Glass is an amorphous solid made by rapidly cooling molten materials such as silica, soda ash, limestone, and lead. It has no definite melting point and softens over a range of temperatures. Glass is produced through melting and refining raw materials, forming the molten glass through various techniques like pressing or blowing, heat treating for strength and annealing, and finishing the surface. The main types of glass are soda-lime glass which is low-cost but not acid resistant, lead glass which has high refractive index, and borosilicate glass which is heat and chemically resistant. Recent developments include bioactive glass for bone repair and replacement.

2. DEFINITION
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.
It possess no definite formula or crystalline structure.
“An inorganic product of fusion which has cooled to a rigid
condition without crystallizing”
Does not have a specific melting point
Softens over a temperature range

3. PROPERTIES OF GLASS
Glass is:
Amorphous
Brittle
Transparent / Translucent
Good electrical insulator
Unaffected by air, water, acid or chemical
reagents
No definite crystal structure means glass
has high Compressive strength
Can absorb, transmit and reflect light

4. RAW MATERIALS USED IN MANUFACTURING GLASS
Raw Materials
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.

5. GLASS PRODUCTION AND PROCESSING
MAJOR STEPS
1.Melting and refining.
2.Forming and shaping.
3.Heat treatment
4.Finishing

6. 1.MELTING AND REFINING
The mixed batch of raw materials, along with broken
or reclaimed glass, is fed into one end of a continuous
–type furnace where it melts and remains molten at
around 1500 deg.
Molten glass is drawn continuously from the furnace
and runs in troughs to the working area, where it is
drawn off for fabrication at a temperature of about
1000 deg.

7. 2.FORMING METHODS
PRESSING
low cost fabrication method.
Used to manufacture table- and ovenware, insulators,
lenses, and reflectors.
Gobs of glass are fed into molds on a rotating press.
The molds are moved beneath a plunger which forces
the glass into final shape.

8. GLASS BLOWING
Used to produce hollow products such as bottles, jars,
and light bulbs.
Molten glass in ribbon form sags through holes as air is
blown in from above.
At the same time, moulds move up from below and
clamp around the molten glass.
More puffs of air forces the glass into the mould to
form final shape.

9. 3.HEAT-TREATMENT OF GLASS
ANNEALING
When glass cools from the forming range to room
temperature , thermal stresses develop that adversely
affect strength properties.
Annealing is done to eliminate this stress.
Involves heating the glass to annealing range, holding
it there for a period of time, and then cooling it slowly
to room temperature.

10. TEMPERING
Involves heating glass to around the softening point
and then cooling it rapidly with blasts of air or by
quenching it in oil.
Heat tempered glass is 3 to 5 times stronger than
annealed glass while still retaining its initial clarity,
hardness, and expansion co-efficient.

11. 4.FINISHING GLASS SURFACE
Hydrofluoric acid is used for polishing and etching.
Also be stained by copper and silver compounds.
Ferric or cerium oxides are used to produce smooth,
accurately finished surfaces.

12. VARIETIES OF GLASS

13. SODALIME OR SOFT GLASS
About 90% of all glass is soda-lime glass made with silica
(sand), Calcium carbonate and soda ash.
The approximate composition is Na2CO3.CaO.6SiO2.
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

14. POTASH LIME OR HARD GLASS
Potash lime glass is made with silica (sand), Calcium
carbonate and potassium carbonate.
The approximate composition is K2CO3.CaO.6SiO2.
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.

15. LEAD GLASS OR FLINT GLASS
It is made up of lead oxide fluxed with silica and K2CO3 is used instead of
sodium oxide.
Its approximate composition is K2Co3.PbO.SiO2.
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.

16. BOROSILICATE / PYREX / JENA GLASS
It is common hard glass containing silica
and boron with small amount of alumina
and less alkaline solids.
It contains SiO2(80.5%), B2O3(13%),
Al2O3(03%), K2O(3%) and Na2O(0.5%).
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.

17. 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.

18. 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.

19. TOUGHENED GLASS
It is made by dipping articles still
hot in an oil bath, so that chilling
takes place. This results in outer
layer of articles shrink and acquire a
state of compression while inner
layer are in state of tension. Such
glass is more elastic to mechanical
and thermal shock. It breaks into a
fine powder.
Uses:
For making window shields of fast
moving vehicles, windows of
furnace and automatic opening
doors.

20. WIRED GLASS
Wired glass does not fall apart into splinters when it breaks and
is fire resistant. It is made by fusing wire in between the two
glass layers.
Uses:
For making fire resistant doors, roofs, skylights and windows

21. PHOTOSENSITIVE GLASS
These are glasses by which a
colored picture may be
developed by exposing the
glass to black and white
negative in ultra violet light.
The appropriate proportions
of potash-alumina glass,
mixed with LiSO3, cerium and
Silver salts have also been
used as photosensitive glass.
Uses:
Photographic development

22. RECENT DEVELOPMENTS
Bioactive glasses : A group of surface reactive glass-ceramic
biomaterials and include the original
bioactive glass, Bioglass. The biocompatibility of these
glasses has led them to be investigated extensively for
use as implant materials in the human body to repair
and replace diseased or damaged bone.