Glass is a homogeneous supercooled liquid. Its very high viscosity gives it an appearance of a solid. Major constituents of glass are sodium silicate, calcium silicate and silica. The arrangement of the constituent particles in the glass does not follow any regular pattern.
Manufacture of Glass
The raw materials used in the manufacture of glass are soda ash (Na2CO3), limestone (CaCO3) and silica (SiO2).
Glass manufacturing process
Finely powdered silica, sodium carbonate and calcium carbonate are mixed with broken glass pieces called cullet and heated at about 1600 K – 1700 K. Cullet increases the fusibility of the raw material evolution of CO2. The most significant part of manufacture of glass is cooling of molten glass which should be carried out carefully at a moderate and uniform rate in order to achieve the required properties of glass.
Manufacture of Glassware
Different methods are adopted for the manufacture of different types of glass ware from molten glass.
Hollow glass articles are manufactured by glass – blowing technology . There are two different methods of glass blowing :
1. Free – blowing
2. Mould – blowing
Free – blowing
The process of free – blowing involves the blowing of air to inflate the molten glass which is gathered at one end of the blowpipe to give the desired shape. Here the perfection depends on the skill of the glass worker.
Mould – blowing
Mould – blowing is an alternate glass – blowing method that was developed after the technique of free – blowing. In this process, molten glass is inflated into a wooden or metal-carved mould with the help of the blow pipe which gives the molten glass the shape and design of the interior of the mould.
In this process , a motorized plunger is used to press the molten glass taken inside the blank mold to make the glass containers and utensils ,
Float Glass Method
Besides the above processes , the flat or sheet glass which is widely used as window panes , glass walls. glass doors , etc. , is manufactured by floating molten glass on a bed of molten tin. This sheet of glass made by this process possesses uniform thickness and very flat surface.
Types of Glass
Since glass has a wide variety of applications, the characteristics of glass are required to be changed to achieve the specific properties suitable for the end products. This can be done by doping different compounds in glass.
The raw material of silica glass is almost 100% pure form of quartz, and hence, it is very expensive. If undergoes very less thermal expansion and has high chemical resistance. Due to these characteristics it is used for the manufacture of laboratory apparatus.
Borosilicate glass is composed of about 80 % silica , 13 % boron trioxide , small amount of oxides of sodium and aluminum . Its coefficient of thermal expansion is low, softening point is very high and it is resistant to a wide variety of chemicals. These properties make it suitable for the manufacture of laboratory ware.
Alkali Silicate Glass
The raw materials of alkali silicate glass are sand and soda . It is also known as water glass because it is soluble in water and used only as a solution. It is generally used for making gums and adhesives.
In lead glass , lead oxide is added which increases the density of the glass thereby increasing its refractive index . This type of glass is used for the manufacture of ornamental glassware, decorative articles, etc.
Optical glass is required to be absolutely flawless because it is used for the manufacture of optical instruments, like, binoculars, spectacles, lenses, prisms, telescopes, microscopes, etc. This type on glass is completely transparent and can be ground into the required shape optical glass generally contains phosphorus and lead silicate with little cerium oxide which absorbs the UV radiation. However, it has the widest range of composition with slight variation in the ingredients which gives rise to slight variation in optical properties and makes the glass suitable for the specific end product.
The colour of the glass can be obtained by the homogeneous distribution of different ions throughout the glass during the manufacture of glass. Presence of different oxides of iron gives green or brown colour, whereas the presence of oxides of copper gives rise to light blue or red colour. This type of glass is used for decorative , technical and scientific purposes.
|Iron Oxides||Green brown|
|Manganese oxides||Deep amber , amethyst , decolonizer|
|Cobalt oxide||Deep blue|
Table 1.1 Compound used in coloured glasses
Properties of glass are not only attributed to the slight alteration of the ingredients but also to the processing of the glass : Some of the examples of processed glass and their applications are given below.
Table 1.2 Different types of processed glass and their uses
|Laminated glass||Doors and windows of automobiles Because it provides good strength to the glass|
|Fibre glass||Used for reinforcing purpose due to its hulle Ligh tensile strength|
|Foam glass||Light weight glans used for civil construction and insulation purposes|
|Opaque glass||Non – transparent glass filters the light entering into it and provides an provides an aesthetic look|