Glass

Glass is a very important chemical compound which is used by humans on a large scale. Today it has acquired such an important place in our daily life that we cannot think to live without it. Many things which we use in our daily life are made up of glass. For example, tumblers, tube lights, plates, electric bulbs, spectacles, laboratory equipments, milk bottles, slides, mirrors, screens of aeroplanes and automobiles are made up of glass. In India it is manufactured in Bangalore, Vadodara, Mumbai, Kolkata, Ferozabad and Delhi.

Chemical Composition of Glass

Ordinary glass which is also known as soda glass or soft glass is a mixture of sodium silicate (Na₂SiO₃), calcium silicate (CaSiO₃), and silica (SiO₂), and its chemical composition is normally represented as Na₂SiO₃.CaSiO₃.4SiO₂ or Na₂O.CaO.6SiO₂.

Raw Materials for Formation of Glass

       Following three substances are required for the manufacture of glass:

• Sodium carbonate
• Calcium carbonate
• Silica

Manufacture of Glass

To manufacture Glass first of all sodium carbonate, calcium carbonate and silica are ground into powder and mixed thoroughly. The homogenous mixture of sodium carbonate, calcium carbonate and silica is called batch. Then a few pieces of broken glass which are called ‘cullet’ are added into the batch. The purpose of adding the cullet in the batch is to lower the melting point of batch because the cullet have low melting point. Thus the addition of cullet into batch helps in early melting of batch during the manufacture of glass. Then a temperature of about 1673 K is produced in the furnace by burning fuel gases. At this high temperature the batch melts and its various components react with each other to form molten glass.

Chemical Reactions Involved in the Formation of Glass

Following are the chemical reactions which takes place during the formation of glass:

1. First of all silica reacts with sodium carbonate to form sodium silicate.
2. In the second step silica reacts with calcium carbonate to form calcium silicate.
   
3. In third step sodium silicate and calcium silicate formed above mixes with silica to form glass.
   

The glass formed here is in molten state, so it can be given any shape by cooling it in the moulds of desired shape and size.

Annealing of glass

As we have already studied that different glass articles are prepared by cooling the hot molten glass in moulds. Special care is taken while cooling the hot glass because if it is cooled at once, it becomes brittle and cracks easily and if it is cooled very slowly then it becomes opaque. To avoid the brittleness of glass articles there annealing is done. Annealing is the process in which a glass article is cooled slowly and uniformly.

Test your understanding and answer these questions:

1. What is annealing of glass?
2. What raw materials are used for manufacture of glass?
3. How is glass manufactured?