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
(1) Collection of raw materials: Depending upon the type of glass to be manufactured, suitable raw materials are collected.
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 color 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).
(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 furnance.
(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.
Pot Furnace: In this furnace, the pots are adopted as units.
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 collection doors are kept projecting outside so that raw materials may be added and molten glass may be taken out conveniently.
The pots are filled with raw materials. The furnace is heated by means of producer gas. When
(i) Blowing: For this purpose, a blow-pipe is used. Its diameter is about 12 mm and its length 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 weights 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 cylinder. It is then heated for few seconds and is blow 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.