As present, it is possible to alter the chemical, electrical, mechanical and optical properties of glass by suitably changing the basic composition of the glass. As a matter of fact, the glass has emerged as a versatile engineering material which can be tailor-made to meet with the requirements of different industries and in construction in the most effective and economic way.
The brief descriptions of some important special varieties of glass are given:
1. Alkali-Barium Silicate Glass:
Without this type of glass watching TV would be very dangerous. A television produces X-rays that must be absorbed; otherwise they could in the long run cause health problems. The X-rays are absorbed by glass with minimum amounts of heavy oxides (lead, barium or strontium). Lead glass is commonly used for the funnel and neck of the TV tube, while glass containing barium is used for the screen.
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2. Alumino Silicate Glass:
This type of glass contains 20% aluminium oxide (alumina – Al2O3) often including calcium oxide, magnesium oxide and boric oxide in relatively small amount, but with only very small amount of soda or potash. It is able to withstand high temperatures and thermal shocks and is typically used in combustion tubes, gauge glasses for high-pressure steam boilers, and in halogen-tungsten lamps capable of operating at temperature as high as 750°C.
3. Bullet-Proof Glass:
This glass is made of several layers of plate glass and alternate layers consist of vinyl-resin plastic. The outer layers of plate glass are made thinner than the inner layers. The special care is to be taken for heating and cooling of layers during manufacture. The thickness of this type of glass may vary from 15 mm to 75 mm or more. It will not allow bullet to pierce through it.
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4. Fibre Glass:
The fibre glass is composed of minute glass rods and each glass rod resembles the parent material in all respects. It is soft to the touch and it is flexible in nature. It does not absorb water and it is proof against fire, vermins, water and acids. It can be prepared either in the form of continuous strands just like silk or in the staple form just like wool.
For making this type of glass, the molten glass is spun at a very high speed to produce continuous fine glass fibres. This is a special type of glass and it is used for thermal insulations, sheets, fibre glass reinforced plastics, etc.
In recent years, great progress has been made in making optical fibres which can guide light and thus transmit images round corners. These fibres are used in endoscopes for examination of internal human organs, changeable traffic message signs now on motorways for speed restriction warnings and communications technology without which telephones and the internet would not be possible.
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Glass fibre has many uses from roof insulation to medical equipment and its composition varies depending on its application.
5. Float Glass or Plate Glass:
The molten glass coming out of the furnace is allowed to float on the molten tin. The glass thus formed is known as the float glass and it is then further annealed to remove all the stresses. It is widely used for residential buildings, commercial complexes, furniture articles, etc.
It is superior to ordinary sheet glass and grants the following advantages:
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(i) It consumes 30% to 40% less energy and is thus environment friendly.
(ii) It is aesthetic in appearance and its use has opened unlimited possibilities of innovation in architectural design.
(iii) It is available in larger sizes and various thicknesses.
(iv) It is cost-effective as compared to the corresponding cost of brick wall, finishing material, paint, maintenance, etc.
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(v) It is ideally suitable for solar applications due to high light transmission and it results in higher efficiency.
(vi) It is tougher and more scratch-resistant.
(vii) It makes the windows that transmit more natural light and the mirrors that give true images.
(viii) It possesses high optical clarity and superior safety properties.
(ix) There is no refraction defect due to uniform thickness and superior optical clarity and thus there is less eye strain.
Some manufacturers produce float glass with a special thin coating on one side which allows the sun’s energy to pass through in one direction while reducing the thermal transfer the other way. It is known as energy efficient glass. The principle behind this is the difference in thermal wavelength of energy transmitted from the sun and that transmitted from the heat within a room.
Some manufacturers produce float glass with a special thin photo catalytic coating on one side. This coating uses the ultraviolet rays from the sun to steadily break down any organic dirt on the surface using the photo catalytic effect and thus loosen the dirt from the glass. This type of float glass is known as self-cleaning glass.
Self-cleaning glass also has hydrophilic properties which means that when rain runs down the pane of glass, it will wash away the dirt previously loosened. Both, the photo catalytic and hydrophilic effects allow the glass to stay cleaner for a longer period than untreated glass.
Small particles of dirt will loosen and (providing there is rain) be washed off fairly quickly, however, bird droppings and other large bits of dirt, will take longer to be cleaned off. Self-cleaning glass may, from time to time, need additional cleaning and great care needs to be taken with such cleaning to avoid damaging the surface coating.
Abrasive cleaning is not permitted. If additional cleaning is carried out, the self-cleaning properties may take a period of time to become active again.
6. Foam Glass:
The foam glass is prepared in the form of rectangular blocks. The finely ground glass and carbon are thoroughly mixed and the mixture is then melted in a furnace. At the time of melting, the mixture expands and assumes the form of black foam. The resulting glass material contains more than 350 million inert air cells per m3.
The foam glass floats in water and it can be cut like wood. It is fire-proof, rigid and an excellent heat insulator. It can be used as a substitute for cork for use in air-conditioning and refrigeration industries.
7. Glass Blocks:
These are partially evacuated and completely sealed hollow units which are formed by fusing together two-halves of pressed glass. The edges are coated with a grit bearing plastic material so that permanent and effective bond with the mortar is ensured.
The glass blocks are available in square sizes with dimensions as 150 mm, 200 mm and 300 mm with the approximate thickness of 100 mm. One or both the faces of the blocks are suitably treated to obscure the glass and to diffuse light. These blocks possess high insulating value and they are set in cement mortar.
The glass blocks possess the following advantages:
(i) They are easy to clean and hence they can be well maintained.
(ii) They are excellent in light transmission.
(iii) They provide very good insulation against cold, heat and noise.
The glass blocks are not intended to carry superimposed loads. But they possess adequate strength to carry their own weight upto a maximum height of 6 m. They are also able to resist the effect of lateral wind pressure for individual panels not exceeding 11 m2 in area.
8. Glass Ceramics:
Some of the glass ceramics, formed typically from lithium alumina silicate glass, are extremely resistant to thermal shock and have found several applications where this property is important, including cooker hobs, cooking ware, windows for gas or coal fires, mirror substrates for astronomical telescopes and missile nose cones.
An essential feature of glass is that it does not contain crystals. However, by deliberately stimulating crystal growth in glass it is possible to produce a type of glass with a controlled amount of crystallisation that can combine many of the best features of ceramics and glass.
9. Heat-Excluding Glass:
This glass allows light to pass through it, but it eliminates heat. It is used for windows of coaches of higher class in railways, in window panels of important buildings, etc.
10. Lead Crystal Glass:
It is made from potassium carbonate, lead oxide and sand. Due to high refractive index, lead crystal glass sparkles. It is used for making expensive glass ware. The surface of lead glass objects is often cut into decorative patterns to reflect light.
11. Obscured Glass or Patterned Glass:
This glass is used at places where light is to be transmitted but vision is to be obscured. It has one surface either ground or made opaque by melting powdered glass upon the surface. It can also be prepared by the abrading or rubbing action of the sand blast. Thus this type of glass will be useful for public toilets, office doors, partitions, etc. The types of obscured glass include figured glass, ground glass, chipped glass and corrugated glass.
12. Optical glass:
Optical glass will be found in scientific instruments, microscopes, fighter aircraft and most commonly in spectacles. The most important properties are the refractive index and the dispersion. The index is a measure of how much the glass bends light. The dispersion is a measure of the way the glass splits white light into the colours of the rainbow. Glass makers use the variations in these characteristics to develop optical glasses.
13. Perforated Glass:
In this type of glass, the perforations are made in sheet glass with the help of rollers. The perforations may be made during the manufacture or after the manufacture. It is used for panels in ventilators.
14. Photo Chromic Glass:
It is a special variety of glass that temporarily darkens when exposed to bright light. It is, therefore, very useful as a sun-shield. This automatic property of photo chromatic glass is because of the presence of silver bromide.
15. Pyrex Glass or Borosilicate Glass:
It is made by fusing a mixture of sand, lime, borax (Na2B4O7.10H2O) and alkali carbonates. It is mainly used as ovenware and other heat-resisting ware. It is also 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.
16. Safety Glass:
This glass is formed by placing celluloid between two sheets of the plate glass and then applying glue to make a single unit. If glass breaks, the flying of splinters does not occur. It is also known as the shatter-proof glass or toughened glass.
17. Sealing Glass:
The glass used to seal metals for electrical and electronic components is called sealing glass. They are grouped according to their thermal expansion which must be matched with the thermal expansions of the respective metals so that sealing is possible without excessive strain.
In making incandescent and discharge lamps, borosilicate alkaline earths-aluminous silicate glasses are suitable for sealing to tungsten. Sodium borosilicate glasses may be used for sealing to molybdenum and the iron-nickel-cobalt alloys.
Where the requirement for electrical insulation is paramount, as in many types of vacuum tube and for the encapsulation of diodes, a variety of lead glasses containing 30% to 60% lead oxide can be used.
18. Shielding Glass:
This is a special variety of glass and it contains heavy elements like lead oxide (PbO), etc. It is used for windows through which high radiation is observed. Depending upon the type of radiation, the quality of shielding glass is determined.
19. Silica Glass:
Silica glass or vitreous silica is of considerable technical importance as it has a very low thermal expansion. This glass is very difficult to make as it contains tiny holes created using acids and is used for filtration. Porous glasses of this kind are commonly known as Vycor.
20. Soluble Glass:
It is prepared by melting quartz sand, grinding and thoroughly mixing it with soda ash, sodium sulphate or potassium carbonate. The melting is carried out in glass tanks at a temperature between 1300°C to 1400°C and it takes about 7 to 10 hours. The resultant glass mass flows out from the furnace and it cool rapidly and break up into pieces, known as the silica lumps. This glass, under normal conditions, is soluble in water.
The soluble glass in the form of silicate lumps is transported in containers and in the form of liquid, it is transported in barrels or glass bottles. It is used for preparing acid-resistant cement.
21. Structural Glass:
These are in the form of glass-crete squares or lenses which are set in cement concrete and reinforced with steel frames. They are popularly known as the glass bricks. The semi-prisms formed on the underside of these bricks collect light and project it on the other dark side.
These bricks can be used as light structural members also. They are widely used for pavement lights, partitions, lantern lights, etc.
22. Technical Glass:
Technical glass is used in the electronics industry.
Following are various types of the technical glass:
(i) Borate Glass:
Without borate glass the computer revolution would not have been possible as it is vitally important in producing electrical components. This type of glass contains little or no silica and is used for soldering glass, metals or ceramics as it melts at the relatively low temperature of 450°C to 550°C.
(ii) Passivation Glass:
This glass is used for protecting silicon semi-conductor components against chemical attack and mechanical damage. It is vital in microelectronics technology and the production of the silicon chips inside computers.
(iii) Phosphate Glass:
It is a semi-conductor, which is used in the construction of secondary electron multipliers.
(iv) Chalcogenide Glass:
This type of glass has also effect of semi-conductor. It is made without the presence of oxygen. It has potential use as infrared transmitting materials and as switching devices in computer memories because their conductivity changes abruptly when particular threshold voltage values are exceeded.
23. Ultra-Violet Ray Glass:
This glass transmits effectively ultra-violet rays even though it is not in the direction of the rays of sun. It is made from the raw mixture with minimum admixtures of iron, titanium and chrome. Such a glass transmits 75 per cent of ultra-violet radiation which is far more than common glass. It is widely used in windows of schools, hospitals, etc.
24. Wired Glass:
In this type of glass, the steel wire mesh is placed in glass during rolling operation. The mesh may have hexagonal or square units. If this glass breaks, the pieces of glass are held by wire in position. This glass is also fire-resistant. The wired glass is used for roofs, skylights, fire-resisting doors and windows, etc.