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54.4 DIFFERENT VARIETIES OF GLASS

Some important types of glasses along with their uses are described briefly as follows:

1.    Lime Glass. This glass involves the use of lime or lime-stone along with the alkali carbonates (Na2CO3 and CO3). This is further classified into:

(i)       Soft Glass and

(ii)      Hard Glass.

(i)   Soft Glass. It is made by fusing soda ash, sand and limestone together. It is believed to have an approximate composition Na2O.CaO.5SiO2 (or Na2O. CaO.6SiO2). It fuses at a low temperature and cannot stand mechanical injuries. It is used for making bottles, tumblers, window panes, cheap glassware, etc.

(ii)   Hard Glass. It is obtained by fusing potassium carbonate and limestone together. It is understood to have a formula K2O.CaO.5SiO2 (or K2O.CaO.6SiO2). It is harder and stronger than the soda glass. It is used for making apparatus capable of standing high
temperature and pressure.

2. Flint or Lead Glass. This is made from potassium carbonate, lead oxide or red lead and sand. This is heavy, shining and extraordinarily durable. This is used for making optical instruments like lenses, prisms, etc. and cut glass articles, like objects of decoration imitation diamonds or jewels.

3. Borosilicate Glass. In this glass a part of the silica is replaced by boric oxide (B2O3). This is super cooled solution of various borates and silicates and hence its name is borosilicate glass. This is made by fusing a mixture of sand, lime, borax and alkali carbonates. Sometimes a little alumina (Al2O3) and zinc oxide are also added to them. The former makes the glass more durable and the latter adds to its thermal and chemical resistance. Pyrex glass and Jena glass are two well-known trade marks of borosilicate glasses, which are widely used for making laboratory apparatus, thermometers, kitchenware, tableware, insulators, etc.

4.  Crooks’ glass. It is a special type of glass containing cerium oxide. It does not allow the passage of ultraviolet rays. It is used for making lenses.

 ETCHING OF GLASS

Glass is attacked by hydrofluoric acid. HF and glass react to form soluble compound hydrofluoric silicic acid (H2SiF6). This property is used in the etching of glass.

The glass to be etched is coated with a thin layer of wax and the design to be produced is scratched with a needle or blade. An aqueous solution of hydrofluoric acid is applied to the exposed part. After some time it is placed in water and wax is removed from the surface. The marks made by blade get engraved on the exposed parts.