The manufacture of glass consists of the following steps;
( 1) Melting of the charge.
(2) Shaping the article desired.
(3) Annealing of article formed.
MELTING The raw materials to be used depend upon the variety of glass to be obtained. The average composition of ordinary soda lime glass is represented by the formula Na2O CaO.SiO2. A mixture of Sand, Sodium Carbonate and Calcium carbonate in the ratio demanded by the above formula is mixed with other raw materials. The resulting mass known as “batch’ is mixed with broken glass called “cullet” and introduced either into the pot furnace or into the tank of a tank furnace.
(i) POT FURNACE
Two types of pots, the open crucible type or the closed covered type are in use. These vessels are used where the melting batch is to be protected from the products of combustion. The closed covered types are specially used. In this furnace the pots are placed in a circle around a central opening in its bottom. Hot gases and the flames come out of this opening. The roof oVer the pots is of refractory material, which reflects the heat on the pots. The pots are heated by supplying heat from external sources. The pot furnace is of batch type.
(b) TANK FURNACE
Melting in a tank furnace is more or less continuous and cheap. The “batch” is melted in a tank placed in the tank furnace. A simple sketch of the tank furnace is shown below:
The “batch” in the tank furnace is heated by burning natural gas. This heating works on the regenerative system of heat economy. The burnt gases are allowed to enter the furnace through fire-brick. Check works “A” and “B”. The flame of the burnt gases is deflected by the roof on the “batch”. As a result, the “batch” gets melted. The hot waste gases from the furnace are allowed to escape through the fire-brick check works “C ” and “D”, which absorb the heat of the escaping gasei. After some times, the direction of the fuel gases is reversed. That is the mixture of air and natural gas is now allowed to enter the furnace from the side of check works “C” and “D”. The waste gases now escape from the side of check works “A” and “B”. In this way, the heat of the escaping gases is utilized.
As the “batch” melts, test samples are taken out from time to time in an iron hook. When the sample is transparent and free from CO2 bubbles, the molten “batch” is taken out for further processing.
Various chemical reactions which take place among the raw materials during melting of the “batch” in the tank furnace are
Molten glossa obtained from the tank furnace is allowed to cool down so that it attains the required degree of plasticity to work. This cooled molten glass is transferred to special moulds and shaped. Articles are formed by air blowing techniques. Glass sheets may be obtained from molten glass by pressing between rollers.
The process of slow and homogeneous cooling of articles made of glass is known as annealing. Glass being a bad conductor of heat, if cooled suddenly, may develop strain. When cooled quickly, the superficial layer cools first, leaving the inner portion in a state of Strain. These strains, if not removed, make the glass liable to crack by it. To reduce the formation of strains, the molten glass is cooled very slowly and gradually to the room temperature in a series of annealing chambers. These chambers operate at different temperatures. Articles to be annealed are placed over annealing belt and passed through the annealing chambers. The time taken from the hot end to the cold end is about six hours.
The finishing operations include cleaning, grinding, polishing, cutting, enameling and grading.