INTRODUCTION
When silica or quartz is heated up to 16500C it melts to a colourless liquid which on cooling gives glass. This fused mass is highly sensitive to temperature change therefore it requires special heat treatment so that ordinary glass can be manufacture which is much stable to temperature change. The glass of various commercial qualities is prepared by heating sand or quartz along with metal oxide or carbonates.
TYPES OF GLASSES
1. Soda-lime or soft glasses
The raw materials are silica (sand), calcium carbonate and soda ash. Their approximate composition is Na2O.CaO.6SiO2. About 90{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} of all glasses produced belong to soda lime glass. The low cost, low melting point soda-lime glass has resistant to de-vitrification and to water. However, they have poor resistance to common reagents like acids.
Uses: They are used as window glass, electric bulbs, plate glass, bottles, jars, building blocks and cheaper tablewares, where high temperature resistance and chemical stability are required.
2. Potash-lime or hard glasses
Silica (sand), calcium carbonate and potassium carbonate are the basic raw material for potash lime glass. Their approximate composition is K2O.CaO.6SiO2. They possess high melting point, fuse with difficulty and have good resistance to acids, alkalis and other solvents compare to ordinary glasses.
Uses: These glasses are costlier than soda-lime glasses and are used for chemical apparatus, combustion tubes, etc., which are to be used for heating operations.
3. Lead glass or Flint glass
Instead of calcium oxide, lead oxide is fused with silica. As much as 80{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} of lead oxide is incorporated for dense optical glasses. In addition, K2O is used instead of sodium oxide. So, its approximate composition is K2O.PbO.6SiO2. Lead glass is more expensive than ordinary lime-soda glass, but it is much easier to shape and to work with. Lead glass has a lower softening temperature and higher refractive index than soda glass. It has excellent electrical properties. It is bright, lustrous and possesses high specific gravity (3 to 3.3).
Uses: Lead glasses are used for high quantity table wares, optical lenses, neon sign tubing, cathode ray tubes, electrical insulators and in the art objects because of their high luster. High lead content glasses are used for extra dense optical glasses, for windows and shields to protect personnel from X-rays and gamma rays in medical and atomic energy fields respectively.
4. Borosilicate glass or Pyrex glass or Jena glass
It is the most common of the hard glasses of commerce which contain virtually only silica and borax with a small amount of alumina and still less alkaline oxides. Borosilicate glass has the following composition.
Component SiO2 B2O3 Al2O3 K2O Na2O
Percentage 80.5 13 3 3 0.5
Boron and aluminium oxides substitutes Na2O and CaO used the lime-soda glasses which results in a glass of low thermal coefficient of expansion, and high chemical resistance. Borosilicate glasses have a very much higher softening point and excellent resistivity to shock.
Uses: They are used in pipelines for corrosive liquids, gauge glasses, superior laboratory apparatus, kitchenwares, chemical plants, television tubes, electrical insulators etc.
5. 96{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} Silica glass
It is produced and shaped as typical borosilicate glass, having dimensions bigger than desired. The heat treatment to the article, separate the glass into two layers, one consisting mainly of silica and the other of the alkali oxides and borates. Then article is dipped in hot acid which dissolves away the alkali oxides and boron oxide layer, leaving behind a fine porous structure consisting of about 96{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} silica, 3{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} B2O3 and traces of other materials. This glass is then washed carefully and annealed to 12000C. The shrinkage of about 14{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} takes place and hard firm shape is produced which is almost gaslight. The translucent 96{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} glass, if it is so desired heated to a higher temperature and made almost transparent or clear.
It is expensive than other types of glasses. The expansion coefficient is very low which accounts for its high resistance to thermal shot. The softening temperature is about 15000C. They possess high chemical resistance to most corrosive agents. They are corroded by only HF, hot H3PO4 and concentrated alkaline solutions.
Uses: They are used where high temperature resistance is required and articles can be safely used at temperature up to 8000C. They are used for the constructed chemical plants, laboratory crucibles, induction furnace linings, electrical insulators.
6. 5{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} silica glass or Vitreosil
It is produced by heating SiO2 to its melting point (1,7500C). Because of absence of fluxing agents, it is extremely difficult to get rid of the bubbles. Shaping of the glass is difficult due to high viscosity at its working temperature. The final product is translucent. It has high softening temperature about 16500C, compare to 96{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} silica glass. Its thermal expansion is very low. Due to their opaque nature, they tend to be mistaken for pipe when dirty and are, therefore, often broken accidentally.
If vitreosil glass is heated for long periods above its melting point, it finally becomes transparent and is then known as ―clear silica glass‖. It has considerable transmission properties e.g. 1mm of this material allows no less than 93{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} of light to pass corresponding figure for good optical glass is only 6{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26}.
Uses: uses are similar to 96{c909ba32dd49e2347b9da76d2471d15615fb19a50a31d6362bb7d8ae14133d26} silica glass. It is exposed for the construction of pipelines for hot concentrated acid. Clear silica glass is used mainly for plant ware, chemical laboratory wares, electrical insulating materials, and in electrical heat furnaces.
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