Page 9 - Curriculum Visions Dynamic Book

Oven glass
Lens glass
Silica glass
80.0
35.0
96.5
4.6
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–
–
–
–
0.4
7.0
–
–
–
–
2.0
–
0.5
–
–
–
13.0
–
3.0
–
58.0
–
lenses for telescopes and glasses. Pure silica glass is made from quartzite and other pure forms of silica rather than sand.
Making glass at a lower temperature than
is needed to make silica melt is one of the most important requirements in glassmaking. This is done by adding a material called a flux. Sodium oxide (Na2O) is the flux used in much glassmaking. That is why soda is used in glassmaking. During heating, the soda changes from sodium carbonate to sodium oxide. The addition of a flux makes the silica molecules move slightly apart and so allows the silica to become more mobile. It also halves the temperature at which silica will melt. Potash (K2O) and lithium oxide (Li2O) are also used as fluxes.
Using a flux comes at a price. Glass made
this way is quite soluble in water. To get over this problem, another ingredient – the stabilizer – is added: usually lime in the form of limestone, but litharge, alumina (Al2O3), magnesia (MgCO3), barium carbonate (BaCO3), strontium carbonate (SrCO3), zinc oxide (ZnO), and zirconium oxide (ZiO) are all also used. Chemical reactions between the stabilizer, flux, and former make the glass almost insoluble.
(Right) Much cut glass contains lead because it adds to the natural sparkle.
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