Vitreous Silica – ‘Quartz’ Glass
Ground quartz materials are melted under vacuum, to remove gas bubbles, at 2000oC with approximately 10-2 % impurities.
The glass possesses a low thermal expansion coefficient (6.7×10-7/K) and is thermally stable to around 1000oC. Devitrification to cristobalite begins between 1150oC and 1200oC. The vitreous melts are highly viscous.
Some vitreous silicas are produced at lower temperatures or without the use of vacuum. These are characteristically opaque due to many small bubbles in the mass of the glass.
Modifications may be made to produce highly reactive forms of silica.
Sodium Silicate – ‘Water Glass’
Sodium silicate is melted from quartz and soda at 1400°C. Dissolution of the glass granulate is carried out at elevated temperature in pressure autoclaves.
The application of sodium silicate as a binder solution in many ceramic systems makes this material attractive as an adhesive.
The content of SiO2 in the initial glass ranges from 66-76 wt.% and the content of sodium silicate varies in the liquid form depending upon the grade.
Potassium silicate water glass is also manufactured for special purposes e.g. acid resistant cements.
Sheet and Container Glass (Soda-Lime-Silica)
The basic formulation of soda lime glasses varies little between flat and container (holloware) applications e.g. 72% SiO2, 14% Na2O(K2O), 9% CaO, 2-4% MgO, 1-2 % Al2O3.
Due to the high alkali content the glasses have relatively high thermal expansion coefficients 8.0-9.0 x10-6 /K and low viscosity at temperature due to the low Al2O3content.
‘Crystal’ Glasses (K2O-CaO-SiO2, K2O-PbO- SiO2)
Lead and potassium oxides are characteristic components of glasses called ‘crystal’. K2O and PbO are also common components of optical, sealing, and other technical glasses.
The term crystal denotes a high-grade clear colourless glass with high gloss and optical transmission. Conventionally only glass containing more than 24% PbO and having a refractive index exceeding 1.545 is termed crystal.
K2O and PbO promote the brilliant appearance of the glass. Glasses are formed with up to 65% K2O in the K2O-SiO2 system and 80 wt.% PbO in the PbO-SiO2 systems.
In most instances, industrially produced crystal glasses are more complex than the basic ternary system. Other components include Na2O, BaO, ZnO, B2O3 and MgO.
Lead crystal glasses contain 24-32 wt.% PbO. Typical compositions for K2O and PbO types are given in table 1.
Refining agents such as sodium sulphate NaSO4, arsenic As2O3, and antimony Sb2O3 are often employed.
Lead glasses are easily shaped, cut and polished and have thermal expansion coefficients range from 7.5-9×10-6/K.
These are glasses which can exhibit improved thermal shock resistance due to relatively low thermal expansion coefficients <5.0×10-7/K. Originally developed for laboratory use, they now find wide application in industrial and domestic situations.
PYREX glasses are of this type.
These glasses tend to have low alkali contents and high SiO2 contents e.g.>80 wt.%.
White Opaque – Opal Glass
Opal glasses show a milky opalescence. Opacity may be due to the presence of a dispersed crystalline, vitreous or gaseous phase. In practice it is usually developed by the introduction of fluorides to the batch. An example formula is given in table