The procedure adopted in 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) Fabrication (5) Annealing.
Glass Manufacturing Process # 1. Collection of Raw Materials:
Depending upon the type of glass to be manufactured, suitable raw materials are collected. Table 14-1 shows the raw materials required for each type of glass.
In addition to the raw materials, the cullet and decolourisers are also added for each type of glass.
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The cullet indicates waste glass or pieces of broken glass. They increase the fusibility of glass and prevent loss of alkali by volatisation during the reaction in forming new glass. They also reduce the cost.
The raw materials generally contain traces of the iron compounds. The ferrous oxide imparts a green colour to glass and ferric oxide imparts a very light yellow tint. To avoid such effects, the decolourisers are added. The usual substances used as decolourisers are antimony oxide (Sb2O3), arsenic oxide (AS2O3), cobalt oxide (CoO), manganese dioxide (MnO2) and nickel oxide (NiO).
Glass Manufacturing Process # 2. Preparation of Batch:
The raw materials, cullet and decolouriser are finely powdered in grinding machines. These materials are accurately weighed in correct proportions before they are 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 the batch or frit and it is taken for further process of melting in a furnace.
Glass Manufacturing Process # 3. Melting in Furnace:
The batch is melted either in a pot furnace or in a tank furnace. The heating is continued until the evolution of carbon dioxide, oxygen, sulphur dioxide and other gases stops.
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(i) Pot Furnace:
In this furnace, the pots are adopted as units. A typical glass melting pot is shown in fig. 14-1.
A pot is a vessel made of fire-clay. This process resembles the crucible steel process. These pots are placed in specially prepared holes in the furnace. The charging and collecting doors are kept projecting outside so that raw materials may be added and molten glass may be taken out conveniently.
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The pots are filled with raw materials. The furnace is heated by means of producer gas. When the mass has melted down, it is removed from the pot and it is taken for the next operation of fabrication. The melting of glass by pot furnace is an intermittent process. It is used to melt small quantities of glass at a time or to prepare special types of glass.
(ii) Tank Furnace:
This furnace resembles the reverberatory furnace adopted for puddling of the wrought-iron. Fig. 14-2 shows the section of a tank furnace adopted for the melting of glass. It is constructed with reinforced masonry. The roof is given special shape to deflect the flames of heated gas.
The ports are provided for the entry of preheated producer gas. The doors are provided for charging and for taking out the molten glass. A bridge separates the tank into two unequal compartments.
The batch is heated in large compartment and it contains somewhat impure glass. It flows through opening of bridge into small compartment. The gall or floating impurities are collected at the top of large compartment. The refractory lining is provided to the interior surface of tank.
The tank is filled with raw materials. The furnace is heated by allowing producer gas through ports. The charging of raw materials and taking out of molten mass are simultaneous. This is a continuous process and it is adopted to melt large quantities of glass at a time.
Glass Manufacturing Process # 4. Fabrication:
The molten glass is given suitable shape or form in this stage. It can either be done by hand or by machine. The hand fabrication is adopted for small scale production and machine fabrication is adopted for large scale production.
Following are the different ways of fabrication:
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(i) Blowing
(ii) Casting
(iii) Drawing
(iv) Pressing
(v) Rolling
(vi) Spinning.
(i) Blowing:
For this purpose, a blow-pipe is used. Its diameter is about 12 mm and its length is about 1.80 m. One end of the blow-pipe is dipped in the molten mass of glass and a lump of about 50 N weight is taken out. This lump of glass will then lengthen to some extent by its own weight.
The operator then blows vigorously from other end of blow pipe. It can also be done with the help of an air compressor. This blowing causes the molten mass to assume the shape of a cylinder. It is then heated for few seconds and is blown again.
The blowing and heating are continued till the cylinder of required size is formed. It is then placed on an iron plate and it is disconnected from blow pipe. The cylinder is then cut vertically by a diamond. It falls into a thin plate by gravity.
(ii) Casting:
The molten glass is poured in moulds and it is allowed to cool down slowly. The large pieces of glass of simple design can be prepared by this method. It is also adopted to prepare mirrors, lenses, etc.
(iii) Drawing:
This process consists in simply pulling the molten glass either by hand or by mechanical equipment. An iron bar is dipped sideways in the molten mass of glass. It is lifted up horizontally and in doing so, it catches up a sheet of molten glass. This sheet is then allowed to pass over a large rotating roller. The roller helps the molten glass to spread in the form of a thin sheet.
(iv) Pressing:
In this process, the molten glass is pressed into moulds. The pressure may either be applied by hand or by mechanical means. This process is adopted for ornamental articles, hollow glass articles etc.
(v) Rolling:
There are two methods of rolling. In one method, the molten mass of glass is passed between heavy iron rollers and flat glass plate of uniform thickness is obtained. In another method, the molten mass of glass is poured on a flat iron casting table and it is then turned flat with the aid of a heavy iron roller.
(vi) Spinning:
In this process, the molten glass is spun at high speed by a machine to form very fine glass fibres. This glass has tensile strength equal to that of mild steel. It does not fade, decay or shrink. It is not attacked by acids, fire and vermins. It is very soft and flexible. It is used for providing insulation against heat, electricity and sound.
Glass Manufacturing Process # 5. Annealing:
The glass articles, after being manufactured, are to be cooled down slowly and gradually. This process of slow and homogeneous cooling of glass articles is known as the annealing of glass.
The annealing of glass is a very important process. If glass articles are allowed to cool down rapidly, the superficial layer of glass cools down first as glass is a bad conductor of heat. The interior portion remains comparatively hot and it is therefore in a state of strain. Hence, such glass articles break to pieces under very slight shocks or disturbances.
Following are the two methods of annealing:
(i) Flue treatment
(ii) Oven treatment.
(i) Flue Treatment:
In this method, a long flue is provided and it is constructed in such a way that there is gradual decrease in temperature from one end of flue to the other. The red-hot articles of glass are allowed to enter at the hot end of flue and they are slowly moved on travelling bands. They become cool when they reach the cool end of flue. This method is useful for large scale production.
(ii) Oven Treatment:
In this method, the red-hot glass articles are placed in ovens in which arrangement is made to control the temperature. After articles are placed in the ovens, the temperature is slowly brought down. This method is useful for small scale production.