Sheet of Glass
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making procedure, and which I will try to set out quite briefly:--|
The principal ingredients of all flat glass are silica soda and lime.
Economic reasons compel us to use these lime soda silicates and the
percentage of the various ingredients is decided by the process or
method of manufacture to be employed. Glass has no definite melting
point; it gradually softens through a very wide range of temperature,
and in determining the mixture from which any particular glass is to
be made, the viscosity curve of the molten glass is an important
consideration. At a dull red heat glass is flexible and can be bent;
at 1000°C. to 1100°C. it is of the consistency of syrup, and
can be gathered or collected on the blow pipe and blown or drawn and
rolled. At 1500°C. it is almost watery, so that even minute bubbles
will rise to the surface, and this is the temperature of founding or
fining. Above a certain temperature glass may be kept in the molten
state indefinitely, but if cooled below such critical temperature,
crystallisation or devitrification begins.
- The softening or viscosity curve.
- Crystallisation or devitrification.
- Resistance to weathering or stability under normal climactic conditions.
In Figure 1 is shown the rate
of growth of crystalline calcium silicate in the devitrification zone for
three different glasses. The ordinates represent temperature in degrees
centigrade and the abcissæ represent rate of growth of
crystallisation. The bulge represents the danger zone of temperature,
through which the glass must be cooled quickly in any manufacturing process.