Manufacture of Glass: Step by Step Process

The Fours Steps for Manufacturing of Glass


1. Collection of raw materials

The raw materials such as silica (in the form of sand or quartz SiO2), soda ash (Na3CO3), limestone (CaCO3), and cullet (broken glass) are collected separately and mixed in a proper proportion. The fusion of cullet (broken glass) is added to bring down the melting point of the charge.


2. Preparation of Batch

The raw materials, cullet, and decolourisers are finely powdered in grinding machines. These materials are accurately weighed in correct proportions before they are mixed.

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. It is taken for further process of melting in a furnace.


3. Melting or heating of the charge

The glass batch is melted either in a pot furnace or in a tank furnace. It is made of fireclay or platinum. The heating is continued until the evolution of carbon dioxide, oxygen, sulfur dioxide, and other gases stops.

(a) Pot furnaces

Pot furnaces are used mainly in the manufacture of optical glass, art glass, and plate glass and in small scale units. The pots are crucibles made of selected clay, high alumina fire clay as mullite (3Al2O3.2SiO2), or platinum.


Pot-furnace-and-glass-furnace


(b) Tank Furnace

In a tank furnace, both materials are charged into one end of a large 'tank' built of refractory blocks. The tank has a capacity of 1400 tons. The glass forms a pool in the heart of the furnace across which the flames play alternately from one side to the other.

Heating is done by burning producer gas mixed with air over the charge. The cullet (broken glass) melts first and helps in the fusion of the rest of the charge. A high temperature of 1500-1800 C is maintained to reduce the viscosity of glass melt and to obtain a homogenous liquid.

The chemical reaction of the formation of glass in a furnace


Chemical reaction of the formation of glass in a furnace


Heating is continued till the glass melt is free from gas bubbles like CO2, SO2, etc. Undecomposed raw materials and impurities form a scum called glass gall which is to be skimmed off.

The clear liquid is now allowed to cool after the necessary decolorizers or coloring agents. It is cooled to 700-1200 C so that it will have the proper viscosity for shaping.


4. Processing of Glass

(a) Shaping

The molten glass is run into molds and automatic machines turn them into desired shapes such as sheets, tubes, rods, wires, etc.

The molten glass is given proper shape according to our need. It can either be done by hand or by machine. Hand fabrication is adopted for small scale production and machine fabrication is adopted for large scale production.

(b) Annealing

After shaping, the glass articles need to be cooled down gradually and slowly. Rapid cooling may cause fracture and crack. Annealing is carried out in special chambers, where the temperature is brought down slowly. 

The entire process of annealing may require a few days. This whole process of slow and homogenous cooling of glass articles is known as the annealing of glass.

Why Annealing is important?

The annealing of glass is a very important process. If the glass is allowed to cool down rapidly then the superficial layer of the glass cools down first and interior portions remain comparatively hot and therefore remain in the state of strain. 

Hence, if annealing is not done then glass articles may break into pieces under very slight shocks or disturbances.

(c) Finishing

After annealing the glass articles are subjected to finishing such as cleaning, grinding, polishing, cutting, etc.

Certain decolorizing agents such as cerium oxide, neodymium oxide, etc. are added to neutralize an undesired color present in the glass.

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