
362
Close view of tridymite plates in glass; PPT, ×300.
(Courtesy of Barry J Hunt.)
