Glass is a supercooled liquid that is transparent and unaffected by chemicals. It is most commonly made from mixtures of silica (sand), soda ash, and limestone. The manufacturing process involves mixing these ingredients and heating them to very high temperatures in a furnace until they melt and form a clear jelly-like substance. As this cools and solidifies, glass is formed. There are several types of glass that vary in properties and uses, such as soda-lime glass commonly used for windows and bottles, colored glass, plate glass, safety glass, laminated glass, optical glass, pyrex glass, photochromic glass, and lead crystal glass.

1. 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 involvesthe 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 becomesinsoluble 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 reactionstake place inside the furnace.
7. The clear jelly-like substance on coolingsets 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
propertiesand 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, bottlesand bulbs.
2. Coloured glass:

2. 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 oxidesgive different coloursto 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 opticalinstruments.
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 colouredwith the intensity of light.)

3. 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.