In this article we will discuss about:- 1. Recent Developments in Glass Industry 2. Classification or types of Glass 3. Composition 4. Properties 5. Types 6. Treatment.
Recent Developments in Glass Industry:
The glass has been used as an engineering material since ancient times. But because of the rapid progress made in the glass industry in recent times, the glass has come out as the most versatile engineering material of the modern times. The first glass objects made by man were of natural glass such as obsidian and rock crystal.
The manufactured glass dates from per-historic times in the Far East, India and Egypt. But its exact place and date of origin are unknown. It is however believed that the ancient Hindus knew the method of glass making long before the Christian era.
With the help of techniques developed in the glass industry, the glass of any type and quality can be produced to suit the requirements of different industries.
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Just to stress the importance of glass in the engineering field of today, few of the recent developments that have taken place in the glass industry are mentioned below:
(i) A modern Boeing 707 jet plane contains more than 5000 components of glass.
(ii) The fibre glass reinforced with plastics can be used in the construction of furniture, lampshades, bathroom fittings, navy boats, aeroplanes, cars, trucks, etc.
(iii) The glass is the only material strong enough to go upto the bottom of ocean and to maintain its buoyancy. It is therefore used in the construction of noses of deep-diving vehicles.
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(iv) The glass linings are applied on equipment likely to be affected by the chemical corrosion such as valves, pumps, pipes, etc.
(v) In the construction of modern homes, the walls and ceilings of hollow glass blocks can be made. Such construction cuts off the glare. But it admits sunlight and controls sound and heat in a better way.
(vi) In the field of fire-arms, the glass is used to forma rifle barrel which is lighter and stronger than the conventional type.
(vii) It will be interesting to note that now-a-days, it is possible to prepare the colour-changing glass. A window with such glass will be transparent during daytime and it will be a source of light at night.
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(viii) The body of a guided missile contains thousands of glass items.
(ix) The development and advancement of sciences of astronomy and bacteriology are mainly due to the use of optical glass.
(x) The mechanical strength of ordinary glass varies from 35 to 70 N/mm2. Due to research in glass industry, it has become possible to produce glass having mechanical strength of about 420 N/mm2.
Classification of Glass:
The glass is a mixture of a number of metallic silicates, one of which is usually that, of an alkali metal. It is amorphous, transparent or translucent. It may also be considered as a solidified super-cooled solution of various metallic silicates having infinite viscosity.
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For the purpose of classification, the glass may be grouped into the following five main categories:
(1) Soda-lime glass or commercial glass
(2) Potash-lime glass
(3) Potash-lead glass
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(4) Common glass
(5) Borosilicate glass.
Composition of Glass:
The glass is not a single compound. It is therefore very difficult to give any particular chemical formula for it. But with reasonable accuracy, it may generally be expressed as follows –
aX2O, bYO, 6SiO2
Where, a and b are numbers of molecules,
X = an atom of an alkali metal such as Na, K, etc.
Y = an atom of a bivalent metal such as Ca, Pb, etc.
With this expression, the chemical formulas for three groups of glass, as classified above, are as follows:
Properties of Glass:
The properties of glass are mainly governed by factors such as composition of the constituents, state of surface, thermal treatment conditions, dimensions of specimen, etc.
Following are the properties of glass which have made the glass popular and useful:
(i) It absorbs, refracts or transmits light.
(ii) It can take up a high polish and may be used as substitute for very costly gems.
(iii) It has no definite crystalline structure.
(iv) It has no sharp melting point.
(v) It is affected by alkalies.
(vi) It is an excellent electrical insulator at elevated temperatures due to the fact that glass can be considered as an ionic liquid. The ions are not easily moved at room temperature because of the high viscosity. But when the temperature rises, the ions are permitted to flow and thus they will sustain an electric current.
(vii) It is available in beautiful colours.
(viii) It behaves more as a solid than most solids in the sense that, it is elastic. But when the elastic limit is exceeded, it fractures instead of deforming.
(ix) It is capable of being worked in many ways. It can be blown, drawn or pressed. But it is strange to note that it is difficult to cast in large pieces.
(x) It is extremely brittle.
(xi) It is not usually affected by air or water.
(xii) It is not easily attacked by ordinary chemical reagents.
(xiii) It is possible to intentionally alter some of its properties such as fusibility, hardness, refractive power, etc. to suit different purposes.
(xiv) It is possible to obtain glasses with diversified properties. The glasses may be clear, colourless, diffused and stained.
(xv) It is possible to weld pieces of glass by fusion.
(xvi) It is transparent and translucent. The transparency is the most used characteristic of glass and it is due to the absence of free electrons. For the same reason, it also works as a good insulator.
(xvii) When it is heated, it becomes soft and soft with the rise in temperature. It is ultimately transformed into a mobile liquid. This liquid, when allowed to cool, passes to all the degrees of viscosity.
This property of glass has made its manufacturing process easy. It can also be formed into articles of desired shape. Thus the amorphousness of glass permits it to be blown, drawn from furnaces and continuously worked.
(xviii) Due to advancement made in the science of glass production, it is possible to make glass lighter than cork or softer than cotton or stronger than steel. The strength of glass however is considerably affected by foreign inclusions, internal defects and cords or chemically heterogeneous areas.
(xix) The glass panes can be cleaned easily by any one of the following methods:
(a) Applying methylated spirit;
(b) Painting the glass panes with lime-wash and leaving it to dry and then washing with clean water;
(c) Rubbing damp salt for cleaning paint spots; and
(d) Rubbing finely powdered chalk.
It can thus be easily appreciated that glass, though used for thousands of years, is just beginning to be understood and it is still possible to get a variety of glasses with certain chemical additives. Further investigations are yet in the process for preparing glass with extraordinary unusual characteristics and thus to increase the utility of this unique and complex material.
As a matter of fact, the glass industry has made enormous progress all over the world and the glass has become very cheap and useful to the poor as well as to the rich.
Types of Glass:
The properties and uses of the following types of glass will now be discussed:
(1) Soda-lime glass or commercial glass
(2) Potash-lime glass
(3) Potash-lead glass
(4) Common glass
(5) Borosilicate glass.
(1) Soda-Lime Glass or Commercial Glass:
This is also known as the soda-glass or soft-glass. It is mainly a mixture of sodium silicate and calcium silicate.
Properties:
Following are the properties of soda-lime glass:
(i) It is available in clean and clear state.
(ii) It is cheap.
(iii) It is easily fusible at comparatively low temperatures.
(iv) It is possible to blow or to weld articles made from this glass with the help of simple sources of heat.
Uses:
It is used in the manufacture of glass tubes and other laboratory apparatus, plate glass, window glass, etc.
(2) Potash-Lime Glass:
This is also known as the Bohemian-glass or hard-glass. It is mainly a mixture of potassium silicate and calcium silicate.
Properties:
Following are the properties of potash-lime glass:
(i) It fuses at high temperatures.
(ii) It is not easily affected by water and other solvents.
(iii) It does not melt so easily.
Uses:
This glass is used in the manufacture of glass articles which have to withstand high temperatures such as combustion tubes, etc.
(3) Potash-Lead Glass:
This is also known as the flint glass. It is mainly a mixture of potassium silicate and lead silicate.
Properties:
Following are the properties of potash-lead glass:
(i) It fuses very easily.
(ii) It is easily attacked by aqueous solutions.
(iii) It possesses bright lustre and great refractive power.
(iv) Its specific gravity is about 3 to 3.30.
(v) It turns black and opaque, if it comes into contact with reducing gases of the furnace during heating.
Uses:
It is used in the manufacture of artificial gems, electric bulbs, lenses, prisms, etc.
(4) Common Glass:
This is also known as the bottle glass. It is prepared from cheap raw materials. It is mainly a mixture of sodium silicate, calcium silicate and iron silicate.
Properties:
Following are the properties of common glass:
(i) It fuses with difficulty.
(ii) It is brown, green or yellow in colour.
(iii) It is easily attacked by acids.
Uses:
It is mainly used in the manufacture of medicine bottles.
(5) Borosilicate Glass:
Most of us are more familiar with this type of glass in the form of ovenware and other heat-resisting ware, better known under the trade name Pyrax. Borosilicate glass is made mainly of 70% to 80% silica and 7% to 13% boric oxide with smaller amounts of the alkalis (sodium and potassium oxides) and aluminium oxide.
Properties:
Following are the properties of borosilicate glass:
(i) It has a relatively low alkali content and consequently has good chemical durability and thermal shock resistance.
(ii) It has high softening point.
(iii) It does not break when temperature changes quickly.
Uses:
This glass is widely used in the chemical industry, for laboratory apparatus, for ampoules and other pharmaceutical containers, for various high intensity lighting applications and as glass fibres used in the reinforced plastics to make protective helmets, boats, piping, car chassis, ropes, car exhausts and many other items and also in textile industry.
Treatment of Glass:
The glass may be given any of the following treatment:
(1) Bending
(2) Cutting
(3) Opaque making
(4) Silvering.
(1) Bending:
The glass may be bent into desired shape by placing it in ovens in which the temperature can be regulated. The glass in the form of rods, sheets or tubes is placed in such ovens and heated. It is then bent when it is suitably heated.
(2) Cutting:
The glass is cut in required sizes with the help of diamond or rough glasses or small wheels of hardened steel.
(3) Opaque Making:
The glass can also be made opaque or impervious to light. It is done by grinding the glass surface with emery. It can also be achieved chemically by the application of hydrofluoric acid.
(4) Silvering:
This process consists in applying a very thin coat of tin on the surface of glass. The silver is deposited on this layer of tin. A suitable paint is then applied to give protection against the atmospheric effects.