Glass is a mixture having no definite boiling of freezing points. It is also called a super cooled liquid. Chemically, most glasses are silicates. It is transparent and not affected by chemicals. It can be moulded into any shape. The ingredients for making glass are:-
1. Limestone (CaCO3),
2. Soda ash (Na2CO3), and
3. Sand (SiO2)
Manufacture of glass
The manufacture of glass involves the following steps:
1. Limestone, sand and soda ash are mixed and poured into a tank furnace. Tank furnace looks like a small swimming pool. It is very hot (about 17000C). It is shallow at one end and deep at the other.
2. The raw material moves slowly towards the deeper end. Silica melts at a very high temperature. In order to lower its melting point, soda ash is added. Thus, energy is saved and a low cost is incurred in the glass-making process.
3. Due to the presence of limestone, glass becomes insoluble in water.
4. As the raw material melts, a clear jelly-like substance is formed; this takes about a week’s time.
5. During this time bubbles of CO2 gas escape and some of the raw material slowly changes into a mixture of silicates.
6. The following reactions take place inside the furnace.
7. The clear jelly-like substance on cooling sets to form glass. This is known as soda-lime glass.
Types of glass
There are nine types of glass according to the minor additions and variations in the ingredients used and according to the methods of manufacturing. The different types of glasses are different in their properties and uses.
1. Soda glass or soda-lime glass:
It is the most common variety of glass. It is prepared by heating sodium carbonate and silica. It is used for making windowpanes, tableware, bottles and bulbs.
2. Coloured glass:
Small amounts of metallic oxides are mixed with the hot molten mixture of sand, sodium carbonate and limestone. The desired colour determines the choice of the metallic oxide to be added, as different metallic oxides give different colours to the glass.
Coloured glass is much in demand. It is used for decorating walls, making sunglasses, and for making light signals for automobiles, trains and aeroplanes.
3. Plate glass:
Plate glass is thicker than ordinary glass. It has a very smooth surface. It is made by floating a layer of molten glass over a layer of molten tin. It is used in shop windows and doors.
4. Safety glass:
It can also be called shatterproof glass. It is made by placing a sheet of plastic such as celluloid between sheets of glass. The special quality of this glass is that in case of breakage the broken pieces stick to the plastic and do not fly off. You must have noticed a broken window-pane of a bus or a car still in its place. It is used in automobiles. It is also used for making bulletproof screens.
5. Laminated glass:
It can also be called bulletproof glass. Several layers of safety glass are bound together with a transparent adhesive. The larger the number of layers used the greater is the strength of the glass. It is stronger than safety glass. It is used in aeroplanes and windshields of cars.
6. Optical glass:
Optical glass is softer than any other glass. It is clear and transparent. Potassium and lead silicates are used in making optical glass. It is also called flint glass. The main use of flint glass is in the manufacture of lenses, prisms and other optical instruments.
7. Pyrex glass:
Pyrex glass is highly heat resistant. In ordinary glass, silica is the main constituent. In pyrex glass some of the silica is replaced by boron oxide. Boron oxide expands very little when heated, thus, pyrex glass does not crack on strong heating. Pyrex glass is also called borosilicate glass. It has a high melting point and is resistant to many chemicals. Laboratory equipment and ovenware are made of pyrex glass.
8. Photo-chromatic glass:
Photochromatic glass acquires a darker shade when exposed to bright light and returns to its original lighter shade in dim light. This happens because silver iodinde is added to this glass. (silver iodide gets coloured with the intensity of light.)
9. Lead crystal glass:
Lead crystal glass has high refractive index, and so has the maximum brilliance. It sparkles and is used for high quality art objects and for expensive glassware. It is also called cut glass because the surface of the glass objects is often cut into decorative patterns to reflect light. In order to increase the refractive index, lead oxide is used as flux in crystal glass, therefore it is also called lead crystal glass.
The major disadvantage of ordinary glass is that it is brittle. It cracks when subjected to sudden changes of temperature. When the glass has been moulded into a finished article, it is cooled very slowly to prevent brittleness. The process in which a finished glass article is cooled slowly is called annealing.