2 Glass

Your are familiar with glass and its common applications. Glass is defined as ‘a rigid, amorphous transparent or translucent supercooled liquid’. It means that glass is not a solid but a rigid material that may allow light to pass through. Further, it is amorphous which means that it lacks regular three dimensional internal structure characteristic of crystalline solids. Glass has extremely high viscosity and normally does not set over a reasonably long period of time.

Glass is obtained by the fusion of sand, alkali metal carbonates alkaline earth carbonates and some other additives. . Glass is a mixture of silicates of alkali and alkaline earth metals with excess of silica. Some verities of glasses may contain small amounts of Al2O3, Fe2O3 , etc. Some common variety of glasses are given in Table 36.3

Nowdays several formulations of glass are known. Silica, soda and lime are main constituents of all glasses. Let us learn about the various raw materials used in manufacture of glass and their roles.

36.2.1 Manufacture of Glass

A large variety of glasses are known today and these are prepared by varying the composition of the raw materials. The basic raw materials and their role in determining the properties of a glass are briefly outlined below :

(a) Raw materials

    (i) Sand It is the source of silica.- the major component of glass. The sand used for manufacture of glass must be almost pure quartz containing about 99.1-99.7 % SiO2. The sand particles should be uniform and of moderate size. Larger particles are slow to react while very fine particles cause violent reaction.

    (ii) Alkali metal compounds: (Na2CO3, K2CO3, NaNO3, KNO3, Na2SO4). These are the sources of Na2O (in soda glass) and K2O (in hard glass).

    (iii) Alkaline earth compounds: (CaO, CaCO3 and BaCO3). These are provided by lime stone and burnt lime from dolomite (CaCO3 .MgCO3). Dolomite also provides MgO which helps in controlling the physical properties of glass. Alkaline Earth compounds contribute towards the high refractive index of glass.

     (iv) Heavy metal oxides (ZnO, PbO, Pb2O4 and Pb3O4): Litharge (PbO) or red lead (Pb3O4) are used as a source of lead in flint glass or crystal glass. It provides lusture and high refractive index to the glass.

    (v) Feldspar is a cheap source of Al2O3. Feldspars have a general formula  I2MI O.Al2O3.6SiO2 where MI represents Na or K or both. Therefore, feldspar has an added advantage as it provides Na2O, K2O and silica also. Alumina (Al2O3) makes glass resistant to sudden changes in temperature.

(vi) Borax is a minor component and is a source of boron oxide (B2O3) and Na2O. Borax reduces the coefficient of expansion of glass and increases its fusibility, chemical durability and refinement.

(vii) Cullets or pieces of broken glass are generally added to the raw material to increase the fusibility of the glass produced from it. In other words it lowers the temperature of fusion so that we can manufacture glass at a lower temperature. The cullets may contribute from 10% to 80% of the raw material and thus provide a good way of utilising the waste glass.

(viii) Besides the above mentioned raw materials certain metallic oxides are also added to impart a colour (if desired) to the glass. Some of the principal glass colorants are given in Table 36.4 .

Table 36.4 : Some common glass colorants

Colorant                                    Formula                            Colour imparted                

Oxides of iron                            FeO, Fe2O3                    Light green or bottle green

Chromium compounds                Cr2O3, K2CrO4            Green tending to yellow

Cobalt oxide                                CoO                                Blue

Cadmium sulphate                        CdSO4                          Yellow

Gold powder                                Colloidal Au                  Shades of ruby

(b) Manufacturing Process: The raw materials including the cullets are powdered in a grinding mill and mixed in proper proportion. This powder is then fused in an open or covered furnace. Certain decolorants like MnO, etc., are added and the heating is continued till the evolution of gases stops. The coloring material is added at this stage and the heating is continued till a homogenous mass is obtained. It is then gradually cooled to a certain degree of plasticity so that it can be handled easily. The glass so obtained is called mother glass. It is then blown or molded to get the desired shape. The articles are then cooled slowly. This process is called annealing. The finished glass product is then obtained after processes like cleaning, cutting, grinding and polishing, etc.

2.2 Properties of Glass

    It is amorphous in nature, having low range of internal order.

    It does not have a sharp melting point. On heating glass gradually becomes softer and softer and eventually starts flowing.

    On cooling, the hot liquid gradually cools to a viscous liquid which can then be transformed into articles of the desired shape.

    It may be transparent or translucent.

    It is quite inert and is not readily affected by ordinary chemicals except hydrogen fluoride and some alkalis.

    It has high compressor strength It lacks an ordered internal structure.

    It is an insulator of heat and electricity.