Manufacture of Glass
Glass is manufactured in the following four steps:
Melting The raw materials - lime, soda and sand - separately cleaned, ground, sieved
(called 'Batch') in definite proportion and mixed with water are fused in a continuous type (tank) furnace or batch-type (pot) furnace. The charge in the first stage melts, forming a bubbly, sticky mass, and as the temperature is raised (1100 o C-1200 o C) it turns to a more watery liquid and the bubbles rise to the surface. The melting process in case of ordinary soda-glass involves the following series of reactions:
CaCO3+SiO2 ¾® CaSiO3 + CO2-
Na2CO3 + SiO2 ¾® Na 2SiO3 + CO2-
When all the carbon dioxide has escaped out of the molten mass, decolourisers such as MnO2 or nitre are added to do away with ferrous compounds and carbon. The colouring salts are added at this stage. Heating is continued till the molten mass is free from bubbles and glass balls. As the glass cools (800 o C), it is ready to be drawn or floated to its desired thickness and size at the other end of the furnace as shown by a flow diagram in Fig..
Forming and Shaping The molten glass can be fabricated to desired shapes by any one of the following methods:
Blowing A 2 m long and 12 mm diameter blow pipe is dipped in the molten glass and taken out. It is held vertically and is vigorously blown by the operator. The sticking molten glass takes the shape of a hollow ball. On cooling it is
reheated and the blowing operation repeated a number of times till the desired articles are ready.
Flat Drawing The process of drawing the glass up into a sheet begins when an grille (bait) is lowered into the glass in the kiln. In a short time the liquid molten glass adheres to the bait, and as the bait is slowly lifted it draws a sheet of glass. The bait and the drawn sheet of glass are then drawn through rollers, the bait is cracked off and a continuous sheet of glass is drawn up. This sheet is then slowly cooled in a chamber and annealed for cutting into proper size. A machine for vertical drawing of glass is shown in Fig..
Compression Moulding In this process moulds are used to obtain the articles of desired shapes.
Spinning A machine is used to spin the molten glass. The fibres so produced are very fine and are used for heat and sound insulation.
Annealing Glass articles are allowed to cool under room temperature by passing through different chambers with descending temperature. If cooled rapidly, the glass being bad conductor of heat, the superficial layer cools down first and strain develops in the interior portions, which causes unequal expansion and the articles are likely to crack.
Finishing After annealing the glass articles are cleaned, ground, polished, cut and sand blasted.