Monday, March 30, 2020

GLASS

Glass as a building material :

Glass is an amorphous, hard, brittle, transparent or translucent super cooled liquid of infinite
viscosity, having no definite melting point obtained by fusing a mixture of a number of metallic
silicates or borates of Sodium, Potassium, Calcium, and Lead.
Manufactured Glass dates from pre-historic times in the Far East, India and Egypt
Little is known about the first attempts to make glass.


However, it is generally believed that glass making was discovered 4,000 years ago, or more, in Mesopotamia.
The Roman historian Pliny attributed the origin of glassmaking to Phoenician sailors.
He recounted how they landed on a beach near Ptolemais (in modern-day Israel), propped a
cooking pot on some blocks of natron (a naturally-occurring alkali substance) they were
carrying as cargo, and made a fire over which to cook a meal.

Properties of glass :

1. It absorbs, refracts or transmits light.
2. It can take up a high polish and may be used as a substitute for very costly gems.
3. Is has no definite crystalline structure.
4. It has no sharp melting point.
5. It is affected by alkalis.
6. 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.
7. It has good workability. It can be blown, drawn or presses. But it is difficult to cast in large
pieces.
8. It is extremely brittle.
9. It is not affected by air or water.
10. It is not easily attacked by ordinary chemical reagents.

Manufacture of glass :

Glass is produced by heating a mixture that consists largely of Sillica(silicon dioxide) and soda ash
(sodium carbonate). Soda ash serves as a flux to reduce the high melting point of silica (approz
1800 degree celcius). the melting that then takes place above 1100deg. cel. is amorphous that is virtually
nocrystals are formed. Because the structure of glass resembles that of fluids, glass is sometimes
called a "supercooled liquid"

#Raw materials used for manufacturing:
• Sodium as Na2Co3 (used in soft glass).
• Potassium as K2Co3 (used in Hard Glass).
• Calcium as lime stone, chalk and lime.
• Lead as litharge, red lead (flint glass).
• Silica arc quartz, white sand and ignited flint.
• Zinc is zinc oxide (Heat and shock proof glass).
• Borates are borax, Boric acid (Heat and shock proof glass).
• Cullets or pieces of broken glass to increase fusibility.



     1. Melting Process: Raw materials in proper proportions are mixed with cullets. It is finely powdered
and intimate mixture called batch is fused in furnace at high temperature of 1800°C this charge melts
and fuses into a viscous fluid. After removal of CO2 decolorizes like MnO2 are added to remove traces of ferrous compounds and
Carbon. Heating is continued till clear molten mass is free from bubbles is obtained and it is then
cooled to about 800°C.
     2. Forming and Shaping: The viscous mass obtained from melting is poured into
moulds to get different types of articles of desired shape by either blowing or pressing between the
rollers.
     3. Annealing: Glass articles are then allowed to cool gradually at room
temperature by passing through different chambers with descending temperatures. This reduces the
internal Strain in the glass.
    4. Finishing: It is the last step in glass manufacturing.Like Cleaning, Grinding, Polishing, Cutting, Sand Blasting.

Following are process for sheet formation of glass:
1. Drawn Clear Sheet Glass: Clear sheet glass is transparent glass with 85% light transmission
with fire finished surface.
2. Vertical Drawing : The VD from a pool of molten glass which when 1m or so above the
pool level is rigid enough to be engaged by a series of asbestos faced rollers that continue to draw
the ribbon of glass up a tower some 10m high after which the ribbon is cut into sheets & washed in a
dilute acid to remove surface deposits.


3. Horizontal Drawing : The glass is initially drawn in the vertical plane but it is turned over
a roller so that it is drawn in the horizontal direction for some 60m & pass in to an annealing furnace
at the cold end of which it is cut in to sheets.
BS 952 recommends for sheet glass:
Ordinary Glazing Quality : this to be used for general glazing purpose.
Selected Glazing Quality : for glazing work requiring a sheet glass above the ordinary glazing
quality.

Classification of glass :

a. According to its manufacturing processes: Float glass, Clear glass, Soda lime glass, lead glass, Rolled glass, Crystal glass, Reinforcement
glass, Wired glass & Opal Glass.
3 steps involve: Melting, Forming & Controlled cooling- Annealing.
b. Post application processes: Offline coating glass, Self cleaning glass, Laminated glass, Chemically strengthened glass,
Thermally Toughened glass, Low- E glass
c. Post manufacturing processes: Edge treatment, Sand blasting, Acid etching, Pigmented glass, Bended glass.

Types of glass :

1. Soda lime or Soft Glass:


About 90% of all glass is soda-lime glass made with silica (sand), Calcium carbonate and
soda ash.
They are low cost, resistant to water but not to acids.
They can melt easily and hence can be hot worked.
Uses: Window glass, Electric bulbs, Plate glass, Bottles, Jars, cheaper table wares, test
tubes, reagent bottles etc.

2. Float Glass: 


Float glass is formed by floating a continuous ribbon of molten glass over a
bath of liquid metal at a controlled rate & temperature .The continuous ribbon of molten glass is then
run in to an annealing chamber in which the temperature is gradually reduced to avoid distortion of
the glass.
Float glass also known as polished plate glass.
This is truly flat glass with undistorted vision.

3. Potash lime or hard glass:


Potash lime glass is made with silica (sand), Calcium carbonate and
potassium carbonate.
They posses high melting point, fuse with difficulty and are less acted upon by acids, alkaline
and other solvents than ordinary glass.
Uses: These glasses are costlier than soda lime glass and are used for chemical apparatus,
combustion tubes and glassware which are used for heating operations.

4. Lead glass or flint glass:


It is made up of lead oxide fluxed with silica and K2CO3 is used instead of sodium oxide.
To get dense optical glasses about 80% lead oxide is used. Lead glasses has a lower
softening temperature than soda glass and higher refractive index and good
electrical properties.
It is bright lustrous and possess high specific gravity.
Uses: High quality table wares, optical lenses, neon sign tubing, cathode ray tubes, electrical
insulators, crystal art objects or cut glass, Windows and Shields for protection against X-rays
and Gamma rays in medical and atomic energy fields etc.

5. Borosilicate / pyrex glass:


It is common hard glass containing silica and boron with small amount of alumina and less
alkaline solids.
These glass have low thermal coefficient of expansion, and high chemical resistance
i.e..shock proof.
Uses: Industrially used for pipeline of corrosive liquids, gauge glasses, superior laboratory
apparatus, kitchen wares, chemical plants, television tubes, electrical insulators etc.

6. Alumino-silicate glass:


This type of glass possess exceptionally high softening temperature.
Uses: It is used for high pressure mercury discharge tubes, chemical combustion tubes and
certain domestic equipments.

7. 96% silica glass:


It is translucent, the coefficient of thermal expansion is very low hence it has high resistance
to thermal shock, have high chemical resistance to corrosive agents and are corroded only
by Hydrofluoric acid, hot phosphoric acids and concentrated alkaline solutions.
Uses: Used only where high temperature resistance is required (800°C). They are used in
construction of chemical plants, laboratory crucibles, induction furnace lining and electrical
insulators.

8. 99.5% silica glass / vitreosil:

It contains pure silica heated to its melting point. It is translucent, the coefficient of thermal
expansion is very low hence it has high resistance to thermal shock, have high chemical
resistance to corrosive agents.
If Vitreosil glass is heated above its melting point, it becomes transparent and is known as
clear silica glass.
Uses: They are used in construction of chemical
plants, laboratory crucibles, induction.

9. Safety glass:
It is made by fusing two to three flat sheets of glass
and in between them alternate thin layer of vinyl plastic
is introduced.
It is heated where both the layers merge together and
glass is toughened.
Uses: It is used as wind shield in automobiles and
airplanes. On breaking it pieces does not fly apart because of the presence of the plastic
layer in between the glass layers.

10. Optical or crook’s glass:


It contains Phosphorus, PbCO3, silicates and Cerium oxide which has the property to absorb
harmful ultra-violet light.
This glass is given through homogeneity by heating it for a prolonged period of time.
These glasses have low melting point and are relatively soft.
Uses: They are used for making optical lenses.