The manufacturing process of glass consists of four major operations: (1) Melting, (2) Shaping, (3) Annealing, (4) Finishing. Each operation is being discussed briefly as follows:
1. Melting. The ingredients called batch materials are mixed in the appropriate proportion and heated to fusion in a furnace. Many designs of glass furnace are in use. The two most commonly used furnaces are: (i) Pot furnace and (ii) Tank furnace.
i. Pot Furnace. In this furnace, the charge is fused in fire clay pots. The pots may be opened or closed. The closed pots are used when the-glass is to be protected from the products of combustion.
The batch materials are put in the pots. They are placed in a circle inside a furnace and heated by burning producer gas around them (Fig. 54.1). When the fusion is complete the pots are removed from the furnace and the fused plastic mass is taken out for shaping. Pot furnace is employed for the production of high quality glass, since the charge remains protected from the products of combustion.
Fig. 54.1. Pot furnace.
(ii) Tank Furnace. It consists of a large rectangular tank built of fire clay blocks. The batch materials are fed into the tank and producer gas is used as a fuel in the furnace. (Fig. 4.2).
The charge is heated at 1400°- 1500°C for 10-12 hours. The chemical reactions involved in both the furnaces are:
Na2CO3 + SiO2 → Na2SiO3 + CO2
2Na2SO4 + 2SiO2 → 2Na2SiO3 + O2 + 2SO2
CaCO3 + SiO2 → CaSiO3 + CO2
At 1400°C silica also in silicates of calcium and sodium
Na2SiO3 + CaSiO3 + 4SiO2 → Na2SiO3.CaSiO3 .4SiO2
During the melting lot of frothing is caused owing to the evolution of the gases like CO2, SO2, O2, etc. When the frothing subsides, the temperature is raised and the molten glass is allowed to stand for some time. This is called refining, and its objective is to form a homogeneous mass free from gas bubbles and bits of undissolved material or batch stones.
Tank furnace is a continuous process and usually employed for the production of large quantities of only one variety of glass at a time.
2. Shaping. The plastic glass formed in the furnace is next shaped or formed into the desired articles. It is accomplished by blowing from mouth or by means of a machine. Glass blowing is a skillful art. The blowing of a glass into bottle is done as illustrated in Fig. 54.3. A lump of the plastic glass is taken on a long iron pipe. It is elongated under its weight when hung downwards. The elongated lump is introduced into a mould and is inflated by blowing air into it from the mouth. On cooling, the bottle is taken out by removing the two-halves of the mould.
Fig. 54.3. Shapinq of glass lump.
3. Annealing. It is a process of cooling slowly the newly shaped articles. If they are cooled quickly they become brittle on account of the high internal strain. Annealing allows the molecules to arrange themselves in such a way that there is no internal strain when the mass is cooled. Annealing is done in a tunnel like oven called lehr which is 50 to 60 feet long. At one end the temperature is a little below the softening point of glass, i.e., 500-600°C and it gradually falls along the length of the oven. At the other end the temperature is almost the same as the room temperature. Immediately after shaping, the articles are introduced into the lehr at the hotter end and travel towards the cooler end by means of a moving belt. It takes a few hours for the articles to move along through the tunnel. Some high quality glasses require long annealing.
4. Finishing. The articles obtained from the lehr are subjected to a number of operations such as cleaning, polishing, grinding, rounding edges, etc., for bringing them to a useable form.