Explain Briefly the Manufacture of Glass

Manufacture of glass

The manufacture of glass may broadly be divided into the following five stages:

1. Collection of raw materials.
2. Preparation of batch.
3. Melting in furnace.
4. Shaping or forming
5. Annealing.
6. Finishing.

(1) Collection of raw materials

• Depending upon the type of glass to be manufactured, suitable raw materials are collected.
• The raw materials required for each type of glass.

S. No	Type of glass	Raw materials
1.	Soda -lime glass	Chalk, soda ash and clean sand
2.	Potash – lime glass	Chalk, potassium carbonate, sand
3.	Potash – lead glass	PbO, potassium carbonate, sand
4.	Common glass	Chalk, salt cake, coke, sand

In addition to the raw materials, the cullet and decolourisers are also added in the glass.

The cullet indicates waste glass or pieces of broken glass.

(2) Preparation of batch

• The raw materials, cullet and decolouriser are finely powdered in grinding machines.
• These materials are taken in correct proportions and mixed together.
• The mixing of these materials is carried out in mixing machines until a uniform mixture is obtained. Such a uniform mixture is known as batch or frit.

(3) Melting in furnace

• The batch is melted either in pot furnace or in a tank furnace.
• The pot furnace is of batch type while the tank furnace works continuously. The tank furnace is adopted to melt large quantities of glass at a time, while pot furnace melts small quantities of glass at a time.
• The tank furnace is filled with raw materials and heated by allowing producer gas through ports . The batch is heated in large compartment.
• Chemical reaction in the furnace. The ordinary glass, also known as winding glass, approximately represented as Na₂ O CaO 6SiO₂ is formed according to the following reactions:

(i) Sand fuses with sodium carbonate to form a glassy mass, known as water glass which is soluble in water.

Na₂ CO₃ + SiO₂ → Na₂ SiO₃ + CO₂

(ii) Lime stone or calcium carbonate reacts with sand to give a glossy substance known as calcium silicate which is insoluble in water and soluble in acids.

CaCO₃ + SiO₂ → CaSiO₃ + CO₂

This is the case when either the lime stone or sodium carbonate is used . The chemical reaction between Na₂SO_{4 ,} C, CaCO₃ and SiO₂ to form glass is as below.

Na₂CO₃ + CaCO₃ + 6SiO₂ → Na₂O.6SiO₂ + 2CO₂

Fig (1) A tank furnace

(4) Shaping or forming

Molten glass is worked into articles and given desired shape or formed either by blowing or moulding.

(5) Annealing

• As soon as the glass is given the desired shape, it is passed through  a series of cooling chambers known as annealing chambers to bring its temperature down gradually to the normal.
• The process of slow cooling is known as annealing.
• The semi- solid and hot glass when exposed suddenly to atmosphere, undergoes unequal cooling, which leads to cracking of the glass.
• The longer the annealing period , The better will be the quality of the glass.

(6) Finishing 

• After annealing , all types of glass articles require finishing process.
• The finishing process includes cleaning, grinding, polishing, cutting , blasting etc.

Read More Topics

Give a brief account of different kinds of glass

Composition and properties of glass

Constituents of glass and their functions